Long Term Evolution
Technology training
(Part 1)
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Outline
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LTE and SAE overview
LTE radio interface architecture
LTE radio access architecture
LTE multiple antenna techniques
Part 1
LTE/SAE OVERVIEW
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Mobile broadband (3GPP)
Release
Standardized
Commercial
Major features
3GPP R99
1999
2000
•Bearer services
•64 kbit/s CS
•384 kbit/s PS
•Location services
•Call services: compatible with GSM
3GPP R5
2002
2006
• IP Multimedia Subsystem (IMS)
• IPv6, IP transport in UTRAN
• Improvements in GERAN
•HSDPA
3GPP R6
2004
2007
• Multimedia broadcast and multicast
•Improvements in IMS
•HSUPA
•Fractional DPCH
3GPP R7
2007
2008
•Enhanced L2
•64 QAM , MIMO
•VoIP over HSPA
•CPC - continuous packet connectivity
•FRLC - Flexible RLC
3GPP R8
2008
2010
•DC-HSPA+ (Dual Cell HSPA+)
•HSUPA 16QAM
3GPP R8 (LTE)
2008
2010
•New air interface (OFDM/SC-FDMA)
•New core network
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3G continues to evolve
Standardized through 3GPP
3G gracefully evolves into 4G –
starting from R7 and R8
Date rates
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R99: 0.4Mbps UL, 0.4Mbps DL
R5: 0.4Mbps UL, 14Mbps DL
R6: 5.7Mbps UL, 14Mbps DL
R7: 11Mbps UL, 28Mbps DL
R8: 50Mbps UL on LTE, 160 Mbps
DL on LTE, 42Mbps DL on HSPA
Two branches of the standards
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HSPA : Gradual performance
improvements at lower incremental
costs
LTE: revolutionary changes with
significant performance
improvements (higher cost, first step
towards IMT advanced)
LTE Releases
Release
3GPP R8 (LTE)
3GPP R9 (LTE)
3GPP R10 (LTE)
LTE Advanced
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5
Standardized
2008
Commercial
2010
Major features
•Multi antenna support
•Channel dependent scheduling
•Bandwidth flexibility
•ICIC (Intercell Interference Coordination)
•Hybrid ARQ
•FDD + TDD support
2009
•Dual layer beam forming
•Network based UE positioning
•MBSFN (Multicast/Broadcast Single Frequency Network)
2010
•Multi antenna extension
•Relaying
•Carrier aggregation
•Heterogeneous networks (HetNet’s)
LTE – has an “evolution path” of its own
Evolution is towards IMT-Advanced (LTE advanced)
LTE advanced – spectral efficiency 30bps/Hz (DL), 15bps/Hz (UL)
Note: This presentation focuses on R8 features
LTE requirements
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Outlined in 3GPP TR 29.913
Seven different areas
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Capabilities
System performance
Deployment related aspects
Architecture and migration
Radio resource management
Complexity, and
General aspects
Capabilities
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DL data rate > 100 Mbps in 20 MHz
UL data rate > 50 Mbps in 20MHz
Rate scales linearly with spectrum
Latency user plane: 5ms (transmission of
small packet from UE to edge of RAN)
Latency control plane: transmission time
from camped state – 100ms, transmission
time from dormant state 50 ms
Support for 200 mobiles in 5MHz, 400
mobiles in more than 5MHz
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System performance
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Baseline is HSPA Rel. 6
Throughput specified at 5% and 50%
Maximum performance for low mobility
users (0-15km/h)
High performance up to 120 km/h
Maximum supported speed 500km/h
Cell range up to 100km
Spectral efficiency for broadcast 1 b/s/Hz
Throughput requirements relative to baseline
Performance
measure
DL target relative to
base line
UL target relative to
baseline
Average throughput
per MHz
3-4 times
2-3 times
Cell edge user
throughput per MHz
2-3 times
2-3 times
Spectrum efficiency
(bit/sec/Hz)
3-4 times
2-3 times
LTE requirements (2)
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Deployment related aspects
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LTE may be deployed as standalone or
together with WCDMA/HSPA and/or
GSM/GPRS
Full mobility between different RANs
Handover interruption time targets
specified
Spectrum flexibility
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Both paired and unpaired bands
