Course 512
LTE Long Term Evolution
Introduction, Air Interface, Core Network, Operation
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 1
512 Course Contents
 Introduction to LTE
• LTE’s place in the family of wireless technologies
• LTE Features, Advantages, Comparison to prior wireless technologies
 The LTE Air Interface
• Basic signal structure, OFDM details, Downlink and Uplink structure
• MIMO, Scheduling, Link Adaptation, Multicast MGSFN, MCH
 LTE Core Network Architecture
• SAE: The Evolved Packet Core Network Architecture
• Network Functional Elements and Standard Interfaces
• The Protocol Stack/Layers: Physical, MAC, RLC, PDCP, RRC, NAS
 LTE Advanced
• Carrier Aggregation, Multi-antenna solutions, relay technology
 Current Hot Topics in LTE
• Voice-over-IP: LTE voice techniques and legacy fallback
• HetNets, Home eNBs, advanced integration
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 2
Introduction to LTE
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 3
Wireless Generations and Sector Data Speeds
EARLY ANALOG
MTS, IMTS
AutoTel
 In the days before analog cellular, various wide-area mobile
telecommunications systems were used
• They covered wide areas with only a few channels available
• Voice calls only - the internet didn't even exist
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 4
Wireless Generations and Sector Data Speeds
1G
EARLY ANALOG
AMPS: Analog Cellular
NMT450, NMT900
MTS, IMTS
AutoTel
LMR, SMR
 1G: When the first cellular systems launched, even though data
wasn't offered by the carriers, a few hardy users provided their own
(MNP10) modems for haphazard, slow data via dialup access
• The internet wasn't a big factor yet!
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 5
Wireless Generations and Sector Data Speeds
2G
1G
EARLY ANALOG
CDMA IS-95, J-Std 008
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
 2G provided digital data but at low bit rates -- 9600 - 32k bps
• Downloading a 2MB file took an hour or more (if it didn't drop in
the middle and require manually re-starting)
• Travel agents with telephones were still faster than online res.
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 6
Wireless Generations and Sector Data Speeds
200+˅
200+˄
153˅
153˄
2.5 G
2G
1G
EARLY ANALOG
CDMA-2000, 1xRTT
CDMA IS-95, J-Std 008
GPRS, EDGE
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
 When 1xRTT, GPRS, and EDGE became available, suddenly it
was possible to do direct IP web access at speeds of 150 kbps or
higher. This was better than dial-up speeds, especially via hotel
switchboards. Nerds and even some normal people on the road
were finally free to stay connected on-line
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 7
Wireless Generations and Sector Data Speeds
153˅
153˄
3G
2.5 G
2G
1G
EARLY ANALOG
3.1M˅
1.8M˄
7M+˅
3M+˄
200+˅
200+˄
1xEV-DO
UMTS WCDMA HSPA
CDMA-2000, 1xRTT
CDMA IS-95, J-Std 008
GPRS, EDGE
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
 When the true 3G services 1xEV-DO and WCDMA/UMTS/HSPA
became available, wireless speeds were boosted into the Mb/s
range for downloading and approaching 1 Mb/s for uploading
 Now mobile users had almost normal internet access, although
many networks had heavy congestion in dense usage areas
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 8
Wireless Generations and Sector Data Speeds
100M˅
50M˄
153˅
153˄
4G
3G
2.5 G
2G
1G
EARLY ANALOG
44M˅
22M˄
HSPA+
3.1M˅
1.8M˄
100M˅
50M˄
7M+˅
3M+˄
200+˅
200+˄
WiMAX
LTE
1xEV-DO
UMTS WCDMA HSPA
CDMA-2000, 1xRTT
CDMA IS-95, J-Std 008
GPRS, EDGE
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
 The first WiMAX and LTE networks brought user speeds of up to
12 Mb/s and even 3G HSPA was enhanced to HSPA+, providing
nearly transparent internet usage for the first time.
 4G Network buildouts were slow, with some carriers still building
only trial networks even in late 2011
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 9
Wireless Generations and Sector Data Speeds
100M˅
50M˄
153˅
153˄
4G
3G
2.5 G
2G
1G
EARLY ANALOG
44M˅
22M˄
HSPA+
3.1M˅
1.8M˄
100M˅
50M˄
1000M˅
500M˄
7M+˅
3M+˄
200+˅
200+˄
WiMAX
LTE
1xEV-DO
LTE adv.
UMTS WCDMA HSPA
CDMA-2000, 1xRTT
CDMA IS-95, J-Std 008
GPRS, EDGE
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
 Within 2 years of initial LTE buildouts,
• Widespread use of MIMO is expected to boost speed 3-4x
• LTE-Advanced technology is expected to boost speeds to 5001000 Mb/s for stationary downlink users
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 10
Wireless Generations and Sector Data Speeds
4G
3G
2.5 G
2G
1G
EARLY ANALOG
7M+˅
3M+˄
200+˅
200+˄
WiMAX
VOIP
VOIP?
153˅
153˄
3.1M˅
1.8M˄
44M˅
22M˄
HSPA+
100M˅
50M˄
LTE
1xEV-DO
VOIP
100M˅
50M˄
1000M˅
500M˄
LTE adv.
UMTS WCDMA HSPA
CDMA-2000, 1xRTT
CDMA IS-95, J-Std 008
GPRS, EDGE
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
 Finally the industry will settle on one or two VOIP standards for
LTE, voice traffic of legacy CDMA and GSM will finally go to LTE
 Nearly all WiMax networks will finally convert to LTE
 CDMA and LTE voice networks won't die until 2017 or even later!
October, 2011
Course 512 v1.0 (c)2011 Scott Baxter
Page 11
Wireless Generations and Sector Data Speeds
4G
3G
2.5 G
2G
1G
EARLY ANALOG




7M+˅
3M+˄
200+˅
200+˄
WiMAX
LTE
1xEV-DO
VOIP
VOIP?
153˅
153˄
3.1M˅
1.8M˄
44M˅
22M˄
HSPA+
100M˅
50M˄
VOIP
100M˅
50M˄
1000M˅
500M˄
LTE adv.
UMTS WCDMA HSPA
CDMA-2000, 1xRTT
CDMA IS-95, J-Std 008
GPRS, EDGE
TDMA: NADC, IS-136
TDMA: GSM, HSCSD
TDMA: IDEN
AMPS: Analog Cellular
NMT450, NMT900
LMR, SMR
MTS, IMTS
AutoTel
1G: Users provided their own modems for haphazard, slow data
2G provided digital data but at low bit rates -- 9600 - 32k bps
3G data users finally passed 1 Mb/s in EV-DO and HSPA
4G users finally get10 Mb/s+
Page 12
Course 512 v1.0 (c)2013
Scott Baxter
October, 2013
LTE Design Objectives
 LTE was intended to be a major leap forward in performance compared to
the 3G technologies HSPA and EV-DO
 LTE objectives as expressed in the early document TR25.913:
• Gross data rate100 Mb/s in 20 MHz. for downlink, 50 Mb/s in 20 MHz.
for uplink, where separate uplink and downlink frequencies are used,
not taking into account multiplying effects available using MIMO
• Control Plane (“setup”) Latency: camped to active <100 ms., dormant
to active <50 ms.
• User Plane (“data”) Latency: 5 ms 1-way on unloaded network
• # Active Users: >200 in 5 MHz., >400 in wider than 5 MHz. block
• Distance: Full performance to 5 km, good to 30 km, up 100 km. is not
specified but to be substantially better than 3G technologies
• Handoff Delay: negligible LTE-LTE, less than 512 ms LTE>GSM
• Bandwidth scalable for incremental transition in existing spectrum
• MBMS (Multimedia Broadcast Multicast Service) to allow about 16 TV
channels simultaneously in 5 MHz. at efficiency of about 1 b/s/hz
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 13
LTE
 The Evolved Packet System (EPS) is purely IP based. Both real time
services and datacom services are carried by the IP protocol.
• The IP address is allocated when the mobile is switched on and
released when switched off.
The new LTE access solution uses OFDMA (Orthogonal Frequency
Division Multiple Access) to reach high data rates and data volumes.
• High order modulation (up to 64QAM), large bandwidth (up to 20
MHz) and MIMO transmission in the downlink (up to 4x4) is also
available. Up to 170 Mbps on uplink and 300 Mbps on the downlink.
 The EPC core network can inter-work with Non-3GPP access such as
WiMAX, WiFi, CDMA and EV-DO.
• Non 3GPP access solutions can be treated as trusted or non-trusted
based on operator requirements.
 The LTE access network is simply a network of base stations (eNodeBs)
in a flat architecture. There is no centralized intelligent controller, and the
eNBs are normally inter-connected by the X2-interface and towards the
core network by the S1-interface.
 Distributing intelligence among eNodeBs speeds up connection set-up
and handovers, especially critical for some types of user traffic.
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 14
LTE vs. LTE Advanced
Characteristic
Peak Data Rate
Latency:
Spectral Width
Peak Spectral Efficiency
Control-Plane
User Capacity
LTE
LTE Advanced
DL: 100 Mbps
UL: 50 Mbps
C-Plane: <100 ms
U-Plane: <5 ms usually
DL: 1 Gbps
UL: 512 Mbps
C-Plane: <50 ms
U-plane: <5 ms always
Multiple Blocks,
up to 100 MHz. +
DL: up to ~30 b/s/hz
UL: up to ~15 b/s/Hz
>300 active in 5 MHz.
without DRX, >600 in 5+
One Block, up to 20 MHz
DL: ~5 b/s/Hz
UL: ~2.5 b/s/Hz
At least 200 active in 5
MHz., 400 in > 5 MHz.
 Many features of LTE-Advanced are already implemented in
current commercial-production network equipment
 Data rate figures above do not include benefits of MIMO
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 15
Multiple Access Methods
FDMA
Power
FDMA: AMPS & NAMPS
•Each user occupies a private Frequency,
protected from interference through physical
separation from other users on the same
frequency
TDMA: IS-136, GSM
TDMA
Power
•Each user occupies a specific frequency but
only during an assigned time slot. The
frequency is used by other users during
other time slots.
CDMA
CDMA
Power
Page 16
•Each user uses a signal on a particular
frequency at the same time as many other
users, but it can be separated out when
receiving because it contains a special code
of its own
512 v1.0 (c) 2013 Scott Baxter
October, 2013
Highly Advanced Multiple Access Methods
OFDM
Power
OFDM, OFDMA
Frequency
• Orthogonal Frequency Division Multiplexing;
Orthogonal Frequency Division Multiple Access
• The signal consists of many (from dozens to
thousands) of thin carriers carrying symbols
• In OFDMA, the symbols are for multiple users
• OFDM provides dense spectral efficiency and robust
resistance to fading, with great flexibility of use
Multiple-Antenna Techniques to Multiply Radio Throughput
MIMO
MIMO
• Multiple Input Multiple Output
• An ideal companion to OFDM, MIMO allows
exploitation of multiple antennas at the base station
and the mobile to effectively multiply the throughput
for the base station and users
SMART ANTENNAS
• Beam forming for C/I improvement and
interference reduction
Page 17
Course 512 v1.0 (c)2013 Scott Baxter
October, 2013
Summary of Major Progress
in Wireless Communications
Cellular Frequency Reuse Concept
with handoffs
From No Frequency Reuse
Progress in
Network Configuration
and Frequency Reuse
to
0.2
104k
0.5
3
1
0.17
0.2
160k
0.8
3
1
0.27
0.2
384k
1.9
3
1
0.63
1.2
1.2
360k 720k
0.3
0.6
1
1
1
1
0.3
0.6
B
C
D
1xEV-DO
EDGE
0.03
28k
0.9
7
1
0.13
1xRTT RC4
GPRS
0.03
9600*
0.3*
7
1
0.04
CDMA
GSM
Signal Bandwidth, MHz =
User Bits/Second =
Signal Efficiency bits/Hz =
Frequency Reuse N =
MIMO factor =
Spectral Efficiency bits/Hz/Area =
TDMA (US)
Progress in
Signal
Technology
Analog*
A
UMTS
HSPA
LTE
1.2
3.1M
2.4
1
1
2.4
3.84
2M
0.5
1
1
0.5
3.84
8M
2.1
1
1
2.1
20
100M
5.5
~3
4
7.3
Progress in
Devices
October, 2013
Course 512 v1.0.1 (c)2013 Scott Baxter
Page 18
Introducing The LTE Air Interface
October, 2013
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Page 19
LTE Uses OFDM
Orthogonal Frequency Division Multiplexing
 An LTE signal is made up of many
small ordinary radio signals
(“subcarriers”) standing together
• The “bundle” could be from a few
dozen to over 1000 subcarriers,
whatever your spectrum can hold
• subcarriers are on 15 kHz. steps
 Each subcarrier can carry whatever bits we put on it
 We can send a large amount of data very fast by splitting it up and
sending over a large number of subcarriers in parallel
 Subcarriers are created and received using Discrete Fourier
Transforms, so they don’t interfere (are “orthogonal”)
 1980’s technology would have needed an individual transmitter and
receiver for each subcarrier – mobiles bigger than suitcases with
car batteries strapped on outside – but modern LTE chipsets keep
a user’s equipment (UE) small and compatible with small batteries
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 20
FDD LTE: Frequency Division Duplex
Uplink
706
eNodeB
Downlink
716
730
740
1.4, 3, 5, 10, 15 or 20 MHz.
1.4, 3, 5, 10, 15 or 20 MHz.
The width of the LTE signal can be set to fill any authorized frequency block
UE  When an operator’s licensed spectrum includes separate frequency
blocks for uplink and downlink, this is called “Frequency Division
Duplex” operation
 The LTE standard contains a list of several dozen “band classes”,
different arrangements of the uplink and downlink blocks and their
frequencies as used in different countries around the world
 Downlink is sometimes called “Forward Link”, and uplink called
“Reverse Link”
 LTE mobiles are called “User Equipment” (UE)
 LTE base stations are “Enhanced Node-Bs” (eNodeB, or eNB)
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 21
Downlink
TDD LTE: Time Division Duplex
Frequency
Uplink
 In TDD, uplink and downlink take turns transmitting in a single block
of spectrum.
 Operators’ choice of FDD or TDD operation is usually dictated by the
frequencies assigned by government
 In FDD, the capacity of uplink and downlink is determined by the
spectrum allocated to each (usually equal)
 In TDD, the relative capacity of uplink and downlink can be adjusted
to most closely match the actual distribution of uplink and downlink
traffic, getting greatest efficiency from available spectrum
 The WiMAX standard was first developed in only a TDD version
 The LTE technology was first developed in only an FDD version
 Today both LTE and WiMAX have FDD and TDD versions
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 22
Orthogonal Frequency Division
Multiple Access - OFDMA
Uplink
Downlink
Uplink spectrum is empty
if no UEs are transmitting
706
Downlink spectrum on active system
usually appears fully occupied
716
1.4, 3, 5, 10, 15 or 20 MHz.
730
740
1.4, 3, 5, 10, 15 or 20 MHz.
 Whether FDD or TDD is used, transmission in each direction on
each subcarrier is scheduled in units of 1 millisecond (or multiples)
 An LTE system dynamically schedules uplink and downlink
subcarriers based user needs and RF conditions to ensure
• Efficiency – each user gets their fair share of the resources and
the total resources are used effectively for greatest throughput
• Quality of Service (QOS) – each user’s type of traffic is
considered when assigning resources, to provide acceptable
quality (both in latency and throughput) for the user
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 23
700 MHz
800
900
PCS
Uplink
PCS
DownLink
1700
1800
1900
Frequency, MegaHertz
2000
AWS
DownLink
2100
 Modern wireless began in the 800 MHz. range, when the US FCC
reallocated UHF TV channels 70-83 for wireless use and AT&T’s
proposed analog technology “AMPS” was chosen.
 Nextel bought many existing 800 MHz. Enhanced Specialized Mobile
Radio (ESMR) systems and converted to Motorola’s “IDEN” technology
 The FCC allocated 1900 MHz. spectrum for Personal Communications
Services, “PCS”, auctioning the frequencies for over $20 billion
 With the end of Analog TV broadcasting in 2013, the FCC auctioned
former TV channels 52-69 for wireless use, the “700 MHz.” band
 The FCC also auctioned spectrum near 1700 and 2100 MHz. for
Advanced Wireless Services, “AWS”.
 Technically speaking, any technology can operate in any band. The
choice of technology is largely a business decision by system operators.
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 24
SAT
AWS
Uplink
AWS?
Proposed AWS-2
SAT
IDEN
CELL DNLNK
700 MHz.
IDEN
CELL UPLINK
Current Wireless Spectrum in the US
2200
The US 700 MHz. Spectrum and Its Blocks
 In the U.S., the former television channels 52-69 have been re-allocated
to wireless operators and public safety entities.
 The “Upper C” block (striped red) is now used by Verizon Wireless in
virtually the entire U.S. with uplink in 776-787 MHz. and downlink in
746-757 MHz. Verizon’s partnership with rural operators has given it a
head-start in completing LTE service along virtually all interstate
highways and many surrounding rural areas.
 AT&T has obtained the lower B and/or lower C block in many areas.
After considerable delay it is now well along in its national rollout.
 Other operators also use lower A, B, and/or C blocks in many areas.
There is controversy over adjacency of lower A to TV channel 51.
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 25
LTE Band
Classes
 The LTE Band Classes
are listed in the ETSI
document 36.101 in the
table shown at left
 Blocks 1-26 are for FDD,
Frequency-DivisionDuplex use
 Blocks 33-43 are for
TDD Time-DivisionDuplex use
 As new frequencies are
purposed for LTE around
the world, new band
classes will be added
 VZW US: Bandclass 14
 ATT US: Bandclass 17
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 26
LTE Subcarriers and Modulation
October, 2013
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Page 27
A Quick Introduction to Digital Modulation
Modulation
Schemes
Q
QPSK
I
Q
16QAM
I
Q
64QAM
I
512 - 28
Modulation
Scheme
BPSK
QPSK *
8PSK
16 QAM *
32 QAM
64 QAM *
256 QAM
Possible
States
2
4
8
16
64
128
256
Efficiency,
Bits/S/Hz
1 b/s/hz
2 b/s/hz
3 b/s/hz
4 b/s/hz
5 b/s/hz
6 b/s/hz
8 b/s/hz
SHANNON’S
CAPACITY EQUATION
C
= B log2
[ 1+
S
N
]
B = bandwidth in Hertz
C = channel capacity in bits/second
S = signal power
N = noise power
 In digital modulation, the signal’s amplitude and
phase are driven among several pre-defined values.
On a vector diagram, these points look like stars in
a constellation. Each dot is called a “symbol”.
 Simple modulation schemes have fewer symbols in
their constellations, and are easy to receive even
through interference and noise. However, each
symbol only carries a few bits of information.
 More complex modulation schemes have more
symbols in their constellations and each symbol
carries many bits of information. However, reception
is vulnerable to errors from interference, noise, or
distortion in amplifiers of the transmitter/receiver.
Course 512 v1.0- (c) 2013 Scott Baxter
October, 2013
One LTE Subcarrier: What Can It Do?
Frequency,
KHz
-30
-15
FSC
+15
+30
 The LTE radio signal is made up of many individual little signals
called subcarriers, spaced 15 kHz apart in spectrum. A subcarrier
can carry information bits or reference signals.
 Bits are carried by a subcarrier by one of three types of modulation.
The system chooses which type to use, reacting to instantaneous
radio conditions between each specific UE and eNB:
• QPSK – rugged but slow, for bad RF conditions
• 16QAM – faster, but only works in fair conditions
• 64QAM – very fast, but only for great conditions
 The smallest “atom” of an LTE signal is one subcarrier during the
time while it transmits one symbol. This is a “resource element”.
 Normal bursts of user data over LTE occupy many subcarriers for
many symbols; we don’t schedule just one resource element alone.
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 29
LTE Symbols’ Weapon against
“Multipath” Reflections: The Cyclic Prefix
LTE Symbol
eNB
LTE Symbol
LTE Symbol
UE
 Radio signals in a mobile environment don’t follow just one direct pathway
from transmitter to receiver. The signal travels over every possible path. The
receiver gets a “jumble” of what was transmitted, “blurred” in time.
 On arrival, the boundary between one symbol and the next is “fuzzy”. A
symbol is sometimes interfered with by overlapping remnants of the symbol
sent just before of it. This is called “intersymbol interference”, ISI.
 LTE exploits Discrete Fourier Transforms to overcome ISI. Each symbol
begins with a preview of its end value, called a “cyclic prefix”.
 If the CP length is longer than the time-blurring of the radio channel, the
Discrete Fourier Transform can eliminate the intersymbol interference.
 LTE systems have a “normal” CP length which nicely fits most situations. The
CP length can also be “extended” to get good performance in very reflective
areas such as big cities and mountain canyons, and in Multicast transmission.
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 30
Normal and Extended Cyclic Prefix
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 31
Generic Frame Sequences
 Each OFDM symbol begins with a cyclic prefix, of duration below:
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 32
The Smallest Assignable Traffic-Carrying
Part of an LTE signal: a Resource Block
A Resource Block is 12 subcarriers
carrying data for one-half millisecond.
Page 33
512 v1.0 (c) 2013 Scott Baxter
October, 2013
LTE Signal Bandwidth in
MegaHertz and Resource Blocks
Signal Bandwidth, MHz.
1.4
1.6
3
3.2
5
10
15
20
Number of Resource Blocks
6
7
15
16
25
50
75
100
 The 1.4 MHz. bandwidth is used only for initial addition of LTE to
cleared spectrum of an existing FDD system which is converting
from another technology to LTE FDD.
 The 1.6 MHz. bandwidth is used only for initial addition of LTE to
cleared spectrum of an existing TDD system which is converting
from another technology to LTE TDD.
 The other bandwidths match frequency blocks authorized by
various countries’ governments for wireless operation.
October, 2013
Course 512 v1.0 (c)2013 Scott Baxter
Page 34
LTE Frame Timing Structure
in Frequency Division Duplex (FDD)
 Each LTE downlink subcarrier operates with radio frames 10
milliseconds long.
 Each frame is made up of 10 subframes, each 1 millisecond long.
 Each subframe contains 2 slots, each 512 microseconds long.
 Normally, each slot carries seven modulated symbols, which could
be QPSK, 16QAM, or 64QAM, whatever is most appropriate for
the prevailing radio conditions.
Page 35
512 v1.0 (c) 2013 Scott Baxter
October, 2013
LTE Frame Timing Structure
in Time Division Duplex (TDD)
 When an LTE system has a single block of frequencies to use, it is
not possible to have simultaneous uplink and downlink.
 Instead, Uplink and downlink must take turns using the available
frequency space. This is called Time Division Duplex, TDD
 The frames for TDD LTE are 10 milliseconds long, just like FDD
 Inside a frame, some subframes are used for uplink and some for
downlink. When transmission direction changes, there is a
“transition” subframe with a pilot timeslot for the ending link direction,
a guard period, and a pilot timeslot for the starting link direction.
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Possible LTE TDD Time Configurations
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More Uplink and Downlink
LTE Transmission Details
 Although it is theoretically possible to achieve OFDM operation
using thousands of individual transmitters and receivers working
together, this has impractical space and power requirements,
especially for mobiles!!
 Fortunately, it is possible both to generate and decode OFDM
signals using digital signal processing (DSP) with discrete fourier
transforms (DFT) in single very-large-scale-integration chips
 The LTE downlink is classical OFDM, and because of the dynamic
assignment of subcarriers to different users, it is often termed
Orthogonal Frequency Division Multiple Access (OFDMA).
 OFDM signals have a very high peak-to-average ratio, requiring
high-quality very linear amplifiers which are not power efficient
 For better battery life, mobiles use a “cousin” of OFDM called
DFTS-OFDM/SC-FDMA: Discrete Fourier Transform Spread
OFDM, Single-Carrier Frequency Division Multiple Access
• Each mobile generates its transmitted signal as a single unit for
lower peak-average power ratio, using DFT
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The LTE Uplink Signal
 The uplink uses SC-FDMA with some dynamic multiple of 4 15-khz
subcarriers to transmit the user’s information
• Modulation can be QPSK, 16QAM or 64QAM for conditions
• SC-FDMA has a low Peak-to-Average Power Ratio (PAPR)
which provides more transmit power and longer battery life
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OFDMA Downlink, SC-FDMA Uplink
 LTE uses a high spectral efficiency multicarrier multiple access approach, OFDM
• Downlink: OFDMA (Orthogonal Frequency Division Multiple Access)
• Uplink: SC-FDMA (Single Carrier - Frequency Division Multiple Access), also
called DFT (Discrete Fourier Transform) spread OFDMA.
 OFDM fills the available bandwidth with many mutually orthogonal narrowband
subcarriers, shared by multiple users.
• OFDMA is spectrum-efficient, but needs fast processors to make and decode
• The OFDMA signal has a high peak-to-average power ratio, needing powerhungry linear amplifiers. It’s no problem for eNBs, but makes handsets costly.
• A near-cousin to OFDMA, SC-FDMA, is used on the uplink because it has the
same multi-carrier structure but a low peak-to-average power ratio.
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MIMO
Multiple Input Multiple Output
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SISO, MISO, SIMO, MIMO
 Single-Input Single-Output is the
default mode for radio links over the
years, and the baseline for further
comparisons.
 Multiple-Input Single Output provides
transmit diversity (recall CDMA2000
OTD). It reduces the total transmit
power required, but does not increase
data rate. It’s also a delicious
Japanese soup.
 Single-Input Multiple Output is “receive
diversity”. It reduces the necessary
SNR but does not increase data rate.
It’s rumored to be named in honor of
Dr. Ernest Simo, noted CDMA expert.
 Multiple-Input Multiple Output is highly
effective, using the differences in path
characteristics to provide a new
dimension to hold additional signals
and increase the total data speed.
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SU-MIMO, MU-MIMO, Co-MIMO
 Single-User MIMO allows
the single user to gain
throughput by having
multiple essentially
independent paths for data
 Multi-User MIMO allows
multiple users on the
reverse link to transmit
simultaneously to the eNB,
increasing system capacity
 Cooperative MIMO allows a
user to receive its signal
from multiple eNBs in
combination, increasing
reliability and throughput
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LTE Channels
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Frequency
All Resource Blocks
Downlink Physical Resources and Mapping
A Physical Resource Block
Time
 A complete view of an FDD LTE Downlink Signal several MHz wide.
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Frequency
Uplink Physical Resources and Mapping
One or more 60-KHz. SC-FDMA carriers
of a UE, as assigned by the system
Time
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LTE Channels – Logical, Transport, Physical
Control
Traffic
Control
Traffic
Control
Traffic
Control
Control
Paging
Overhead
Shared
Random
Access
MultiMedia
Shared
Paging
Broadcast
Overhead
Shared
Control
Random Access
MultiMedia
Control
Format
HARQ
Paging
Broadcast
Overhead
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Individual
User
Public
MultiMedia
Individual
User
Public
Types of Channels in LTE
 Logical Channels
• A logical channel carries a specific traffic or control messaging
between the RLC and an upper-level entity
 Transport Channels
• The Transport channels carry information between Medium
Access Control (MAC) and higher layers.
 Physical Channels
• A physical channel holds content with bits mapped into the
appropriate format to be transmitted over the air interface
• In addition to physical channels carrying user and control bits,
there are also physical signals
– PSS: downlink Primary Synchronization Signal
– SSS: downlink Secondary Synchronization Signal
– RS: downlink demodulation Reference Signal
– Uplink demodulation Reference Signal
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Downlink Physical Signals and Channels
 Downlink Physical Signals
• Reference Signal (RS)
–
Pilot used for DL channel estimation. Derived from cell ID (one of 3x168=504 PN Sequences)
• Primary Synchronization Signal (P-SCH)
–
Signal used by UE for initial cell acquisition – codes 0, 1, or 2
• Secondary Synchronization Signal (S-SCH)
–
Signal used by UE for initial cell acquisition – 168 different codes
 Downlink Physical Channels
• Physical Broadcast Channel (PBCH)
–
Broadcasts system information, including MIB and SIBs
• Physical Downlink Shared Channel (PDSCH)
–
Shared channel for user data, radio/core network, System information (BCH), paging messages.
• Physical Downlink Control Channel (PDCCH)
–
Shared signaling channel for allocation of resources for the PDSCH.
• Physical Control Format Indicator Channel (PCFICH)
–
Defines number of PDCCH OFDMA symbols per Sub-frame (1, 2, or 3)
• Physical Hybrid-ARQ Indicator Channel (PHICH)
–
Carries HARQ ACK/NACK
• Physical Multicast Channel (PMCH)
–
Carries the MCH Transport channel
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Uplink Physical Signals and Channels
 Uplink Physical Signal
• Reference signal (RS)
– Reference signal used for demodulation and sounding
– Used for synchronization to the UE and UL channel estimation
 Uplink Physical Channels
• Physical Uplink Shared Channel (PUSCH)
– Shared channel used to carry user data..
• Physical Uplink Control Channel (PUCCH)
– Shared signaling channel for UE to request PUSCH resources
• Physical Random Access Channel (PRACH)
– Shared channel used for the access procedure, Call setup
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Downlink LTE Physical Channels
PBCH: The Physical Broadcast Channel
4 Frames
PBCH
 The Physical Broadcast Channel carries system
information for UEs needing to access the network.
• It carries only the Master Information Block, MIB.
• The modulation is always QPSK.
• The information bits are coded, rate matched, and then
scrambled using a cell-specific sequence to prevent
confusion with data from other cells
• It’s carried in the central six resource blocks of the LTE
signal (72 subcarriers) regardless of the overall system
bandwidth.
• The PBCH message is repeated every 40 ms, i.e. one
TTI of PBCH includes four radio frames.
• One PBCH transmission contains 14 information bits,
10 spare bits, and 16 CRC bits.
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Downlink LTE Physical Channels
PCFICH: The Physical Control Format Indicator Channel
PCFICH symbols
 The Physical Control Format Indicator Channel tells the
UE the format of the PDCCHs (1, 2, or 3 symbols).
• This information is crucial since without it the UE has
no idea of the size of the control region.
 The PCFICH rides on the first symbol of every sub-frame
and carries a Control Format Indicator (CFI) field.
• The CFI contains a 32 bit code word that represents 1,
2, or 3. CFI 4 is reserved for possible future use.
 The PCFICH uses 32,2 block coding, giving a 1/16
coding rate, and it always uses QPSK modulation to
make the reception as robust as possible.
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Downlink LTE Physical Channels
PDCCH: The Physical Downlink Control Channel
PDCCH symbols
 The Physical Downlink Control Channel carries Downlink
Control Information DCI (scheduling information for UEs):
• Downlink resource scheduling, telling UEs which
resource blocks and subframes are theirs
• Uplink power control commands for UE transmitters
• Uplink resource grants for UE uplink transmission
• Indications for paging or system information
 The Downlink Control Information (DCI) can be in any of
several formats, which are indicated by the PCFICH. The
format types include types:
• 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3, 3A, and 4.
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Downlink LTE Physical Channels
PHICH: The Physical Hybrid ARQ Indicator Channel
PHICH symbols
 The Physical Hybrid ARQ Indicator Channel carries the
HARQ ACK/NACK signal telling a UE whether an
uplink transport block has been correctly received. The
HARQ indicator is 1 bit long - "0" indicates ACK, and
"1" indicates NACK.
 The PHICH is transmitted within the control region of
the subframe and is typically only transmitted within the
first symbol. If the radio link is poor, then the PHICH is
extended to several symbols for robustness.
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Downlink LTE Physical Channels
PDSCH: The Physical Downlink Shared Channel
All of the orange-colored space in the signal is shared space of
the PDSCH, to serve as the downlink channel for different UEs.
 Physical Downlink Shared Channel carries the actual
traffic from eNB to the UEs.
 It is allocated by resource blocks in 1-ms. scheduling
increments.
 UEs transmit their traffic back to the eNBs on the
equivalent uplink channel, the PUSCH.
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Physical Uplink Shared Channel - PUSCH
 Physical Uplink Shared Channel (PUSCH) : This physical
channel carries actual user traffic. It’s the equivalent of the
PDSCH on the downlink.
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Uplink LTE Physical Channels
Uplink Physical Control Channel - PUCCH
 Physical Uplink Control Channel (PUCCH) : The Physical
Uplink Control Channel carries control signaling.