IMT 2000 bands (co-existence with
WCDMA and GSM)
Channel bandwidth from 1.4-20MHz
Handover interruption time
Non-real time
services (ms)
Real time services
(ms)
LTE to WCDMA
500
300
LTE to GSM
500
300
LTE duplexing options
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LTE requirements (3)
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Architecture and migration
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Single RAN architecture
RAN is fully packet based with support
for real time conversational class
RAN architecture should minimize
“single points” of failure
RAN should simplify and reduce
number of interfaces
Radio Network Layer and Transport
Network Layer interaction should not
be precluded in interest of performance
QoS support should be provided for
various types of traffic
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Radio resource management
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Support for enhanced end to end QoS
Support for load sharing between different
radio access technologies (RATs)
Complexity
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LTE should be less complex than
WCDMA/HSPA
SAE design targets
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SAE – Service Architecture Evolution
SAE = core network
Requirements placed into seven categories
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High level and operational aspects
Basic capabilities
Multi-access and seamless mobility
Man-machine interface aspects
Performance requirements for Evolved 3GPP system
Security and privacy
Charging aspects
SAE requirements mainly non access related
(highlighted ones have impact on RAN)
Basic principles – Air interface
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Downlink OFDM
OFDM = Orthogonal Frequency
Division Multiplexing
OFDM = Parallel transmission on
multiple carriers
Advantages of OFDM
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High PAPR and lower power amplifier
efficiency
Uplink DFTS-OFDM (SC-FDMA)
DFTS = DFT spread OFDM
SC-FDMA = Single carrier FDMA
Advantages (all critical for UL)
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Avoid intra-cell interference
Robust with respect to multi-path propagation
and channel dispersion
Disadvantage of OFDM
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Signal has single carrier properties
Low PAPR
Similar hardware as OFDM
Reduced PA cost
Efficient power consumption
Disadvantage
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Equalizer needed (not critical from UL)
UL modulation
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DL modulation
Basic principles – Air interface
• Shared channel transmission
– Only PS support
– No CS services
• Fast channel dependent
scheduling
– Adaptation in time
– Adaptation in frequency
– Adaptation in code
Scheduler takes the
advantage of timefrequency variations
of the channel
• Hybrid ARQ with soft combining
– Chain combining
– Incremental redundancy
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ARQ reduces
required Eb/No
One shared channel
simplifies the overall
signaling
Basic principles – air interface
• MIMO support
– MIMO = Multiple Input Multiple Output
– Use of multiple TX / RX antennas
– Three ways of utilizing MIMO
Outline of spatial multiplexing
idea
• RX diversity/TX diversity
• Beam forming
• Spatial multiplexing (MIMO with space time
coding)
– MIMO transmission in Rayleigh fading
environment increases theoretical
capacity by a factor equal to number of
independent TX RX paths
– As a minimum LTE mobiles have two
antennas (possibly four)
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Note: Rayleigh fading de-correlates
the paths and provides multiple
uncorrelated channels
Basic principles – air interface
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ICIC – Inter-cell interference
coordination
LTE affected by inter-cell
interference (more than HSDPA)
In LTE interference avoidance
becomes scheduling problem
By managing resources across
multiple cells inter-cell
interference may be reduced
Standard supports exchange of
interference indicators between
the cells
One possible
implementation of ICIC. Cell
edge implements N=3. Cell
interior implements N=1.