 There are several different PUCCH formats so the
channel can carry information most efficiently for particular
scenarios. It includes the ability to carry SRs, Scheduling
Requests.
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Physical Random Access Channel - PRACH
 Physical Random Access Channel (PRACH) : This uplink
physical channel is used for random access functions. This
is the only non-synchronized transmission that the UE can
make within LTE. The downlink and uplink propagation
delays are unknown when PRACH is used and therefore it
cannot be synchronized.
 The PRACH instance is made up from two sequences: a
cyclic prefix and a guard period. The preamble sequence
may be repeated to enable the eNodeB to decode the
preamble when link conditions are poor.
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Downlink Resource Grid Details
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Resource Allocation in LTE
 Resources in LTE
• Resource Element, Resource Block, Slot, Sub-frame
• Resource Grid
 Control Information
• Physical Channels, PDCCH, DCI
 Resource Allocation
• Resource Block Group (RBG) based
• RBG Subset based
• Virtual Resource Block (VRB)-based
 Interactive LTE downlink signal demonstration:
• http://paul.wad.homepage.dk/LTE/lte_resource_grid.html
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LTE Resource Grid Interactive Example
http://paul.wad.homepage.dk/LTE/lte_resource_grid.html
 cc
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Resource Element Groups (REG)
 For the overall LTE signal structure, both uplink and downlink, a
Physical Resource Block (PRB) is the main allocated “chunk” of
signal.
 However, control channels are mapped into smaller units called
Resource Element Groups (REG). Because control channel
information is usually very compact in size, an REG easily fits
inside a PRB.
 An REG is just one symbol long, and it takes up either 4 or 6
subcarriers – depending on whether pilot subcarriers are included.
 Several REG may be grouped into a Control Channel Element
(CCE).
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The Downlink Reference Signal
 The downlink reference signals consist of known reference symbols
inserted in the first and third last OFDM symbol of each slot.
 There is one reference signal transmitted per downlink antenna port.
• The number of downlink antenna ports equals 1, 2, or 4.
 Frequency hopping can be applied to the downlink reference signals. The
hopping pattern period is one frame (10 ms). Each frequency hopping
pattern corresponds to one cell identity group.
 The downlink MBSFN reference signals consist of known reference
symbols inserted every other sub-carrier in the 3rd, 7th and 11th OFDM
symbol of sub-frame in case of 15kHz sub-carrier spacing and extended
cyclic prefix
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Example of Downlink Control Signal Mapping
 This figure shows a typical
example of mapping the
various downlink control
signals to the slots and
resource elements which hold
them
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Example of RS Sequences for 4 Antennas
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Downlink Control and Data Regions
 The PCFICH tells the length of the control region.
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Control Region Mapping:
Resource Element Groups (REGs)
 The Resource Element Groups define Control Channel mapping.
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PDCCH Mapping
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What’s a DCI?
 The Downlink Control Indicator (DCI) carries the information a UE
needs to know
• Which resource blocks carry your data?
• What modulation scheme is used for your data?
• What’s the starting resource block for your data?
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User Identification using DCI Scrambling
 Which UE owns a particular
PDCCH?
 With Radio Network Temporary
Identifier (RNTI) as User ID, it can
be calculated:
DCI
CRC
Attachment
DCI +
16 bit CRC
scrambled with RNTI
DCI
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(DCI + RNTI) mod 2
Page 71
LTE Resource Allocation and PDCCH Support
 There are 10 DCI formats for indicating downlink scheduling, in
three broad types.
 There is one DCI format for assigning uplink scheduling.
 A Control Channel Element (CCE) consists of 9 Resource Element
Groups (REG).
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DCI Formats and Resource Allocation
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Resource Allocation type 0
 In type 0 resource allocation, a bit map represents a resource
block group (RBG) allocated to a UE. The size of RBG is given by
P, which can be deduced from TS 36.213 Table 7.1.6.1-1 for given
system bandwidth. The numbers of bits in “Bitmap” field are equal
to. Each bit in the “Bitmap” will select a small contiguous group
whose size depends on the bandwidth (RBG: 1…4). The maximum
resource block (RB) coverage of any type 0 allocation is the entire
bandwidth i.e. a type 0 allocation with all the bits in bitmap set to
‘1’ is equivalent to the entire bandwidth.
 Example – 50 RB Bandwidth, the number of bits in “Bitmap” are
17. Each bit in the 17 bit bitmap selects a group of 3 RB (apart
from the last group which will only contains 2 RB for this BW) i.e.
each bit is associated with a group of RE with the same color.
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Resource Allocation Type 1
 Type 1 resource allocation uses a bit map to indicate physical resource blocks
inside an RB subset “p”, where 0 ≤ p < P. Even with all the bits in the “Bitmap” set
to ‘1’, it does not span the whole signal bandwidth. Each bit in the bitmap selects a
single RB from ‘islands’ of small contiguous groups whose size (RBG) and
separation depend on the total bandwidth. This allows selecting individual RBs.
 Resource block assignment signaling is split into 3-parts:
• RBSubset, Shift (whether to apply an offset when interpreting), and Bitmap
indicating the specific physical resource block inside the resource block group
subset. This makes Type 1 bitmap sizes smaller by [log2 (P)]+1 than Type 0.
 Example – 50 RB Bandwidth, the number of bits in “Bitmap” are 14. Each bit
selects one RB inside a selected subset. If all bits are set to one, we get:
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Resource Allocation type 2
 In Type 2 resource allocation, physical resource blocks are not
directly allocated. Instead, virtual resource blocks are allocated
which are then mapped onto physical resource blocks. Type 2
allocation supports both localized and distributed virtual resource
block allocation differentiated by one bit-flag. The information
regarding the starting point of virtual resource block and the length
in terms of contiguously allocated virtual resource block can be
derived from Resource Indication Value (RIV) signaled within the
DCI.
 Example – 50 RB Bandwidth, a UE shall be assigned an allocation
of 25 resource blocks (LCRBs = 25), starting from resource block 10
(RBstart = 10) in the frequency domain. Now to calculate the RIV
value refer to the formula given in TS 36.213 Section 7.1.6.3,
which yields RIV = 1210. This RIV is signaled in DCI and the UE
could unambiguously derive the starting resource block and the
number of allocated resource blocks from RIV again.
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Non-Hopping and Hopping
Uplink Resource Allocation
 Non-Hopping Uplink Resource Allocation
• Type 2 localized resource allocation rules apply for deriving the
resource allocation from the RIV value.
 Uplink Hopping Resource Allocation – two types of hopping exist:
• Type 1 PUSCH Hopping
– Type 1 PUSCH Hopping is calculated using the RIV value and a
number of parameters signalled by higher layers;
• Type 2 PUSCH Hopping (not to be confused with downlink resource
allocation type 1 and type 2 described earlier).
– Type 2 PUSCH Hopping is calculated using a pre-defined pattern
(a function of subframe/frame number) defined in TS36.211 5.3.4.
 The fundamental set of resource blocks is calculated from the rules for
type 2 localized resource allocation from the RIV value, except either 1 or
2 hopping bits deduced from bandwidth and resource allocation bitmap.
– These hopping bits specify whether Type 1 or Type 2 PUSCH
Hopping is to be used, and for the case of 2 bits, variations of the
position of the Type 1 hopping in the frequency domain. The
definition of the hopping bits is in TS 36.213 Table 8.4-2.
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The LTE Core Network
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The EPC – Evolved Packet Core
 The EPC is the latest evolution of the 3GPP core network.
 GSM architecture is circuit-switched (CS). Steady circuits are
established between the calling and called parties throughout the
whole network (radio, core mobile network, and landline network)
 In GPRS, packet-switching (PS) is added. Data is transported in
packets without using dedicated circuits. This is more flexible and
efficient. Voice and SMS still are carried in a circuit-switched
mode, so the core network is has two domains: circuit and packet.
 In UMTS (3G), this dual-domain concept remains about the same.
 For the next generation, it was decided to use IP (Internet
Protocol) as the key protocol to transport all services. The EPC
does not have a circuit-switched domain anymore. Only IP-based
packet data can be carried.
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Architecture of the EPC
 The EPC was first introduced in Release 8 of the ETSI standards.
 The EPC has a "flat architecture“, to leverage all the advantages of
IP to handle the data traffic efficiently from a performance and
costs perspective. Few network nodes are involved in the handling
of the traffic and protocol conversion is avoided.
 User data (called the user plane) and the signaling (called the
control plane) are independent. Thanks to this functional split, the
operators can dimension and adapt their network easily.
 Shown above is the basic EPS architecture with the User
Equipment (UE) connected to the EPC over E-UTRAN (LTE
access network). The elements are introduced in following pages.
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EPC Elements
 HSS The HSS (Home Subscriber Server) is a database that contains
user and subscriber information. It provides support functions in mobility
management, call and session setup, user authentication and access
authorization. It’s a combination of Home Location Register (HLR) and
Authentication Center (AuC) functions.
 Serving GW, PDN GW The Serving and PDN gateways transport the IP
data traffic between User Equipment (UE) and external networks.
• The Serving GW connects the radio-side and the EPC.
• The PDN GW connects EPC and external IP networks (PDN).
 MME The Mobility Management Entity handles the control plane, in
particular signaling related to mobility and security for UEs. It handles UE
tracking and paging, and is the termination point of the NAS.
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The Evolved Packet System and the
Evolved Packet Core
E-UTRAN
eNB
Inter-cell RMM
EPC
RB Control
Connection Mobility Ctrl
MME
Radio Admission Ctrl.
NAS Security
eNB Measurement
Config. & Provision
Idle State Mobility
Handling
Dynamic Resource
Allocation (scheduler)
EPS Bearer
Control
RRC
PDCP
S-GW
RLC
MAC
PHY
October, 2013
Mobility
Anchoring
S1
P-GW
UE IP Address
Allocation
Internet
Packet Filtering
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Support of Multiple Access Technologies
 Other Multiple access technologies are supported, along with handover to
and from LTE. The EPC can interface with existing technologies:
• 3GPP GERAN: GSM, GPRS, and EDGE are supported.
• 3GPP UTRAN: WCDMA and HSPA
• non-3GPP: WiMax, fixed networks
• Non-3GPP: cdma2000®, 1xRTT, EVDO, WLAN
 Non-3GPP networks considered “trusted” by their operator can interact
directly with the EPC.
 Non-3GPP networks considered “untrusted” by their operator can
interwork with the EPC via an ePDG (for Evolved Packet Data Gateway).
It can handle security, such as IPsec tunnels
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Networking Functional Elements
(eNB; MME; Anchors/Gateways, PCRF; HSS)
Legacy GSM radio Networks
Gb
GERAN
Policy and Charging Rules Function
SGSN GPRS CORE
S3
WCDMA /HSPA radio Networks
Mobility Management Entity
User Plane Entity
Evolved
RAN: eNB
LTE radio
Networks
S1
Ref Pt.
MME
UPE
S4
Ref Pt.
3GPP
Anchor
Home Subscriber Server
“Super HLR”
S6a
SAE
Anchor
HSS
SGi
Operator’s
IP Services
IASA
Inter Access System Anchor
Evolved Packet Core
Uu
S2a
1xRTT, CDMA2000,
EV-DO networks
October, 2013
Rx+
S7
S5b
Iu
S5a
UTRAN
PCRF
Non-3GPP
IP access
S2b,c
WLAN 3GPP
IP access
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Key Network Interfaces (1)
 Uu – The LTE physical layer interface connecting the UE with the
eNodeB on both uplink and downlink directions (GTP-U Protocol)
 S1-MME – The Control Plane (command and control) connection
from the eNB to the MME managing user mobility (GTP)
 S1-U – The User Plane (traffic-carrying) connection from the eNB
to the serving gateway (GTP protocol)
 S2a – PDN link to trusted non-3GPP networks (CDMA EVDO)
(based on proxy mobile IP, can use client mobile IP FA mode)
 S2b – PDN link to serving gateway for an untrusted network GTP
(based on proxy mobile IP)
 S2c – PDN link to trusted non-3GPP network (CDMA, EVDO) GTP
(based on client mobile co-location)
 S3 – Connection between 2G/3G SGSN and SAE MME (GTP)
 S4 -- Provides user plane connection and mobility support
between a 2G/3G SGSN and the SGW (based on Gn reference
point defined between SGSN and GGSN) (GTP protocol)
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Key Network Interfaces (2)
 S5 – Provides user plane tunneling and tunnel management
between SGW and PDN GW. Handles S GW relocation for UE
mobility if the S GW must connect to a non-collocated PDN GW.
S5 is the intra PLMN variant of S8.
 S6a – Carries subscription and authentication data between the
MME and the HSS (often called a ‘super HLR’)
 S7 – Carries policy and charging rules information between the
PDN gateway and the PCRF
 S8 – Inter-PLMN reference point providing user and control plane
between the Serving GW in the VPLMN and the PDN GW in the
HPLMN. S8 is the inter PLMN variant of S5.
 S9 - Transfers (QoS) policy and charging control information
between Home/Visited PCRF to support local breakout function.
 S10 -- Reference point between MMEs for MME relocation and
MME to MME information transfer
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Key Network Interfaces (3)
 S11 -- Reference point between MME and Serving GW
 S12 – Connection from UTRAN to Serving GW during user plane
Direct Tunnel. Based on Iu-u/Gn-u ref. point and GTP-U protocol
SGSN-to-UTRAN or SGSN-to-GGSN. Optional by Operator.
 S13 – Enables UE identity check between MME and EIR
 SGi -- Reference point between PDN GW and packet data
network. Packet data network can be external public, private, or
intra-operator packet data network, e.g. for provision of IMS.
Corresponds to Gi interface for 3GPP accesses.
 Rx -- The Rx reference point resides between the AF and the
PCRF in the TS 23.203 [6].
 Wn* The reference point between the Untrusted Non-3GPP IP
Access and the ePDG. Traffic on this interface for a UE initiated
tunnel must be forced towards the ePDG.
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Key Network Interfaces (4)
 X2 -- The X2 interface can provide
• inter-connection of eNBs supplied by different manufacturers;
• support of continuation between eNBs of the E-UTRAN
services offered via the S1 interface;
• separation of X2 interface Radio Network functionality and
Transport Network functionality to facilitate introduction of
future technology.
 SBc:- Reference point between CBC and MME for warning
message delivery and control functions
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X2 and S1 Interfaces
 Another advantage with the distributed solution is that the MAC
protocol layer, which is responsible for scheduling, is represented
only in the UE and in the base station leading to fast
communication and decisions between the eNB and the UE. In
UMTS the MAC protocol, and scheduling, is located in the
controller and when HSDPA was introduced an additional MAC
sub-layer, responsible for HSPA scheduling was added in the NB.
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The Radio Protocol Stack
Control Plane
(messaging)
NAS
Layer 2
RLC
MAC Layer
(user
Equipment)
October, 2013
NAS
Radio Resource Control
Radio Signaling
Radio Bearer
Logical Channel
Transport Channel
Physical Layer
UE
Control Plane
(messaging)
Non-Access Stratum
Core<>UE signaling
RRC
PDCP
User Plane
(user data)
RRC
PDCP
Layer 2
User Plane
(user data)
RLC
MAC Layer
Physical Layer
Physical Channel
Course 512 v1.0 (c)2013 Scott Baxter
eNodeB
(base
station)
Page 90
Processes Within Layer 2
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Some Key Points and Definitions
Upper layer
SDU
Lower layer
Lower layer
PDU
HEADER
SDU
 A message or data sent from one protocol layer to its counterpart
on the other side is called a Protocol Data Unit (PDU).
• It includes only data and the other side knows fully how to
process it. It does not contain a header.
 A PDU from a higher layer in transit in a lower layer is called a
Service Data Unit (SDU). It is regarded just as “freight” by the layer
it is passing through, and is given a header for its destination on
the other side. The SDU from the higher layer is now contained
within a PDU at the lower layer.
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SDUs and PDUs in the Protocol Stack
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RRC Protocol and NAS Signaling
User Plane
(user data)
Control Plane
(messaging)
NAS
RRC
PDCP
RLC
MAC Layer
Physical Layer
UU Air Interface
October, 2013
 In LTE, Radio Resource Connection
(RRC) protocol is handled by
intelligence in the eNodeB and UE. No
radio network controller is needed, as
in previous technologies. This was
one of the objectives of System
Architecture Evolution (SAE), to flatten
the core network.
 The RRC provides exchange of two
types of messages:
• Radio Signaling related to radio
access, paging, setting up and
maintaining RRC connections,
including identifiers, bearers, etc
• Non-Access stratum signaling
(between higher layers of UE and
the LTE core network)
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Radio Resource Control:
UE Connection States
 RRC_Connected
• UE is connected to the RAN
• Date can be immediately exchanged between network and UE
• Network know UE location down to the cell level
• The Network will maintain the connection by managing
handovers when necessary
 RRC_Idle
• The UE is not connected to the network; there is no traffic
being sent in either direction between UE and RAN
• The network knows the UE is present, and the location area
where it can be paged to deliver an incoming call
• The UE is monitoring the network discontinuously to save radio
resources and its battery; the system knows when it will be
listening for pages and other orders
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Packet Data Convergence Protocol (PDCP)
User Plane
(user data)
Control Plane
(messaging)
NAS
RRC
PDCP
RLC
MAC Layer
Physical Layer
UU Air Interface
October, 2013
 True to its name, the Packet Data
Convergence Protocol (PDCP)
accepts data of various types from
user and control plane entities,
combines and manages the flow of
data to and from the lower layers
 Some of these functions include:
• 20-byte Packet header
compression down to 1-2 bytes
and decompression, ciphering,
transferring, and during handoffs,
managing in-sequence delivery
• Control plane ciphering and
protection for core network
signaling
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Radio Link Control Protocol
PDCP
RLC
MAC Layer
In-Sequence
Delivery
RRC
Managing
RLC PDUs
NAS
 The RLC maps radio bearers into
logical channels and does the
dirtywork of segmenting and resegmenting SDUs and PDUs
 RLC offers three data transfer modes:
Managing
RLC SDUs
Control Plane
(messaging)
ARQ Error
Correction
User Plane
(user data)
TM – Transparent Mode
• For real-time services like voice, video
• No retransmission of failed packets
• No error statistics maintained
UM – Unacknowledged Mode
Physical Layer
UU Air Interface
• Useful especially for signaling
• No retransmission of failed packets
• Block Error statistics are maintained
Yes
AM – Acknowledged Mode
• Useful for non-real-time high quality
services like web browsing
• Retransmission of failed packets
October, 2013
Yes
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Yes Yes Yes Yes
Page 97
The MAC Layer Protocol
User Plane
(user data)
Control Plane
(messaging)
NAS
RRC
PDCP
RLC
 The Media Access Control (MAC)
layer maps Logical Channels into
Transport Channels and handles
multiplexing/demultiplexing of RLC
PDUs.
 It dynamically schedules the uplink
and downlink resources such as
resource blocks and slots based on
measurement reports
MAC Layer
Physical Layer
UU Air Interface
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The Downlink Scheduler
 The Downlink Scheduler must manage the assignment of
resource blocks to users for the downlink shared channel, and the
Modulation and Coding Schemes (MCS) to be used on
transmissions to individual UEs.
 The scheduler is ultimately responsible for maximizing the overall
throughput through each EnodeB and the data delivered to the
users.
 In order to correctly manage the air resources, the Downlink
Scheduler must be aware of the data waiting to be sent and
frequently receive channel RF condition details from the UEs.
• Amount and type of data waiting to be sent to each UE
• Channel RF condition (CQI) measurements from each UE
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The Uplink Scheduler
 The Uplink Scheduler must manage the assignment of resource
blocks to users for the uplink shared channel
 The mechanism is similar to the Downlink Scheduler but the
directions are reversed
• Uplink Channel quality measurements are made by the
eNodeB
• Mobiles report the data in their buffers ready to be sent and
request authority to begin transmission
• The uplink scheduler applies QOS and throughput
maximization strategies to achieve an optimum user
experience
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Intercell Interference Coordination
 LTE signals are unlike CDMA – the traffic channels of different
cells are not coded orthogonally different from each other
 Cochannel interference will result if adjacent cells use adjacent
frequncies to serve distant UEs in the border areas
 The LTE standards provide methods for cells to communicate their
present loading to one another
 LTE manufacturers are allowed to develop their own algorithms for
cells to dynamically coordinate the subcarriers used to serve their
various mobiles to avoid interference as much as possible
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Scrambling in LTE
 LTE is conceived assuming a
frequency reuse rate of 1, using all
available frequencies in all cells of
the system.
• Although LTE does not use
CDMA codes to differentiate
cells, it does perform
information scrambling at the bit
level.
 LTE scrambling codes are Pseudorandom sequences defined by a
length-31 Gold code.
 Each type of physical channel uses
a different scrambling code. The
scrambling code used in the
downlink is not the same all the
time.
 It is determined by UE Identity, and
also related with the channel
type/format associated with service.
 The table at right shows the
scrambling methods by channel.
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Waking Up with a UE:
LTE ‘Call Processing’
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System Acquisition
Searching In Frequency
Searching In Time
 At power-up, the UE notes its LTE band class capabilities and begins
exploring all the possible center frequencies that might be hold the SCH
 The UE first looks for the primary synchronization signal (P-SCH) in the
last OFDM symbol of the first time slot of the first subframe (subframe 0)
in each radio frame. It reads symbol timing, and learns which of three cell
identities is being transmitted, and locks its frequencies to the eNB.
 The UE next searches for the (S-SCH) secondary synchronization signal,
and learns which of 170 cell identities it carries. From this it decodes the
PCI, physical cell identity, and the frame boundaries
 The UE next finds the RS sequence and learns antenna port configuration
 Now the UE can decode the P-BCH and apply cell selection and
reselection criteria
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Cell Reselection (Idle Mode Handover)
 The mobile is in power-conservation mode
• Does not inform network of every cell change; rather, just when
it detects entry into a new Tracking Area
• UE-terminated calls are paged in the UE’s last reported TA
 TA organization and procedures have been widely debated
• Static non-overlapping TAs were used in earlier technologies
• New techniques reduce ping-ponging, distribute TA update
load more evenly across cells, and reduce aggregate TA
update load
• Mechanisms include overlapping TAs, multiple TAs, and
distance-based schemes
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Cell Search Measurements
 An LTE UE measures reference signal RSRP (Reference Signal Received
Power) and RSRQ (Reference Signal Received Quality).
 RSRP is a RSSI type of measurement. It measures the average received
power over the resource elements that carry cell-specific reference signals
within certain frequency bandwidth.
 RSRQ is a C/I type of measurement and it indicates the quality of the
received reference signal, defined as (N*RSRP)/(E-UTRA Carrier RSSI),
• N ensures the nominator and denominator are measured over the
same frequency bandwidth;
• carrier RSSI measures the average total received power observed
only in OFDM symbols containing reference symbols for antenna port
0 in the measurement bandwidth over N resource blocks. The total
carrier RSSI includes all incoming RF from all sources.
 RSRP is applicable in both RRC_idle and RRC_connected modes, while
RSRQ is only applicable in RRC_connected mode.
 In the procedure of cell selection and cell reselection in idle mode, RSRP
is used. In the procedure of handover, the LTE specification provides the
flexibility of using RSRP, RSRQ, or both.
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Physical Layer Measurements Definition
 The physical layer measurements to support mobility are classified
as:
• within E-UTRAN (intra-frequency, inter-frequency);
• between E-UTRAN and GERAN/UTRAN (inter-RAT);
• between E-UTRAN and non-3GPP RAT (Inter 3GPP access
system mobility).
 For measurements within E-UTRAN at least two basic UE
measurement quantities shall be supported:
• Reference symbol received power (RSRP);
• E-UTRA carrier received signal strength indicator (RSSI).
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LTE Measurement: RSSI
LTE Carrier Received Signal Strength Indicator (RSSI)
 Definition: The total received wideband power observed by the UE
from all sources, including co-channel serving and non-serving
cells, adjacent channel interference and thermal noise within the
bandwidth of the whole LTE signal.
 Uses: LTE carrier RSSI is not used as a measurement by itself,
but as an input to the LTE RSRQ measurement.
LTE Downlink
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LTE Measurement: RSRP
LTE Reference Signal Received
Power (RSRP)
 Definition: RSRP is the linear
average power of the
Resource Elements (REs)
carrying a specific cell’s RS
within the considered
measurement frequency
bandwidth.
 Uses: Rank cells for
reselection and handoff.
 Notes: Normally based on the
RS of the first antenna port, but
the RS on the second antenna
port can also be used if they
are known to be transmitted.
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LTE Measurement: RSRQ
RB RB RB RB RB RB RB RB RB RB RB RB
 LTE Reference Signal Received Quality (RSRQ)
 Definition: RSRQ = N · RSRP / RSSI
• N is the number of Resource Blocks (RBs) of the LTE carrier
RSSI measurement bandwidth. Since RSRQ exists in only one
or a few resource blocks, and RSSI is measured over the
whole width of the LTE signal, RSRQ must be “scaled up” for a
fair apples-to-apples comparison with RSSI.
 Uses: Mainly to rank different LTE cells for handover and cell
reselection decisions
 Notes: The reporting range of RSRQ is defined from −19.5 to −3
dB with 0.5 dB resolution. -9 and above are good values.
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‘S’ Cell Selection and Reselection criteria
 After finding a cell, the UE may or may not be permitted to use it,
based on various signal quality criteria broadcast by the eNB.
 Here are two procedures for cell qualification:
• In the initial cell selection procedure, no knowledge about RF
channels carrying an E-UTRA signal is available at the UE.
– In that case the UE scans the supported E-UTRA
frequency bands to find a suitable cell. Only the cell with
the strongest signal per carrier will be selected by the UE.
• The second procedure relies on information about carrier
frequencies and optionally cell parameters received and stored
from previously-detected cells.
– If no suitable cell is found using the stored information the
UE starts with the initial cell selection procedure.
 S is the criterion defined to decide if the cell is still suitable . This
criterion is fulfilled when the cell selection receive level is Srxlev >
0. Srxlev is computed based on the following Equation:
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‘S’ Cell Selection and Reselection criteria
Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation
Where Pcompensation = max (PEMAX – PUMAX, 0)
All in db
 Qrxlevmeas is the UE-measured receive level value for this cell, i.e.
the Reference Signal Received Power (RSRP
 Qrxlevmin is the minimum required receive level in this cell, in dBm.
 Qrxlevminoffset is an offset to Qrxlevmin that is only taken into
account as a result of a periodic search for a higher priority PLMN
while camped normally in a Visitor PLMN (VPLMN).
 PCompensation is a maximum function. PEMAX is maximum power
allowed for a UE in this cell. PUMAX is maximum for power class
 A UE may discover cells from different network operators.
• First the UE will look for the strongest cell per carrier,
• Then the PLMN identity from the SIB Type 1 to see if suitable,
• Then it will compute the S criterion and decide if suitable
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Special Details for TDD
 In TDD, the Primary synchronization signal (PSS) is placed at the
third symbol in subframes #1 and #6.
 The Secondary Synchronization signal (SSS) is placed at the last
symbol in subframes #0 and #5.
 The S-RACH is transmitted on the UpPTS within the special frame
 The Primary Broadcast Channel (PBCH) and the Dynamic
Broadcast Channel (D-BCH) are located just as in LTE FDD.
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Getting Needed Cell Parameters:
Information Blocks
inter
 The Master Information Block (MIB) gives the basic signal configuration
and bandwith
 System Information Block 1 declares what other information blocks exist,
and the mobile goes about collecting all their contents
 The MIB and SIB1 are carried by the BCH channel; all the other SIBS are
carried by the DL-SCH
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UE (Mobile) Categories
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LTE UE Categories
Rationale:
 The LTE UE categories or UE classes are needed to ensure that
the base station, or eNodeB, eNB can communicate correctly with
the user equipment. By relaying the LTE UE category information
to the base station, it is able to determine the performance of the
UE and communicate with it accordingly.
 As the LTE category defines the overall performance and the
capabilities of the UE, it is possible for the eNB to communicate
using capabilities that it knows the UE possesses. Accordingly the
eNB will not communicate beyond the performance of the UE.
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LTE UE Category Definitions
Data Rates by UE Category
Modulation Types by UE Category
MIMO Capabilities by UE Category
 Five different LTE UE categories are defined with a wide range of
supported parameters and performance.
 Bandwidth for all categories is 20 MHz.
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LTE Power Save Operation
 In wireless data communication, the receiver uses significant
power for the RF transceiver, fast A/D converters, wideband signal
processing, etc. As LTE increases data rates by a factor of 50 over
3G, wireless device batteries are still the same size, so substantial
improvements in power use are necessary to operate at these very
high rates and wide bandwidths. Some of that savings comes from
hardware, some from system architecture and some from the
protocol.
 Wireless standards employ power save mechanisms. The
objective is to turn off the radio for the most time possible while
staying connected to the network. The radio modem can be turned
off “most” of the time while the mobile device stays connected to
the network with reduced throughput. The receiver is turned on at
specific times for updates.
 Devices can quickly transition to full power mode for full
performance.
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DRX and DTX
 LTE power save protocols include Discontinuous Reception (DRX)
and Discontinuous Transmission (DTX). Both involve reducing
transceiver duty cycle while in active operation. DRX also applies
to the RRC_Idle state with a longer cycle time than active mode.
However, DRX and DTX do not operate without a cost: the UE’s
data throughput capacity is reduced in proportion to power
savings.
 The RRC sets a cycle where the UE is operational for a certain
period of time when all the scheduling and paging information is
transmitted. The eNodeB knows that the UE is completely turned
off and is not able to receive anything.
 Except when in DRX, the UE radio must be active to monitor
PDCCH (to identify DL data). During DRX, the UE radio can be
turned off. This is illustrated in the figure above.
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Long and Short DRX
 In active mode, there is dynamic transition between long DRX and short
DRX. Durations for long and short DRX are configured by the RRC. The
transition is determined by the eNodeB (MAC commands) or by the UE
based on an activity timer. The figure shows DRX cycle operation during a
voice over IP example. A lower duty cycle could be used during a pause in
speaking during a voice over IP call; packets are coming at a lower rate,
so the UE can be off for a longer period of time. When speaking resumes,
this results in lower latency. Packets are coming more often, so the DRX
interval is reduced during this period.
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UE (Mobile) States
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UE States
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Idle Mode Operation
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Tracking Area Update
 Consider a UE in idle state (RRC idle and ECM idle)
• This UE is free to travel and only do a Tracking Area Update
(TAU) when it discovers it has landed on a cell in a different TA
• If data arrives for the UE, the system must page the UE
throughout the TA where it last registered
• The mobile responds to the page, implicitly revealing its cell
location and re-establishing its connection to the network
– When a mobile is switched on it always has at least a
default bearer with the IP address that comes with it
 A UE is in ECM-IDLE state when no NAS signaling connection
exists between the UE and the network
• The mobile only performs cell selection and PLMN selection
• There is no UE context, no S1_MME and no S1_U connection
• The UE will perform the TA procedure when the TAI in the
EMM isn’t on the UE’s registered list of Tas
• The UE will then be in ECM-CONNECTED state again
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More EMM
 EPS also includes the concept of TAL, the Tracking Area List.