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SAE-Architecture
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SAE – flat architecture
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Single element simplifies RAN
No single point of failure
Core network provides two planes
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LTE Network layout
RAN consist of single elements:
eNode B
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Core network,
RAN
User plane (through SGSN)
Control plane (through MME)
Interfaces
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S1-UP (eNode B to SGSN)
S1-CP (eNode B to MME)
X2 between two eNode Bs (required for
handover)
Uu (UE to eNode B)
SAE = System Architecture Evaluation
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UE – user equipment (i.e. mobile)
eNode B – base station
SGSN – Support GPRS Serving Node
GGSN – Gateway GPRS Serving Node
MME – Mobility Management Entity
PCRF - Policy and Charging Rules function
LTE protocol-control plane
NAS
RRC
PDCP
RLC
MAC
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– Non Access Stratum
– Radio Resource Control
– Packet Data Convergence Protocol
– Radio Link Control
– Medium Access Control
S1-AP – S1 Application
SCTP – Stream Control Transmission Prot.
IP
– Internet Protocol
Note: LTE control plane is almost the
same as WCDMA (PDCP did not exist in
WCDMA control plane)
LTE protocol- user plane
PDCP – Packet Data Convergence Protocol
RLC
– Radio Link Control
MAC – Medium Access Control
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GTP-U - GPRS Tunneling Protocol
Note: LTE user plane is identical to UMTS PS
side. There is no CS in LTE – user plane is
simplified.
LTE protocol – X2
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Connects all eNodeB’s that are
supporting end user active
mobility (handover)
Supports both user plane and
control plane
Control plane – signaling
required for handover execution
User plane – packet forwarding
during handover
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Control plane
GTP-U: GPRS tunneling protocol
STCP: Stream Transmission Control Protocol
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User plane
Channel structure
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Channels – defined on Uu
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Logical channels
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Transport channels
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Formed by MAC
Characterized by how the data are
organized
Physical channels
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Formed by RLC
Characterized by type of information
Formed by PHY
Consist of a group of assignable radio
resource elements
Uu interface
Note: LTE defines same types of channels as WCDMA/HSPA
LTE - channel mapping
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Logical channels
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BCCH – Broadcast Control CH
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System information sent to all UEs
PCCH – Paging Control CH
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Paging information when addressing UE
CCCH – Common Control CH
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Access information during call establishment
DCCH – Dedicated Control CH
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User specific signaling and control
DTCH – Dedicated Traffic CH
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User data
MCCH – Multicast Control CH
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Signaling for multi-cast
MTCH – Multicast Traffic CH
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Multicast data
Red – common, green – shared, blue - dedicated
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LTE Channels
Transport channels
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BCH – Broadcast CH
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PCH – Paging CH
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Used for multicast transmission
UL-SCH – Uplink Shared CH
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Transport of user data and signaling.
Used by many logical channels
MCH – Multicast channel
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Transport for PCH
DL-SCH – Downlink Shared CH
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Transport for BCCH
Transport for user data and signaling
RACH – Random Access CH
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Used for UE’s accessing the network
Red – common, green – shared
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LTE Channels
PHY Channels
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PDSCH – Physical DL Shared CH
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Uni-cast transmission and paging
PBCH – Physical Broadcast CH
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Broadcast information necessary for accessing the network
PMCH – Physical Multicast Channel
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Data and signaling for multicast
PDCCH – Physical Downlink Control CH
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Carries mainly scheduling information
PHICH – Physical Hybrid ARQ Indicator
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Reports status of Hybrid ARQ
PCIFIC – Physical Control Format Indicator
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Information required by UE so that PDSCH can be
demodulated (format of PDSCH)
PUSCH – Physical Uplink Shared Channel