• A uE does not need to initiate a TAU when it enters a new Tracking
Area, if that area is already in its present Tracking Area List
• Provisioning different lists to the UEs can avoid signaling peaks when
a large nujmber of Ues cross a TA border, for example on a train or
other public transport
 EMM Connection Management Procedures
• Service request UE initiates to begin NAS signaling connection
• Network-initiated paging on NAS to UE to send service request
• Transport of NAS messages for SMS (CS fallback)
• Generic transport of NAS messages, various others
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Management and Control Functions
 UE management and control is handled in Radio Resource Control
(RRC). Functions handled by RRC include:
• Processing broadcast system information, so a device can decide to
connect to the network from access stratum (AS) and/or non access
stratum (NAS)
– The access stratum is the functional grouping of the parts in the
infrastructure and the UE, and protocols between them, for
access. The access stratum provides transmission of data over
the radio interface and management of the radio interface to the
other parts of UMTS
• Paging, indicating to an idle device that it may have an incoming call
• RRC connection management between the UE and the eNodeB
• Protection/ciphering RRC messages (different keys than user plane)
• Radio Bearer control (logical channels at the top of the PDCP)
• Mobility functions (handover when active, cell reselection when idle)
• UE measurement reporting and control of signal quality, both for the
current base station and other base stations that the UE can hear
• QoS management maintains the uplink scheduling to maintain QoS
requirements for different active radio bearers
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EPS Session Management
 EPS Session Management Protocol establishes and handles user data in the NAS
 Two EPS concepts define IP connectivity between UE and packet data network:
• PDN connection
• EPS bearer
 A PDN connection includes a default EPS bearer and possibly additional “dedicated
bearers” to give specific QoS handling for the traffic data flows
 A UE can have multiple simultaneous PDN connections (one for web, one IMS, etc)
 EPS procedure Categories:
• Network-initiated EPS procedures to activate, deactivate or modify bearers
• Transaction-related procedures initiated by the UE for
– PDN connection establishment and disconnection
– Requests for bearer resource allocation and modification
– Release requests
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Access Barring During System Overload
 Every UE is in one of ten randomly allocated Access Classes (AC) 0 to 9,
stored in the SIM/USIM. A UE can also be in one or more of 5 special
categories (Access Classes 11 to 15), in the SIM/USIM:
• 0-9: Regular users, 10: Users calling emergency numbers
• 11 - For PLMN special use, 12 - Security Services
• 13 - Public Utilities (e.g. water/gas suppliers)
• 14 - Emergency Services, 15 - PLMN Staff
 During overload, the network can cope by changing the SIB2 (System
Information Block Type 2). The UE generates a random number “Rand”
and must pass a “persistence” test before making an access attempt.
• By setting ac-Barring to a lower value, normal UEs are randomly
delayed while priority users with AC11 – 15 have no restriction
• Regular users AC 0 – 9 obey ac-Barring Factor and ac-Barring Time.
• Emergency calls (AC10) use ac-Barring For Emergency – on or off
• UEs of AC11- 15 use ac-Barring For Special AC – on or off
• The eNB transmits ‘mean duration of access control’ and the barring
rate for each type of access attempt (data origination, signaling orig.)
• Service Specific Access Control (SSAC) can restrict attempts by
service type.
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Cell Reselection (Idle Mode Handover)
 The mobile is in power-conservation mode
• Does not inform network of every cell change; rather, just when
it detects it is entering a new Tracking Area
• UE-terminated calls are paged over the UE’s last reported TA
 TA organization and procedures have been widely debated
• Static non-overlapping TAs were used in earlier technologies
• New techniques reduce ping-ponging, distribute TA update
load more evenly across cells, and reduce aggregate TA
update load
• Mechanisms include overlapping TAs, multiple TAs, and
distance-based schemes
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Flow Examples
Random Access
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What is Random Access?
 An LTE UE uses the random access process to gain access to a
cell for any the following reasons:
• Initial access to the network from the idle state
– For performing an initial attach
– For initiating a new call
– For responding to a page
• Regaining access to the network after a radio link failure
• During the handover process to gain timing synchronization
with a new cell
• Before uplink data transfers when the UE is not time
synchronized with the network
 The random access process allows multiple user equipment to
gain simultaneous access to a cell by using different random
access preamble sequence codes. User equipment on the uplink
in specific Physical Random Access Channel (PRACH) subframes
transmits these codes.
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Contention-Based Random Access (CBRA)
 The UE initiates the Contention Based Random Access (CBRA)
process to gain access to the network. It involves the UE selecting
a random access preamble code from a list of codes available for
selection by all UE in the cell.
 Unfortunately, Contention can occur when multiple UEs just
happen to pick the same PRACH subframe and use the same
preamble code. CBRA additional messaging is required to resolve
such conflicts.
 Random Access is Contention-Based in all of the following
situations: Initial network access, Access following a radio link
failure, Handover between cells, and data transfers on either uplink
or downlink when UE synchronization must be established
 Random Access is NOT Contention-Based during handoffs, since
the system can assign a specific preamble for the UE to use in
accessing the new site and there is no danger another UE will
intrude or compete
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The Steps of the Random Access Process
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eNB Announces the Rules,
1. UE Transmits the first
Random Access Preamble
 All the UEs learn the necessary details of the Random Access
process before they even need to use it. The network transmits it in
overhead messages. The key details include:
• Which Preamble Format to use
– Usually Preamble Format 0 providing range up to about 14
kM. Other formats are available if greater range is needed.
• When the PRACH occurs, usually once per 10 ms. radio frame
• How the UE should calculate its “open loop” transmit power for its
initial transmissions before the eNB acknowledges it
– When the eNB finally responds, it will take over using “closed
loop” power control
 Step 1: Now the UE transmits its first Random Access Preamble.
 3GPP TS 36.321 contains more information on power control.
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2. eNB sends Random Access
Response Message
 When the eNB hears the UE’s random
access preamble, it generates and sends a
Random Access Response Message on the
Physical Downlink Shared Channel (PDSCH)
• It’s addressed to a specific Random Access Radio Network
Temporary Identifier (RA-RNTI) address.
• There’s room in the RARM for multiple RA-RNTI addresses in
case multiple UEs were heard and need to be acknowledged
 The UE watches the PDCCH for its specific RA-RNTI address to
recognize its random access response message, which contains:
• Random access preamble sequence code identifying the
preamble sequence code which has been detected by the eNB
• Initial uplink schedule grant used for transmitting subsequent
data on the uplink channel
• Timing Alignment information so packet collisions won’t occur
• A Cell Radio Network Temporary Identifier (C-RNTI) for the UE
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CBRA Contention Resolution:
Steps 3 and 4
 Contention resolution steps (3 and 4) are
used whenever multiple UEs are detected
attempting random access using the same
preamble code sequence.
 Step 3: The UE hears the RARM and makes its first scheduled
uplink transmission on Physical Uplink Shared Channel (PUSCH).
The UE gives the network a unique identifier in this message.
 Step 4: The eNB repeats back the UE identity provided in step 3. A
UE which hears a match with the identity it transmitted now
declares the random access procedure successful. It transmits an
acknowledgment in the uplink.
 UEs which don’t hear a match know they have failed the random
access procedure. They have to start over again at step 1.
 Both step 3 and step 4 use the Hybrid Automatic Repeat Request
(HARQ) process. Further details on the contention resolution
process and the HARQ process are in Chapter 5.1 of 3GPP TS
36.321.
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Flow Examples
Tracking Area Update
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Tracking Area Update from LTE to GSM
 Consider a UE in idle state (RRC idle and ECM idle)
• This UE is free to travel and only do a Tracking Area Update
(TAU) when it discovers it has landed on a cell in a different TA
• If data arrives for the UE, the system must page the UE
throughout the TA where it last registered
• The mobile responds to the page, implicitly revealing its cell
location and re-establishing its connection to the network
– When a mobile is switched on it always has at least a
default bearer with the IP address that comes with it
 A UE is in ECM-IDLE state when no NAS signaling connection
exists between the UE and the network
• The mobile only performs cell selection and PLMN selection
• There is no UE context, no S1_MME and no S1_U connection
• The UE will perform the TA procedure when the TAI in the
EMM isn’t on the UE’s registered list of TAs
• The UE will then be in ECM-CONNECTED state again
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Case VII. LTE>GSM Tracking Area Update
 The UE is operating in LTE using the eNodeB, Old MME, SGW
and PGW.
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Case VII. LTE>GSM Tracking Area Update
 The UE moves away from the LTE network and into the
UTRAN/GERAN service area
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Case VII. LTE>GSM Tracking Area Update
 The UE sends a Routing Area Update to the Gn/Gp SGSN
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Case VII. LTE>GSM Tracking Area Update
 The Gn/Gp sends a Context Request to the Old MME
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Case VII. LTE>GSM Tracking Area Update
 The Old MME sends an SGSN Context Response to the Gn/Gp
SGSN
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Case VII. LTE>GSM Tracking Area Update
 Security Processes are applied
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Case VII. LTE>GSM Tracking Area Update
 The Gn/Gp SGSN sends a SGSN Context ACK to the Old MME
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Case VII. LTE>GSM Tracking Area Update
 The Gn/Gp SGSN sends an Update PDP Context Request to the
PGW
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Case VII. LTE>GSM Tracking Area Update
 The PGW sends an Update PDP Context Response to the Gn/Gp
SGSN
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Case VII. LTE>GSM Tracking Area Update
 The Gn/Gp SGSN sends an Update Location Request to the HSS
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Case VII. LTE>GSM Tracking Area Update
 The HSS sends an Insert Subscriber Data message to the Gn/Gp
SGSN
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Case VII. LTE>GSM Tracking Area Update
>GSM
 The Gn/Gp SGSN sends an Insert Subscriber Data Ack to the
HSS
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Case VII. LTE>GSM Tracking Area Update
 The HSS sends an Update Location Ack to the Gn/Gp SGSN
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Case VII. LTE>GSM Tracking Area Update
 The Gn/Gp SGSN sends a Routing Area Accept to the UE
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Case VII. LTE>GSM Tracking Area Update
 The Old MME sends a Delete Session Request to the SGW
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Case VII. LTE>GSM Tracking Area Update
 The UE sends a Routing Area Complete to the Gn/Gp SGSN
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Case VII. LTE>GSM Tracking Area Update
 The SGW sends a Delete Session Response to the Old MME
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Case VII. LTE>GSM Tracking Area Update
 The Old MME sends an S1 Release message to the Gn/Gp SGSN
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Flow Examples
Initial Attach
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LTE Initial Attach
 The S1 interface is initialized by request from the eNB to the MME
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LTE Initial Attach
 The MME confirms setup of the S1AP interface by sending an S1
Setup Successful Outcome message to the eNB
 S1 Setup: This is where eNB is attached to the network. As long
the eNB is functioning the S1 setup remains.
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LTE Initial Attach
 The UE sends an RRC connection request message to the eNB
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LTE Initial Attach
 The eNB sends an RRC Connection Setup message to the UE
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LTE Initial Attach
 The UE sends an RRC Connection Setup Complete message to
the eNB
• The message contains an NAS attachment request and a
PDN connectivity request
 RRC Connections: Once UE comes up a RRC connection is
established for communication with the network.
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LTE Initial Attach
 The eNB sends the requests on to the MME
• NAS Attach Request
• PDN connectivity request
 NAS: After RRC is established then the NAS signaling begins .
• UE sends Attach request along with PDN connectivity request
to network.
• Attach is for attaching to the network and the other message
are for establishing the bearers.
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LTE Initial Attach
 The MME sends an Authentication Info Request to the HSS
 HSS: This is Home Subscriber System and it understands
diameter protocol. Once MME receives Attach Request, it queries
HSS for authentication details. HSS sends the authentication
vectors to MME in Authentication Info Answer
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LTE Initial Attach
 The HSS responds to the MME with an Authentication Info Answer
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LTE Initial Attach
 The MME now has sufficient information to begin authentiation of
the UE
 The MME sends an S1AP DL NAS Transport and NAS message
containing the Authentication Request
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LTE Initial Attach
 The eNB sends a RRC DL info Transfer and NAS message to the
UE, containing the Authentication Request
 Authentication/Security: Networks request Authentication Vectors
from UE. Once UE provides them, MME compares them with what
HSS has sent. If they match UE is authenticated. Next is security.
After the security all the NAS messages are encrypted using the
security algorithms that were exchanged.
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LTE Initial Attach
 The UE replies with an RRC UL info transfer and NAS message
including an NAS Authentication Response
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LTE Initial Attach
 The eNB sends an S1AP UL NAS transport and NAS message
containing the Authentication Response
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LTE Initial Attach
 The MME processes the authentication response and if
successful, sends a DL NAS Transport and NAS message
containing a Security Mode Command to the eNB.
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LTE Initial Attach
 The eNB sends a DL Info Transfer and NAS message including
the Security Mode Command to the UE.
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LTE Initial Attach
 The UE confirms it has applied the Security Mode Command by
sending to the eNB a UL Info Transfer and NAS message
containing Security Mode Complete
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LTE Initial Attach
 The eNB forwards a UL NAS Transport and NAS message to the
MME with the Security Mode Complete details.
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LTE Initial Attach
 Now the MME is able to send a Create Session Request to the
SGW.
 After security mode is complete, all the NAS messages are
encrypted using the security algorithms that were exchanged.
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LTE Initial Attach
 The PGW sends a Proxy Binding Update/ACK message to the
SGW using PMIP
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LTE Initial Attach
 The SGW sends a Create Session Response to the MME using
GTP
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LTE Initial Attach
 MME sends eNB the Initial Context Setup Request and NAS
message containing Attach Accept and Activate Default EPS
Bearer Context Request
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LTE Initial Attach
 eNB sends RRC Connection Reconfiguration and NAS message
to UE containing Attach Accept, Activate Default EPS Bearer
Context Request.
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LTE Initial Attach
 UE sends RRC Configuration Complete message to eNB
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LTE Initial Attach
 MME sends Initial Context Setup Response message to the eNB
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LTE Initial Attach
 Security: network creates the EPS bearers (GTP messages). Then
radio bearers created, RRC connections modified, radio bearers
created, eNB downlink addresses sent to SGW in GTP messages
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LTE Initial Attach
 eNB sends UL NAS transport and NAS Attach Complete message
to MME, and Activate Default EPS Bearer Context Accept
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LTE Initial Attach
 MME sends Modify Bearer Request by GTP to the SGW
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LTE Initial Attach
 Attach complete
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Flow Examples
UE Detach
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LTE UE Detach
 The UE is attached to this network. It decides to detach.
 In the following pages,
• It sends a detach request message to network.
• Network deletes the EPS bearers
• then the radio bearers are torn down.
• Finally RRC connection is released.
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LTE UE Detach
 The UE sends an RRC UL Info Transfer + NAS containing a
detach request.
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LTE UE Detach
 The eNB sends to the MME an UL NAS Transport + NAS
message containing a Detach request
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LTE UE Detach
 The MME sends a Delete Session Request to the SGW using GTP
protocol.
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LTE UE Detach
 The SGW sends the PGW a PMIP Proxy Binding Update, deleting
the EPS bearers.
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LTE UE Detach
 The PGW sends a PMIP Proxy Binding ACK to the SGW
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LTE UE Detach
 The SGW sends a Delete Session Response message by GTP to
the MME.
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LTE UE Detach
 The MME updates the HSS on the UE’s detachment with a Notify
Request
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LTE UE Detach
 The HSS confirms it has received the notification by sending a
Notify Answer to the MME
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LTE UE Detach
 Now the MME sends the eNB a DL NAS Transport + NAS Detach
Accept
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LTE UE Detach
 The eNB sends the UE an RRC Connection Reconfiguration
message
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LTE UE Detach
 The UE confirms to the eNB by sending an RRC Connection
Reconfiguration Complete message
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LTE UE Detach
 The MME sends the eNB a UE Context Release Command
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LTE UE Detach
 The eNB responds to the MME with a UE Context Release
Complete message
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LTE UE Detach
 The eNB sends the UE an RRC Connection Release message
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Radio System Identifiers,
Tunnels, Connections, Bearers
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3. Radio System Identifiers and Parameters
 UE Identifiers (IMSI, TMSI, GUTI …)
• Random Access Radio Network
Temporary Identifier (RA-RNTI)
– contained in the MAC subheader of each random access
response
• LCID Logical channel identifier
• RRC layer in the Enb allocates celllevel temporary identifiers
• S-TMSI SAE Temporary Mobile
Station Identifier
 UTRAN and EPC Identifiers
• ECGI E-UTRAN Cell Global
Identifier
• one or multiple 'PLMN identity' in a
given cell
• CSG identity: broadcast by cells in a
CSG to allow authorized CSG
member UEs to access
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• C-RNTI (Cell Radio Network Temporary
Identifier)
• PCI Physical Cell Identifier
• QCI QoS Class Identifier
• RNTI Radio Network Temporary
Identifier
• SystemInformationBlockType9 contains
a home eNB identifier (HNBID);
• eNB Identifier (eNB ID): used to identify
eNBs within a PLMN.
• Tracking Area identity (TAI): used to
identify tracking areas
• NAS UE identifier
• NAS (EPC/UE) level AKA procedure
(KASME) and identified with a key
identifier (KSIASME).
• MME includes a session identifier
• SI-RNTI System Information RNTI
• CID Context Identifier
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E-UTRAN Network Identities
 PLMN Identity
• A Public Land Mobile Network is uniquely identified by its PLMN Identity.
 Globally Unique MME Identifier (GUMMEI)
• The Globally Unique MME Identifier consists of a PLMN Identity, a MME Group
Identity and a MME Code
• An MME logical node may be associated with one or more GUMMEI, but each
GUMMEI uniquely identifies an MME logical node.
 Global eNB ID
• The Global eNB ID is used to globally identify an eNB
 E-UTRAN Cell Global Identifier (ECGI)
• The ECGI is used to globally identify a cell.
 Tracking Area Identity (TAI)
• Each Tracking Area (a defined group of local cells) has an assigned TAI
 E-RAB ID
• An E-RAB uniquely identifies the combination of an S1 bearer and the
corresponding Data Radio Bearer. Under an E-RAB, there is a one-to-one
mapping between this E-RAB and an EPS bearer of the Non Access Stratum.
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E-UTRAN UE Identifiers (1)
 RNTI
• Radio Network Temporary Identifiers (RNTI) are used as UE
identifiers within E-UTRAN and in signaling messages between
UE and E-UTRAN. Some types of RNTI exist:
• C-RNTI Connected Radio Network Temporary Identifier
– The C-RNTI provides a unique UE identification at the cell
level identifying RRC Connection
• RA-RNTI Random-Access Ratio Network Temporary Identifier
– The RA-RNTI is used during some transient states, the UE
is temporarily identified with a random value for contention
resolution purposes
• S-TMSI S-Temporary Mobile Subscriber Identity (S-TMSI)
– The S-TMSI is a temporary UE identity in order to support
the subscriber identity confidentiality. This S-TMSI is
allocated by MME.
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E-UTRAN UE Identifiers (2)
 Transport Layer Addresses
• The transport layer address parameter is sent in radio signaling
procedures to establish the transport bearer connections.
• The transport layer address parameter is not interpreted in the
radio network application protocols
 An eNB UE context is a block of information about one active UE
held by the eNB.
• The block contains
– UE state information, security information, UE capability
information, identities of the UE’s logical S1-connection
– An eNB UE context is established when the transition to
active state for a UE is completed or in target eNB after
completion of handover resource allocation during
handover preparation.
 LCID Logical channel identifier
 RRC layer in the eNB allocates cell-level temporary identifiers
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4. Tunnels, Connections and Bearers
 Default Bearers, Dedicated Bearers
 GPRS Tunneling Protocol (GTP) and Proxy Mobile IP (PMIP)
 Tunnel parameters (TEID; F-TEID …)
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LTE Bearers
 In LTE, data plane traffic travels over virtual connections called
service data flows (SDFs).
 SDFs travel over bearers: Virtual containers with unique QoS
characteristics.
 A bearer is a datapath between UE and PDN, in three segments:
• Radio bearer between UE and eNodeB
• Data bearer between eNodeB and SGW (S1 bearer)
• Data bearer between SGW and PGW (S5 bearer)
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LTE PMIP TEID
Tunnel Endpoint ID
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LTE QoS Architecture
 LTE architecture supports “hard QoS,” with end-to-end quality of
service and guaranteed bit rate (GBR) for radio bearers. Just as
Ethernet and the internet have different types of QoS, for example,
various levels of QoS can be applied to LTE traffic for different
applications. Because the LTE MAC is fully scheduled, QoS is a
natural fit.
 Evolved Packet System (EPS) bearers provide one-to-one
correspondence with RLC radio bearers and provide support for
Traffic Flow Templates (TFT). There are four types of EPS
bearers:
• GBR Bearer – resources permanently allocated by admission
control
• Non-GBR Bearer – no admission control
• Dedicated Bearer – associated with specific TFT (GBR or nonGBR)
• Default Bearer – Non GBR, “catch-all” for unassigned traffic
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QoS Parameters and TFTs (1)
 A Traffic Flow Template (TFT) is all the packet filters associated with an EPS bearer.
• A packet filter may be associated with a protocol.
• Several packet filters can be combined to form a Traffic Flow Template.
• EBI+Packet filter ID gives us a "unique" packet filter Identifier. The following is the
TFT for FTP protocol.
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QoS Parameters and TFTs (2)
 Bearer level QoS is associated with a bearer and all traffic mapped
to that will receive same bearer level packet forwarding treatment.
 QoS parameter values of the default bearer are assigned by the
network based on the subscription data received from HSS.
 In LTE the decision to establish or modify a dedicated bearer is
taken by EPC and bearer level QoS parameters are assigned by
EPC. These values are not modified by MME but are forwarded
transparently to EUTRAN. However MME may reject the
establishment of dedicated bearer if there is any discrepancy.
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QoS Parameters and TFTs (2)
 A default bearer may or may not be associated with a TFT. But a
dedicated bearer is always associated with TFT.
• So we have bearers, the QoS values for them and TFT which
indicate what type of application should run over them. This
defines the LTE QoS. We have Uplink TFT and Downlink TFT
which are used by UE and PDN
 The UE routes uplink packets to the different EPS bearers based
on uplink packet filters in the TFT's assigned to those EPS
bearers.
• We have evaluation packet precedence index in packet filter
which is used by UE to search for a match (to map the
application traffic).
• Once the UE finds a match it uses that particular packet filter to
transmit the data.
• If there is no match UE transmits the data on bearer to which
no TFT has been assigned.
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Flow Examples
Default Bearer Establishment
Incoming
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LTE Default Bearer Establishment,
Incoming (1)
 UE is in RRC_IDLE condition
 MME has traffic for specific UE. It sends Page message to all
eNBs in UE’s current tracking area (TA).
 eNB sends page message over air interface for UE
 UE recognizes the page and responds by sending RRC
Connection Request message to eNB
 eNB sends RRC Connection Setup message to UE
 UE sends eNB a RRC Connection Setup Complete message and
NAS message including Attach Request and PDN Connectivity
Request
 eNB sends Initial UE Message + NAS attach request and PDN
connectivity request to MME
 eNB sends Initial UE Message + NAS attach request and PDN
connectivity request to MME
 MME sends Create Session Request to SGW using GTP
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LTE Default Bearer Establishment, Incoming
 UE is in RRC_IDLE condition
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LTE Default Bearer Establishment, Incoming
 MME has traffic for specific UE. It sends Page message to all
eNBs in UE’s current tracking area (TA).
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LTE Default Bearer Establishment, Incoming
 eNB sends page message over air interface for UE
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LTE Default Bearer Establishment, Incoming
 UE recognizes the page and responds by sending RRC
Connection Request message to eNB
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LTE Default Bearer Establishment, Incoming
 eNB sends RRC Connection Setup message to UE
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LTE Default Bearer Establishment, Incoming
 UE sends eNB a RRC Connection Setup Complete message and
NAS message including Attach Request and PDN Connectivity
Request
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LTE Default Bearer Establishment, Incoming
 eNB sends Initial UE Message + NAS attach request and PDN
connectivity request to MME
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LTE Default Bearer Establishment, Incoming
 MME sends Create Session Request to SGW using GTP
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LTE Default Bearer Establishment, Incoming
 SGW sends PGW a PMIP Proxy Binding Update
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LTE Default Bearer Establishment, Incoming
 PGW responds to SGW with PMIP Proxy Binding ACK
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LTE Default Bearer Establishment, Incoming
 SGW sends Create Session Response to MME
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LTE Default Bearer Establishment, Incoming
 MME sends eNB Initial Context Setup request + NAS Activate
Default EPS Bearer Context Request and Attach Accept
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LTE Default Bearer Establishment, Incoming
 eNB sends UE an RRC Connection Reconfig and NAS Activate
Default EPS bearer context request and Attach Accept
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LTE Default Bearer Establishment, Incoming
 UE responds with RRC Connection Reconfiguration Complete
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LTE Default Bearer Establishment, Incoming
 eNB sends Initial Context Setup Response to MME
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LTE Default Bearer Establishment, Incoming
 UE sends eNB an RRC UL Info Transfer and NAS Activate Default
EPS bearer context accept and Attach Accept
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LTE Default Bearer Establishment, Incoming
 eNB sends to MME UL NAS Transport and NAS Activate Default
EPS Bearer Context Accept and Attach Accept
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LTE Default Bearer Establishment, Incoming
 MME sends Modify Bearer Request to SGW using GTP
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LTE Default Bearer Establishment, Incoming
 SGW responds to MME with Modify Bearer Response over GTP
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Flow Examples
Default Bearer Establishment
Outgoing
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LTE Default Bearer Establishment, Outgoing
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UE is in RRC_Idle mode
UE has data and needs connection to network
UE sends RRC Connection Request to eNB
eNB sends RRC Connection Setup to UE
UE sends RRC Connection Setup Complete and NAS Attach
Request and PDN Connectivity Request to eNB
eNB sends Initial UE Message and NAS Attach Request and PDN
Connectivity Request to MME
MME sends Create Session Request to SGW using GTP
SGW sends PMIP Proxy Binding Update to PGW
PGW sends PMIP Proxy Binding Ack to SGW
SGW sends Create Session Response to MME by GTP
MME sends eNB an Initial Context Setup Request and NAS
Activate Default EPS Bearer Context request and Attach Accept
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LTE Default Bearer Establishment, Outgoing
 UE is in RRC_Idle mode
 UE has data and needs connection to network
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LTE Default Bearer Establishment, Outgoing
 UE sends RRC Connection Request to eNB
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LTE Default Bearer Establishment, Outgoing
 eNB sends RRC Connection Setup to UE
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LTE Default Bearer Establishment, Outgoing
 UE sends RRC Connection Setup Complete and NAS Attach
Request and PDN Connectivity Request to eNB
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LTE Default Bearer Establishment, Outgoing
 eNB sends Initial UE Message and NAS Attach Request and PDN
Connectivity Request to MME
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LTE Default Bearer Establishment, Outgoing
 MME sends Create Session Request to SGW using GTP
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LTE Default Bearer Establishment, Outgoing
 SGW semds PMIP Proxy Binding Update to PGW
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LTE Default Bearer Establishment, Outgoing
 PGW sends PMIP Proxy Binding Ack to SGW
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LTE Default Bearer Establishment, Outgoing
 SGW sends Create Session Response to MME by GTP
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LTE Default Bearer Establishment, Outgoing
 MME sends eNB an Initial Context Setup Request and NAS
Activate Default EPS Bearer Context request and Attach Accept
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LTE Default Bearer Establishment, Outgoing
 eNB sends UE an RRC Connection Reconfiguration and NAS
Activate Default EBS Bearer Context Request and Attach Accept
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LTE Default Bearer Establishment, Outgoing
 UE sends eNB RRC Connection Reconfiguration Complete
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LTE Default Bearer Establishment, Outgoing
 eNB sends MME an Initial Context Setup Response message
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LTE Default Bearer Establishment, Outgoing
 UE sends eNB RRC UL Info Transfer NAS Activate Default EPS
Bearer Context Accept and Attach Accept
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LTE Default Bearer Establishment, Outgoing
 eNB sends MME a UL NAS Transport + NAS Activate Default EPS
Bearer Context Accept and Attach Complete
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LTE Default Bearer Establishment, Outgoing
 MME sends SGW a Modify Bearer request by GTP
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LTE Default Bearer Establishment, Outgoing
 SGW sends MME a Modify Bearer Response message by GTP
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LTE Scheduling
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Resource Allocation in LTE
 Resources in LTE
• Resource Grid, Resource Block, Slot, Sub-frame
 Control Information
• Physical Channels, PDCCH, DCI
 Resource Allocation
• Resource Block Group (RBG) based
• RBG Subset based
• Virtual Resource Block (VRB)-based
 Helpful Link: very useful utility showing LTE resource grid
• http://paul.wad.homepage.dk/LTE/lte_resource_grid.html
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LTE Scheduling
 The eNodeB allocates physical layer resources for the uplink and
downlink shared channels (UL-SCH and DL-SCH). Resources are
composed of Physical Resource Blocks (PRB) and Modulation
Coding Scheme (MCS). The MCS determines the bit rate, and
thus the capacity, of PRBs. Allocations may be valid for one or
more TTIs; each TTI interval is one subframe (1 ms).
 Semi-persistent scheduling reduces control channel signaling. If
every allocation was individually signaled, the overhead would be
unacceptable. In an application such as voice over IP, for example,
a downlink frame occurs every 10 to 20 milliseconds. If each
downlink frame were signaled individually, it would cause a lot of
traffic on the control channel and the control channel would need a
lot more bandwidth than necessary. Semi-persistent scheduling
lets you set up an ongoing allocation that persists until it is
changed. Semi-persistent schedules can be configured for both
uplink and downlink.
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Scheduling: Transmission Time Interval (TTI)
 The scheduler is the main player in rapidly utilized radio resource. The
smallest Transmission Time Interval (TTI) is only 1 ms.
 During each TTI the eNB scheduler:
• considers the physical radio environment per UE. The UEs report
received radio quality to the scheduler which decides which
Modulation and Coding scheme to use. The scheduler rapidly adapts
to channel variations, using HARQ (Hybrid Automatic Repeat
Request), soft-combining, and rate adaptation.
• prioritizes QoS requirements among the UEs. Both delay sensitive
and rate-sensitive data services are accomodated.
• informs UEs of their allocated downlink and uplink radio resources.
 Each UE scheduled in a TTI gets a Transport Block (TB) carrying its data.
• On downlink there can be a maximum of two TBs generated per UE if
using MIMO. The TBs are delivered over a transport channel.
• The user plane has only one shared channel in each direction. The TB
can contain bits from several services, multiplexed together.
• In theory the highest number of users that can be scheduled during 1
ms is 440, presuming 20 MHz band and 4x4 Multi User MIMO.
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Downlink: Dynamic Scheduling
 The PDCCH carries the Cell Radio Network Temporary Identifier
(C-RNTI), the dynamic UE identifier. The CRNTI indicates that an
upcoming downlink resource has been demultiplexed by the MAC,
passed on to higher layers and is now scheduled for this UE.
 Semi-persistent scheduling periodicity is configured by RRC.
Whether scheduling is dynamic or semi-persistent is indicated by
using different scrambling codes for the C-RNTI on PDCCH. The
PDCCH is a very low-bandwidth channel; it does not carry a lot of
information compared to the downlink shared channel.
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Downlink Semi-Persistent
and Dynamic Scheduling
 This figure adds semipersistent scheduling information to the
information already presented. Here, the RRC configures some of
the semipersistent scheduling. This shows a four-TTI example.
 The first time it actually occurs there is signaling on the PDCCH.
After that, every four TTIs there is a transmission which occurs
without any signaling on the control channel. You can still use
dynamic scheduling at the same time for other purposes if
necessary; this carries on until changed by another indication on
the control channel.
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Downlink Scheduling with HARQ
 Again, the C-RNTI is found on the PDCCH, indicating that an upcoming
downlink resource is scheduled for this UE.
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Downlink Scheduling with HARQ
 This figure shows the ACK/NACK process. HARQ generates an ACK or
NACK, sent on L1/L2 control channel (PUCCH) on subframe n+4, for
each downlink transport block. Here there is a negative
acknowledgement, so a subframe needs to be retransmitted using HARQ.
The retransmission is signaled dynamically and downlinked, then decoded
and sent up to higher layers. Finally the subframe has to be
acknowledged again. The process can become fairly complicated when
both acknowledgements and semipersistent scheduling are involved.
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Uplink Scheduling with HARQ
 As with the downlink, uplink scheduling information is found on the
PDCCH. The C-RNTI indicates that an upcoming uplink resource
is scheduled for this UE in 4 TTI. The 4 TTI delay gives the UE
time to dequeue, determine the proper priority and determine the
best way to pack that transport block with information based on the
QoS requirements of the scheduler that it’s running locally.
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ACK/NACK Process in Uplink Scheduling
 This figures shows the ACK/NACK process. The Physical HARQ Indicator Channel
(PHICH) is a special channel for providing feedback from the eNodeB back to the
UE on the uplink HARQ process. It carries ACK/NACK messages for uplink data
transport blocks. HARQ is synchronous, with a fixed time of 4 TTI from uplink to
ACK/NACK on the downlink from the eNodeB. The eNodeB responds back with an
opportunity to retransmit which is then scheduled and retransmitted. Although this
illustration does not show the positive acknowledgement after that, it would occur.