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Uplink user data and signaling
PUCCH – Physical Uplink Control Channel
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Reports Hybrid ARQ acknowledgements
PRACH – Physical Random Access Channel
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Used for random access
Red – common, green – shared
LTE Channels
Time domain structure
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Two time domain structures
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Type 1: used for FDD transmission (may be full duplex or half duplex)
Type 2: used for TDD transmission
Both Type 1 and Type 2 are based on 10ms radio frame
Radio frame :
Type 1
Radio frame :
Type 2
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TDD frame configurations
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Different configurations
allow balancing between
DL and UL capacity
Allocation is semi-static
Adjacent cells have same
allocation
Transition DL->UL
happens in the second
subframe of each halfframe
Note: TDD frame
structure allows coexistence between LTE
TDD and TD-SCDMA
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Allocatable resources
• LTE – radio resource = “time-frequency chunk”
Resource Block (RB) = 12
carriers in one TS
(12*15KHz x 0.5ms)
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Time domain
 1 frame = 10 sub-frames
 1 subframe = 2 slots
 1 slot = 7 (or 6) OFDM
symbols
Frequency domain
 1 OFDM carrier = 15KHz
Note: In LTE resource management is along
three dimensions: Time, Frequency, Code
Bandwidth flexibility
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LTE supports deployment from 6RBs to 110 RBs in 1 RB increments
6RBs = 6 x 12 x 15KHz = 1080KHz -> 1.4MHz (with guard band)
110RBs = 110 X 12 X 15KHz = 19800KHz -> 20MHz (with guard band)
Typical deployment channel bandwidths: 1.4, 3, 5, 10, 15, 20 MHz
Straight forward to support other channel bandwidths (due to OFDM)
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UE needs to support up to the largest bandwidth (i.e. 20MHz)
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UE States
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UE may be in three states
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Detached: not connected to the network
Idle: attached to the network but not active
Connected: attached and active
Note: Both the UE states and UE
tracking are simpler than in UMTS
UE tracking
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Detached state: UE position unknown
Idle state: UE position know with the Tracking Area (TA) resolution
Connected: UE location known to the eNodeB resolution
3GPP Specifications
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All 3GPP specs are available at http://www.3gpp.org
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RAN 1
RAN2
RAN3
RAN4
RAN5
36.2xx series
36.3xx series
36.4xx series
36.1xx series
36. 5xx series
Example specs organization
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PHY layer
Layers 2 and 3
S1 and X2 interfaces
Core performance requirements
Terminal conformance testing
Section review
1. What are 3GPP broadband
cellular technologies?
2. What releases of 3GPP standard
contains LTE?
3. What were target DL and UL
throughputs for LTE?
4. What does SAE stand for?
5. What are components of the CS
part of the LTE core network?
6. What is the access scheme used
on the DL?
7. What is the role of fast scheduler
on LTE DL?
8. What is the smallest allocateable
resource in LTE DL?
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9. What is Radio Block (RB)?
10. What are spectrum bandwidth
deployment options for LTE?
11. How many radio blocks are in
20MHz deployment?
12. Does LTE support TDD
deployment?
13. What are three UE States
supported by LTE?
Part 2
LTE RADIO ACCESS
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Overview
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Overview of OFDM/OFDMA
LTE Downlink transmission
Overview of DFTS-OFDM
LTE Uplink transmission
Multi-antenna transmission
Single carrier transmission
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Data are used to modulate amplitude/phase (frequency) of a single carrier
Higher data rate results in wider bandwidth
Over larger bandwidths ( > 20KHz), wireless channel is frequency selective
As a result of frequency selectivity the received signal is severely distorted
Channel equalization needed
Complexity of equalizer increases rapidly with the signal bandwidth requirements
Transmission of single
carrier in mobile
terrestrial environment
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Note: over small
portion of the signal
spectrum, fading
may be seen as flat
Multi-carrier transmission
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Channel fading over smaller frequency bands – flat (no need for equalizer)
Divide high rate input data stream into many low rate parallel streams
At the receiver – aggregate low data rate streams
Signal for each
stream experiences
flat fading
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FDM versus OFDM
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OFDMA minimizes separation between
carriers
Carriers are selected so that they are
orthogonal over symbol interval
Carrier orthogonality leads to frequency
domain spacing Df=1/T, where T is the
symbol time
In LTE carrier spacing is 15KHz and
useful part of the symbol is 66.7
microsec
Note: orthogonality between
carriers in time domain allows
closer spacing in frequency
domain.
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FDM versus OFDM
OFDM transmitter/receiver
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Practically OFDM TX/RX is implemented using IFFT/FFT
Use of the IFFT/FFT at the baseband means that there is no need for
separate oscillators for each of the OFDM carriers
FFT (IFFT) hardware is readily available – TX/RX implementation is simple
Guard time
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Duration of the OFDM symbol is chosen to be much longer than the multi-path
delay spread
Long symbols imply low rate on individual OFDM carriers
In multipath environment long symbol minimizes the effect of channel delay spread
To make sure that there is no ISI between OFDM symbols – guard time is inserted
OFDM symbols without guard time
OFDM symbols with guard time
Cyclic prefix
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Guard time eliminates ISI between OFDM symbols
Multipath propagation degrades orthogonality between carriers within an
OFDMS symbol
To regain the orthogonality between subcarriers – cyclic prefix is used
Cyclic prefix fills in the guard time between the OFDM symbols
Block diagram of full OFDM TX/RX
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LTE supports numerous AMC schemes
AMC adds additional level of adaptation to the RF channel
Size of CP depends on the amount of dispersion in the channel
Two CP are used: normal (4.7 us) and extended (16.7 us)
OFDMA time-frequency scheduling
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Minimum allocateable resource in
LTE is Resource Block pair
Resource block pair is 12 carriers
wide in frequency domain and lasts
for two time slots (1ms)
Depending on the length of cyclic
prefix RB pair may have 14 or 12
OFDM symbols
PHY channels consist of certain
number of allocated RB pairs
Overhead channels are typically in a
predetermined location in time
frequency domain
Within a RB different AMC scheme
may be used
Allocation of the radio block is done
by scheduler at eNode B
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Part 3
LTE DOWNLINK TRANSMISSION
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LTE OFDM
Parameter
Bandwidth (MHz)
Value
1.4
3
5
10
Frame /subframe
duration
10/1 ms
Subcarrier spacing
15KHz
Useful symbol part
66.7us
15
20
FFT size
128
256
512
1024
1536
2048
Resource blocks
6
15
25
50
75
100
Number of used
subcarriers
72
180
300
600
900
1200
Cyclic prefix length
Normal: 5.1us for first symbol in a slot and 4.7us for other symbols , Extended: 16.7us
OFDM symbols /slot
7 (normal CP), 6 (extended CP)
Error coding
1/3 convolutional (signaling); 1/3 turbo (data)
Basic timing unit: Ts = 1/(2048 x 15000) ~ 23.552 ns
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Detailed time domain structure
Need for two different CP:
1. To accommodate environments
with large channel dispersion
2. To accommodate MBSFN (MultiCast Broadcast Single Frequency
Network) transmission
In case of MBSFN it may be
beneficial to have mixture of
sub-frames with normal CP and
extended CP. Extended CP is
used for MBSFN sub-frames
TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for
other six symbols
TCP-e: 512 Ts (16.7 us) for all symbols
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Exercise – OFDM data rate
capability at the PHY
Case 1.
Case 2.