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LTE Handover and Roaming
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Introduction to Handover
 In modern wireless systems, “seamless handover” is expected by
users as they move between sites and networks.
 Handover occurs in the active state; it is controlled by the network
(the eNodeB).The network uses measurements from the UE and
its own knowledge of the network topology to determine when to
handover a UE, and to which eNodeB.
 Don’t confuse handover with the cell re-selection which occurs
when the UE is in the idle state. Reselection is controlled by the
UE using previously received parameters and does not involve
communication between the UE and eNodeB, unless the UE
enters a new tracking area and must do a tracking area update..
 In this chapter we briefly explain the procedures executed by the
user equipment (UE) and the various network elements to provide
the handover services requested by the UE. We cover Intra-LTE
handover and handovers from LTE to/from UMTS.
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Handover Measurement
 In a single-radio architecture, it is challenging to monitor other
networks on other frequencies while the receiver is active. The
radio can only receive on one frequency at a time. The radio needs
to listen to other frequencies to determine if a better base station
(eNodeB) is available.
 In the active state, the eNB provides measurement gaps in the
scheduling of the UE where no downlink or uplink scheduling
occurs. Ultimately the network makes the decision, but the gap
provides the UE sufficient time to change frequency, make a
measurement, and switch back to the active channel. This can
normally occur in a few TTIs. This has to be coordinated with DRX,
which also causes the system to shut off the radio for periods of
time to save power.
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Handover: Neighbor Lists
 The LTE network provides the UE with neighbor lists.
• The eNodeB provides the UE with neighboring eNB’s
identifiers and their frequency.
 During measurement gaps or idle periods, the UE measures the
signal quality of the neighbors it can receive.
 The UE reports results back to the eNodeB and the network
decides the best handover (if any), based on signal quality,
network utilization, etc.
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Handover Procedures - Objectives
 Objectives of Handover Procedures
• It is important that QoS is maintained, not just before and after
a handover, but during the handover as well.
• Handover must not unduly drain the UE battery power.
• Service continuity shall be maintained (i.e., minimal handover
latency).
• Seamless handoff is required to 3G / 2G / CDMA technology.
 There are two ways a handoff can be decided:
• Network Evaluated: the network makes the handover decision
• Mobile Evaluated: the UE makes the handoff decision and
informs the network about it.
– In this instance, the final decision will be made by the
network based upon on the Radio Resource Management.
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Handover Types
 In 3G and LTE networks, a hybrid approach is used to decide on
the handover.
• The UE will assist in the handoff decision by measuring the
neighboring cells and reporting the measurements to the
network
• The network decides upon the handoff timing and the target
cell/node.
• The parameters to measure and the thresholds for reporting
are decided by the network.
 In LTE there are three types of handovers:
• Intra-LTE: Handover happens within the current LTE nodes
(intra-MME and Intra-SGW)
• Inter-LTE: Handover happens toward the other LTE nodes
(inter-MME and Inter-SGW)
• Inter-RAT: Handover between different radio technology
networks, for example GSM/UMTS and UMTS
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Flow Examples
Intra-LTE (Intra-MME / SGW)
Handover
Using the X2 Interface
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 Consider Intra-LTE handovers with X2AP signaling and S1AP
signaling first, then Inter-RAT handovers in LTE (i.e., handover
between LTE and UMTS).
 Intra-LTE (Intra-MME / SGW) Handover Using the X2 Interface:
 This procedure is used to handover a UE from a source eNodeB
(S-eNB) to a target eNodeB (T-eNB) using the X2 interface when
the Mobility Management Entity (MME) and Serving Gateway
(SGW) are unchanged. It is possible only if direct connectivity
exists between the source and target eNodeB’s with the X2
interface.
 The X2 handover procedure is performed without Evolved Packet
Core (EPC) involvement, i.e. preparation messages are directly
exchanged between the S-eNB and T-eNB. The release of the
resources at the source side during the handover completion
phase is triggered by the T-eNB. The message flow is shown in
Figure 1 followed by the description
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The Data call is already established between the UE, S-eNB and
network elements.
 Data packets are already flowing to/from the UE on both DL & UL.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The Network sends a MEASUREMENT CONTROL REQ message
to the UE to set the measurement parameters and thresholds.
 The UE is instructed to send measurement report when thresholds
are met.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The UE sends a MEASUREMENT REPORT to the S-eNB as soon
as thresholds are met.
 The S-eNB decides to hand UE off to a T-eNB using network
operators’ handover algorithm.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 Optionally S-eNB issues RESOURCE STATUS REQUEST
message to determine the load on T-eNB.
 Based on received RESOURCE STATUS RESPONSE, the S-eNB
can decide whether to continue the handover procedure using the
X2 interface.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The S-eNB issues a HANDOVER REQUEST message to the TeNB with UE and RB contexts to prepare handover at the target.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 T-eNB checks availability, reserves resources and sends back
HANDOVER REQUEST ACKNOWLEDGE message including a
transparent container for the UE as an RRC message to perform
the handover.
 The container includes a new C-RNTI, T-eNB security algorithm
identifiers for the selected security algorithms, and may include a
dedicated RACH preamble and possibly some other parameters
(i.e., access parameters, SIBs, etc.).
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The S-eNB generates the RRC message to perform the handover,
i.e, RRCCONNECTION RECONFIGURATION message including
the mobility Control Information. The S-eNB performs the
necessary integrity protection and ciphering of the message and
sends it to the UE.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The S-eNB sends the eNB STATUS TRANSFER message to the
T-eNB to convey the PDCP and HFN status of the E-RABs.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The S-eNB starts forwarding the downlink data packets to the TeNB for all the data bearers (which are being established in the TeNB during the HANDOVER REQ message processing).
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 In the meantime, the UE tries to access the T-eNB cell using the
non-contention-based Random Access Procedure.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 If it succeeds in accessing the target cell, it sends the RRC
CONNECTION RECONFIGURATION COMPLETE to the T-eNB.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The T-eNB sends a PATH SWITCH REQUEST message to the
MME to inform it that the UE has changed cells, including the
TAI+ECGI of the target.
 The MME determines that the SGW can continue to serve the UE.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The MME sends a MODIFY BEARER REQUEST (eNodeB
address and TEIDs for downlink user plane for the accepted EPS
bearers) message to the SGW. If the PDN GW requested the UE’s
location info, the MME also includes the User Location Information
IE in this message.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The SGW sends one or more “end marker” packets on the old path
to the S-eNB and then can release any user plane / TNL resources
toward the S-eNB.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 15. The MME responds to the T-eNB with a PATH SWITCH REQ
ACK message to notify the completion of the handover.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 User data packets now flow between the SGW and the UE.
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Case I. Intra-LTE (Intra-MME / SGW) Handover
Using the X2 Interface
 The T-eNB now requests the S-eNB to release the resources
using the X2 UE CONTEXT RELEASE message. With this, the
handover procedure is complete.
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Flow Examples
Intra-LTE (Intra-MME / SGW)
Handover
Using the S1 Interface
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 An S1-based handover procedure is used when the X2-based
handover cannot be used
• no X2 connectivity to the target eNodeB;
• by an error indication from the T-eNB after an unsuccessful X2based handover
• by dynamic information learned by the S-eNB using the
STATUS TRANSFER procedure.
 The S-eNB initiates the handover by sending a Handover required
message over the S1-MME reference point. The EPC does not
change the decisions taken by the S-eNB.
 The availability of a direct forwarding path is determined in the SeNB (based on the X2 connectivity with the T-eNB) and indicated
to the source MME.
• If a direct forwarding path is not available, indirect forwarding
will be used. The source MME uses the indication from the SeNB to determine whether to apply indirect forwarding or not.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 Based on the MEASUREMENT REPORT from the UE, the S-eNB
decides to Handover the UE to another eNodeB (T-eNB). The
handover procedure in this section is very similar to that in the
previous section (Intra-LTE Handover Using the X2 Interface),
except the involvement of the MME in relaying the handover
signaling between the S-eNB and T-eNB.
 There are two differences here:
• No need for the PATH SWITCH Procedure between the T-eNB
and MME, as MME is aware of the Handover.
• The SGW is involved in the DL data forwarding if there is no
direct forwarding path available between the S-eNB and TeNB.
 Once the Handover is complete, the MME clears the logical S1
connection with the S-eNB by initiating the UE CONTEXT
RELEASE procedure.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The UE is sending and receiving user data on both the uplink and
downlink.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The S-eNB sends an RRC: Measurement Control message to the
UE, instructing it to take certain measurements at specific intervals
and to report the results when specific criteria are met.
 The UE sets to work taking the requested measurements and
performing comparisons against the specified criteria.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The UE notices that measurements have satisfied the specified
criteria. It sends an RRC: Measurement Report to the Currently
Serving eNB.
 The handover procedure in this section is very similar to that in the
previous section (Intra-LTE Handover Using the X2 Interface),
except the involvement of the MME in relaying the handover
signaling between the S-eNB and T-eNB.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The serving eNB sends a Handover Required message to the
MME
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 MME sends Handover Request to Target eNB
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The Target eNB sends a Handover Request Acknowledgment to
the MME
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The MME sends a Handover Command to the serving eNB
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The Serving eNB sends an RRC Connection Reconfiguration
Request to the UE
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The Serving eNB sends an eNB Status Transfer message to the
MME
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The Serving eNB sends a Forward User Data message to the
SGW by GTP protocol
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The MME sends an MME Status Transfer message to the Target
eNB
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The UE performs the Non-Contention RACH Process on the
Target eNB
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The SGW sends Forward User Data to the Target eNB
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The UE sends an RRC Connection Reconfiguration Complete
message to the Target eNB
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The Target eNB sends a Handover Notify message to the MME
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The MME sends a Modify Bearer Request message to the SGW
by GTP
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The SGW sends a Modify Bearer Response to the MME by GTP
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 User data packets now flow between the UE and the SGW.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The T-eNB sends an S1AP UE Context Release Command to the
the S-eNB.
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Case II. Intra-LTE (Intra-MME / SGW) Handover
Using the S1 Interface
 The S-eNB confirms the requested UE context release by sending
the MME an S1AP UE Context Release Complete message.
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Flow Examples
Inter-MME Handover (Intra-SGW)
(no change in Gateway)
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Case III. Inter-MME Handover (Intra-SGW)
(no change in Gateway)
 In an inter-MME handover, two MMEs are involved in the
handover, the source MME (S-MME) and target MME (T-MME).
The S-MME controls the S-eNB and the T-MME controls the TeNB; both MMEs are connected to the same SGW. This handover
is triggered when the UE moves from one MME area to another
MME area.
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Case III. Inter-MME Handover (Intra-SGW)
 The UE is sending and receiving user data on both the uplink and
downlink.
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving eNB sends a Handover Request to the Serving MME
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving MME sends a Forward Relocation Request to the
Target MME by GTP
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Case III. Inter-MME Handover (Intra-SGW)
 The Target MME sends a Handover Request to the Target eNB
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Case III. Inter-MME Handover (Intra-SGW)
 The Target eNB sends a Handover Request Acknowledgment to
the Target MME
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Case III. Inter-MME Handover (Intra-SGW)
 The Target MME sends a Forward Relocation Response to the
Serving MME
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving MME sends a Handover Command to the Serving
eNB
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving eNB sends a RRC Connection Reconfiguration
Request to the UE
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving eNB sends an eNB Status Transfer to the Serving
MME, which forwards it to the Target MME
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Case III. Inter-MME Handover (Intra-SGW)
 The Target MME sends an eNB Status Transfer to the Target eNB
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving eNB sends Forward User data to the SGW by GTP
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Case III. Inter-MME Handover (Intra-SGW)
 The SGW sends Forward User Data to the Target eNB by GTP
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Case III. Inter-MME Handover (Intra-SGW)
 The UE performs the Non-Contention RACH procedure on the
Target eNB
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Case III. Inter-MME Handover (Intra-SGW)
 The UE sends RRC Connection Reconfiguration Complete to the
Target eNB
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Case III. Inter-MME Handover (Intra-SGW)
 The Target eNB sends a Handover Notify message to the Target
MME
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Case III. Inter-MME Handover (Intra-SGW)
 The Target MME sends a Modify Bearer Request to the SGW by
GTP
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Case III. Inter-MME Handover (Intra-SGW)
 The SGW sends a Modify Bearer Response to the Target MME by
GTP
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Case III. Inter-MME Handover (Intra-SGW)
 The Target MME sends a Forward Relocation Complete message
to the Serving MME
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Case III. Inter-MME Handover (Intra-SGW)
 The Serving MME sends a Forward Relocation Complete
Acknowledgment to the Target MME
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Case III. Inter-MME Handover (Intra-SGW)
 User Packets now flow directly from UE to SGW in both directions
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Case III. Inter-MME Handover (Intra-SGW)
 The S-MME sends a UE Context Release Command to S-eNB
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Case III. Inter-MME Handover (Intra-SGW)
 The S-eNB responds with a UE Context Release Complete
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Flow Examples
Inter-MME / SGW Handover
Using the S1 Interface
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Case IV. Inter-MME / SGW Handover
Using the S1 Interface
 Inter-MME / SGW Handover Using the S1 Interface
 This scenario is similar to the previous section with the difference
being the Source and Target eNodeBs are served by different
MME / SGW nodes. Figure 4 depicts the procedures and is
followed by the explanation.
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Case IV. Inter-MME / SGW Handover
Using the S1 Interface
 1 Based on the MEASUREMENT REPORT from the UE, the S-eNB
decides to handover the UE to another eNodeB (T-eNB). The procedure is
like earlier ones except for involvement of two SGWs (S-SGW and TSGW) to transfer data packets during handover.
 2. After receiving GTP: FORWARD RELOCATION REQ from S-MME, TMME detects SGW change, starts bearer creation toward target T-SGW
using GTP: CREATE SESSION REQ message.
 3. After creation of requested bearers, T-SGW responds back to MME
with a GTP: CREATE SESSION RESPONSE message.
 4. From here on, message flow is very similar to Inter-MME, Intra- SGW
handover except for these differences:
• While processing the S1 HANDOVER NOTIFY message from the TeNB, the T-MME updates the T-eNB endpoint information to the TSGW using GTP: MODIFY BEARER REQ.
• After updating T-eNB information in the bearers T-SGW sends GTP:
MODIFY BEARER RESPONSE message to the T-MME.
 When Handover Complete, S-MME releases bearer resources with the SSGW for this UE by GTP: DELETE SESSION procedure
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Case IV. Inter-MME / SGW Handover
 The UE is sending and receiving user data on both the uplink and
downlink.
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Case IV. Inter-MME / SGW Handover
 The S-eNB sends RRC Measurement Procedures to the UE
 The UE performs the requested measurements
 The S-eNB receives information when specified thresholds are
exceeded, triggering need for a handover
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Case IV. Inter-MME / SGW Handover
 The Serving eNB sends a Handover Request to the serving MME
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Case IV. Inter-MME / SGW Handover
 The serving MME sends a Forward Relocation Request to the
target MME
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Case IV. Inter-MME / SGW Handover
 The Target MME sends a Create Session Request to the Target
SGW by GTP
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Case IV. Inter-MME / SGW Handover
 The Target SGW sends a Create Session Request to the Target
MME by GTP
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Case IV. Inter-MME / SGW Handover
 The Target MME sends a Handover Request to the Target eNB
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Case IV. Inter-MME / SGW Handover
 The Target eNB sends a handover Request Acknowledgment to
the Target MME
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Case IV. Inter-MME / SGW Handover
 The Target MME sends a Forward Relocation Request to the
Serving MME using GTP
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Case IV. Inter-MME / SGW Handover
 The Serving MME sends a Handover Command to the Serving
eNB
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Case IV. Inter-MME / SGW Handover
 The Serving eNB sends an RRC Connection Reconfiguration
Request to the UE
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Case IV. Inter-MME / SGW Handover
 The Serving eNB sends an eNB Status Transfer to the Target
MME
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Case IV. Inter-MME / SGW Handover
 The Target MME sends an eNB Status Transfer to the Target eNB
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Case IV. Inter-MME / SGW Handover
 The Serving eNB sends Forward User Data to the Target eNB
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Case IV. Inter-MME / SGW Handover
 The UE performs the Non-Contention RACH Procedure on the
Target eNB
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Case IV. Inter-MME / SGW Handover
 The UE sends an RRC Connection Reconfiguration Complete
message to the Target eNB
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Case IV. Inter-MME / SGW Handover
 The Target eNB sends a Handover Notify message to the Target
MME
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Case IV. Inter-MME / SGW Handover
 The Target MME sends a Modify Bearer Request to the Target
SGW using GTP
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Case IV. Inter-MME / SGW Handover
 The Target SGW sends a Modify Bearer Response to the Target
MME by GTP
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Case IV. Inter-MME / SGW Handover
 The Target MME sends a Forward Relocation Complete message
to the Serving MME
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Case IV. Inter-MME / SGW Handover
 The Serving MME sends a UE Context Release Command to the
Serving eNB
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Case IV. Inter-MME / SGW Handover
 The Serving MME sends a Forward Relocation Completion
acknowledgment to the Target MME
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Case IV. Inter-MME / SGW Handover
 The Serving eNB sends a UE Context release Complete to the
Serving MME
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Case IV. Inter-MME / SGW Handover
 The Serving MME sends a Delete Session Request to the Serving
SGW
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Case IV. Inter-MME / SGW Handover
 The S-SGW sends a Delete Session Response to the S-MME
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Case IV. Inter-MME / SGW Handover
 User data packets flow from UE to T-SGW in both UL and DL
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LTE InterRAT Handover
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LTE Security
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LTE Security Objectives
 LTE security is extremely important. LTE must required security
without impacting the user experience.
 Users must operate freely and without fear of attack from hackers
and the network must also be secure against a variety of attacks.
 LTE security basics: Requirements for LTE security
• provide at least same level of security as in 3G services.
• LTE security measures must not affect user convenience.
• provide defense from attacks from the Internet.
• LTE security functions should not impede the transition from
existing 3G services to LTE.
• The USIM currently used for 3G services should still be used.
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Basic Development of LTE Security
 Additional LTE measures have been implemented in all areas of
the system from the UE through to the core network. In summary:
• A new hierarchical key system has been introduced in which
keys can be changed for different purposes.
• security functions for the Non-Access Stratum, NAS, and
Access Stratum, AS have been separated.
• NAS functions are processed between the core network and
the mobile terminal or UE.
• AS functions encompass communications between the network
edge, i.e. the Evolved Node B, eNB and the UE
• The concept of forward security has been introduced for LTE
security.
• LTE security functions have been introduced between the
existing 3G network and the LTE network.
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The LTE USIM
 The Subscriber Identity Module
(SIM) is one of the key security
elements of GSM, UMTS and
now LTE. This card holds identity
of the subscriber in an encrypted
fashion while phone or device.
 In transition from 2G/GSM to
3G/UMTS, the SIM concept was
upgraded and the USIM/UMTS
Subscriber Identity Module is
used. It has more functionality,
larger memory, etc.
 For LTE, only the USIM may be
used - the older SIM cards are not
compatible and may not be used.
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Voice over LTE
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Why Voice Over LTE?
 Originally LTE was seen as a completely IP cellular system just for
carrying data, and operators would be able to carry voice either by
reverting to 2G / 3G systems or by using VoIP.
 The Voice over LTE, VoLTE scheme was devised by operators
looking for a standardized system for carrying voice over LTE.
 But to Operators, the lack of a defined voice format seemed to be
a major omission for the system.
• lack of standardization may cause problems in roaming.
• SMS is a key requirement since it used to set-up many mobile
broadband connections. Lack of SMS is a show-stopper
 Mobile operators still receive over 80% of their revenues from
voice and SMS traffic. A viable and standardized scheme is
essential to provide these services and protect this revenue.
• LTE can more efficiently deliver these services due to its much
higher spectral efficiency
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Options for Voice over LTE
 There are several options for delivering Voice over LTE:
• CSFB, Circuit Switched Fall Back: automatically falling back
the old 2G or 3G system when an LTE UE initiates a call. This
spec also allows SMS to be carried over an interface known as
SGs, so messages to be sent over an LTE channel.
• SV-LTE - simultaneous voice LTE: SV-LTE can run packet
switched LTE services simultaneously with circuit switched
voice service.However,it requires two radios to run at the same
time within the handset, with serious battery drain
• VoLGA, Voice over LTE via GAN: The VoLGA standard is
based on existing 3GPP Generic Access Network (GAN)
standards, aiming to deliver a consistent user services while
the network transitions to LTE (low-risk, popular with operators)
• One Voice / later called Voice over LTE, VoLTE: Provides
voice over the LTE system using IMS as part of a rich media
solution which can handle multimedia as well
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Issues for Voice Services over LTE
 Unlike previous standards (GSM, CDMA), LTE does not have
dedicated channels for circuit switched telephony. LTE is an all-IP
system providing an end-to-end IP connection from the mobile
equipment to the core network and out again.
 In order to provide some form of voice connection over a standard
LTE bearer, some form of Voice over IP (VoIP) must be used.
 The aim for any voice service is to exploit the LTE low latency and
QoS features so that any LTE voice service is better than 2G/3G
 However to achieve a full VoIP offering on LTE poses some
significant problems which will take time to resolve. With the first
deployments having taken place in 2010, it is necessary that a
solution for voice is available within a short timescale.
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Voice over LTE (VoLTE) Basics
 The One Voice profile for Voice over LTE (VoLTE) was developed
by a collaboration between over forty operators and manufacturers
including AT&T, Verizon Wireless, Nokia and Alcatel-Lucent.
• At the 2010 GSMA Mobile World Congress, GSMA announced
their support for the VoLTE solution to provide Voice over LTE.
• VoLTE, Voice over LTE is an IMS-based specification.
Adopting this approach, it enables the system to be integrated
with the suite of applications that will become available on LTE
 Three interfaces are being defined to provide VoLTE:
• User Network interface, UNI: between the user's equipment
and the operators network.
• Roaming Network Network Interface, R-NNI: located between
the Home and Visited Network.
• Interconnect Network Network Interface, I-NNI: located
between the networks of the two parties making a call.
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Continuing Work on LTE
 Work to define Voice over LTE (VoLTE) is ongoing, including the
following elements:
• ensuring continuity of Voice calls as a user moves from an LTE
coverage area to an area where a fallback to another
technology is required. This form of handover will be achieved
using Single Radio Voice Call Continuity, or SR-VCC).
• Providing optimal routing of bearers for voice calls when
customers are roaming.
• establishing commercial frameworks for roaming and
interconnect for services implemented using VoLTE definitions,
necessary to set up roaming agreements
• Providing capabilities ror roaming hubbing
• Providing security and fraud threat measures to prevent
hacking and unauthorized network penetration..
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IMS
IP Multimedia Subsystem
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What is IMS?
IP Multimedia Core Network Subsystem
 The IP Multimedia Subsystem or IP Multimedia Core Network
Subsystem, IMS is an architectural framework for delivering
Internet Protocol, IP multimedia services. It enables a variety of
services to be run seamlessly rather than having independent
applications operating concurrently.
 IMS, or IP Multimedia Subsystem is having a major impact on the
telecommunications industry, both wired and wireless.
 Although IMS was originally created for mobile applications by
3GPP and 3GPP2, its use is more widespread as fixed line
providers are also being forced to find ways of integrating mobile
or mobile associated technologies into their portfolios.
 As a result the use of IMS, IP multimedia subsystem is crossing
the frontiers of mobile, wire-less and fixed line technologies.
Indeed there is very little within IMS that is wireless or mobile
specific, and as a result there are no barriers to its use in any
telecommunications environment.
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IMS Basics
 IMS, IP multimedia subsystem is an architecture, not a technology
• It uses Internet standards to deliver services on new networks.
• It uses Session Initiation Protocol (SIP) for establishing, managing and
terminating sessions on IP networks.
 The overall IMS architecture uses several components to enable multimedia
sessions between two or more end devices.
• One element is a presence server to handle user status
– a key element in Push to talk over Cellular (PoC) where the presence, or
user status is key to enabling one user to be able to talk to another.
 Users often need many concurrent simultaneous sessions of different applications
• IMS provides a common IP interface for simplified signaling, traffic, and
application development
• In addition, under IMS architecture subscribers can connect to a network using
multiple mobile and fixed devices and technologies. With new applications
such as Push to talk over Cellular (PoC), gaming, video and more, it is
seamless integration is necessary for users to get the full benefits.
 IMS has advantages for operators too. In addition to maximum services for
maximum revenues, functions like billing, and "access approval" can be unified
across network applications, greatly simplifying deployment and management
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IMS Architecture Basics
 The architecture of an IMS system can be split into a number of
main elements or areas:
• User equipment: As the name implies, the user equipment or
UE is part of the IMS architecture resides with the user - it is
the endpoint.
• Access network: This is the portion of the IMS architecture
through which the overall network is accessed.
• Core network: This is a major element within the IMS
architecture and provides all the core functionality.
• Application layer: The application layer contains the web
portal and the application servers, which provide the end user
with service and enhanced service controls. T
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IMS Architecture Functional View
Elements of overall IMS architecture:
 Server CSCF: session control for endpoint
devices; maintains state.
 Proxy CSCF: entry point to IMS for the UE;
forwards SIP messages to user's home S-CSCF;
controls inter-working security; QoS mgt.
 Interrogating CSCF: a session control for endpoint
devices.
 Home Subscriber Server, HSS: provides
subscriber database for the home network.
 Breakout gateway control function, BGCF: selects
the network in which a PSTN breakout is to occur.
If on in the same network as the BGCF, also
selects a media gateway control function, MGCF
 Media gateway control function, MGCF:
interworks the SIP signalling. manages sessions
across multiple media gateways
 Media server function control, MSCF: manages
the use of resources on media servers.
 SIP applications server, SIP-AS: execution
platform to deploy more services
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LTE Advanced
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LTE Advanced
 The driving force to further develop LTE towards LTE–Advanced,
LTE R-10 is to provide higher bitrates in a cost efficient way, and
at the same time completely fulfil the requirements set by ITU for
IMT Advanced, also referred to as 4G.
 In LTE-Advanced focus is on higher capacity:
 - increased peak data rate, DL 3 Gbps, UL 1.5 Gbps
 - higher spectral efficiency, from a maximum of 16bps/Hz in R8 to
30 bps/Hz in R10
 - increased number of simultaneously active subscribers
 - improved performance at cell edges, e.g. for DL 2x2 MIMO at
least 2.40 bps/Hz/cell.
 The main new functionalities introduced in LTE-Advanced are
Carrier Aggregation (CA), enhanced use of multi-antenna
techniques and support for Relay Nodes (RN).
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LTE Advanced (2)
Carrier Aggregation
 The simplest way to increase
capacity is to add more bandwidth.
 To keep backward compatibility with
R8 and R9 mobiles the increase in
bandwidth in LTE-Advanced is
provided through aggregation of
R8/R9 carriers. Carrier aggregation
can be used for both FDD and TDD.
 Each aggregated carrier is referred
to as a component carrier.
 A component carrier can have a
bandwidth of 1.4, 3, 5, 10, 15 or 20
MHz Up to five component carriers
can be aggregated.
 R10 UEs can use DL and UL on up
to five Component Carriers (CC).
R8/R9 UEs can use any ONE of the
CCs. The CCs can be of different
bandwidths.
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 The maximum aggregate
bandwidth is 100 MHz.
 The number of aggregated
carriers can be different in DL
and UL, but UL is never larger
than DL. The individual
component carriers can have
different bandwidths.
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LTE Advanced (3)
Continuous and Non-Continuous Aggregation
 Contiguous component carriers in the same operating frequency band are called
intra-band contiguous. This simplest arrangement is not always possible..
 Non-contiguous allocation can be intra-band, i.e. the component carriers belong to
the same operating frequency band, but are separated by a gap
 Non-contiguous allocation can be inter-band, in which case the component carriers
belong to different operating frequency bands
 Each component carrier is present on certain cells. Not all cells have all carriers.
The coverage of serving cells may differ due to different frequencies and powers
 RRC connection is handled by one cell, the Primary serving cell, using the Primary
component carrier (DL and UL PCC). The other component carriers are called
Secondary component carriers (DL and UL SCC), on secondary serving cells.
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Differing Coverage of Different Carriers
 Different component carriers can have different coverage
 In inter-band carrier aggregation the component carriers will
experience different pathloss, due to different frequencies.
 In the example above, carrier aggregation on all three component
carriers can only be used by the black UE. The white UE is not
within the coverage area of the red component carrier. Note that
for UEs using the same set of CCs, they can have different PCC.
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Main Differences in LTE Protocols
to Support Carrier Aggregation
 Introduction of carrier aggregation influences mainly MAC and the
physical layer protocol, but also some new RRC messages are
introduced.
 In order to keep R8/R9 compatibility the protocol changes are kept
to a minimum.
• Basically each component carrier is treated as an R8 carrier.
• However some information is necessary, such as new RRC
messages in order to make SCC and MAC able to handle
scheduling on a number of CCs.
• Major changes on the physical layer are for example that
signaling information about scheduling on CCs as well as
HARQ ACK/NACK per CC must be carried.
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Main Differences in LTE Protocols
to Support Carrier Aggregation
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Cross-Carrier Scheduling
 Regarding scheduling there are two main alternatives for CA, either
resources are scheduled on the same carrier as the grant is received, or
so called cross-carrier scheduling may be used
 Figure 5. CA scheduling (FDD). Cross- carrier scheduling is only used to
schedule resources on SCC without PDCCH. The CIF (Carrier Indicator
Field) on PDCCH (represented by the red area) indicates on which carrier
the scheduled resource is located
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More References on Carrier Aggregation
 TR 36.808 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier
Aggregation; Base Station (BS) radio transmission and reception
 TR 36.814 Evolved Universal Terrestrial Radio Access (E-UTRA); Further
advancements for E-UTRA physical layer aspects
 TR 36.815 Further Advancements for E-UTRA; LTE-Advanced feasibility studies in
RAN WG4
 TR 36.823 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier
Aggregation Enhancements; UE and BS radio transmission and reception
 TR 36.912 Feasibility study for Further Advancements for E-UTRA (LTE-Advanced)
 TR 36.913 Requirements for further advancements for Evolved Universal Terrestrial
Radio Access (E-UTRA) (LTE-Advanced)
 TS 36.211 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels
and modulation
 TS 36.212 Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and
channel coding
 TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer
procedures
 TS 36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved
Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2
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LTE SON:
Self Organizing/Optimizing Networks
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Major Elements of LTE SON
 LTE has created a lot of interest in Self Optimizing Networks,
although the idea can be applied to other technologies too
 The main elements of SON include:
• Self configuration: to enable new base stations to become
essentially "Plug and Play" items. They need little manual
attention for RF or backhaul configuration
• Self optimization: After setup, the eNodeB will autonomously
optimize its operational characteristics for best performance
• Self-healing: Autonomously detecting network problems and
changing network characteristics to mask the problem until
manual repairs can be made - for example, adjacent cell
boundary manipulation when a cell goes down
 Typically an LTE SON system is a feature and software package
with relevant options that an operator buys from the network
manufacturer
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LTE SON Development
 The Next Generation Mobile Networks (NGMN) alliance introduced
the term SON when it became obvious that LTE networks were
going to use large numbers of cells, microcells, and femtocells.
• With revenue per bit falling, deployment costs must be reduced
at the same time network performance demands are increasing
 Third Generation Partnership Program (3GPP) has created the
standards for SON. Since LTE is the first technology to use them,
they are often referred to as LTE SON.
 While 3GPP has generated the standards, they have been based
upon long term objectives for a 'SON-enabled broadband mobile
network' set out by the NGMN.
 NGMN has defined the necessary use cases, measurements,
procedures and open interfaces to ensure that multivendor
offerings are available. 3GPP has incorporated these aspirations
into useable standards.
 Deployment of LTE SON features is in very early stages now
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LTE SON and 3GPP Standards
 LTE Son has been standardized in the various 3GPP standards. It
was first incorporated into 3GPP release 8, and further
functionality has been progressively added in the further releases
of the standards.
 One of the major aims of the 3GPP standardization is the support
of SON features is to ensure that multi-vendor network
environments operate correctly with LTE SON. As a result, 3GPP
has defined a set of LTE SON use cases and the associated SON
functions.
 As the functionality of LTE advances, the LTE SON
standardization effectively track the LTE network evolution stages.
In this way SON will be applicable to the LTE networks.
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