Normal CP (no MIMO)
Resource block: 12 carriers x 14 OFDM symbols = 168 resource elements
Each resource element carries one modulation symbol
For 64 QAM: 1 symbol = 6 bits
Number of bits per subframe = 168 x 6 = 1008 bits/subframe
Raw PHY data rate = 1008/1ms = 1,008,000 bits/sec/resource block (180KHz)
For 20MHz, Raw PHY data rate = 100 RB x 1,008,000 bits/sec/RB = 100.8Mbps
Extended CP (no MIMO)
Resource block: 12 carriers x 12 OFDM symbols = 144 resource elements
Each resource element carries one modulation symbol
For 64 QAM: 1 symbol = 6 bits
Number of bits per subframe = 144 x 6 = 864 bits/subframe
Raw PHY data rate = 864/1ms = 864,000 bits/sec/resource block (180KHz)
For 20MHz, Raw PHY data rate = 100 RB x 864,000 bits/sec/RB = 86.4Mbps
Note: with the use of MIMO the rates are increased
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Downlink reference signals
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For coherent demodulation – terminal needs channel estimate for each subcarrier
Reference signals – used for channel estimation
There are three type of reference signals
1. Cell specific DL reference signals
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Every DL subframe
Across entire DL bandwidth
2. UE specific DL reference signals
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Sent only on DL-SCH
Intended for individual UE’s
3. MBSFN reference signals
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Support multicast/broadcast
Note: Reference signals are staggered in time
and frequency. This allows UE to perform 2-D
complex interpolation of channel timefrequency response
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Cell specific reference signals
Two port TX
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DL transmission may use up to four antennas
Each antenna port has its own pattern of reference
signals
Reference signals are transmitted at higher power in
multi-antenna case
Reference signals introduce overhead
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4.8% for 1 antenna port
9.5% for 2 antenna ports
14.3 % for 4 antenna ports
Four port TX
Reference symbols vary from position to position and
from cell to cell – cell specific 2 dimensional sequence
Period of the sequence is one frame
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One port TX
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Cell specific reference signals (2)
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There are 504 different Reference Sequences (RS)
They are linked to PHY-layer cell identities
The sequence may be shifted in frequency domain – 6 possible shifts
Each shift is associated with 84 different cell identities (6 x 84 = 504)
Shifts are introduced to avoid collision between RS of adjacent cells
In case of multiple antenna ports – only three shifts are useful
For a given PHY Cell ID - sequence is the same regardless of the bandwidth used –
UE can demodulate middle RBs in the same way for all channel bandwidths
Shifts for
single port
transmission
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UE Specific RS
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UE specific RS – used for beam forming
Provided in addition to cell specific RS
Sent over resource block allocated for DL-SCH (applicable only
for data transmission)
Note: additional reference signals
increase overhead. One of the most
beneficial use of beam forming is at the
cell edge – improves SNR
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PHY channels supporting DL TX
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SCH – allows mobile to synchronize
to the DL TX during acquisition
PBCH – used to broadcast static
portion of the BCCH
PDSCH – carries user information
and signaling from upper layers of
protocol stack
PDCCH – channel used by MAC
scheduler to configure L1/L2 and
assign resources (DL scheduling
and UL grants)
PCFICH – explains to the UE the
format of the DL transmission
PHICH – support for HARQ on the
uplink
PUCCH – support for HARQ on the
downlink
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Channels required for DL
transmission
Summary of PHY DL channels
L1/L2 signaling
L1/L2 Control
Coding scheme
PHY Channel
Modulation
CFI (Channel format
Indicator)
Block code R=1/16
PCFICH
QPSK
HI (HARQ information)
Repetition 1/3
PHICH
BPSK
DCI (Downlink control
Information)
Convolutional 1/3 with
rate matching
PDCCH
QPSK
Services to upper layers
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Transport channel
Coding scheme
PHY Channel
Modulation
DL-SCH
Turbo 1/3
PDSCH
QPSK, 16-QAM, 64-QAM
BCH
Convolutional 1/3
PBCH
QPSK
PCH
Turbo 1/3
PDSCH
QPSK
MCH
Turbo 1/3
PMCH
QPSK, 16-QAM, 64-QAM
Downlink L1/L2 signaling
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Signaling that supports DL transmission
Originates at L1/L2 (no higher layer data
or messaging)
Consists of
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Scheduling assignments and associated
information required for demodulation and
decoding of DL-SCH
Uplink scheduling grants for UL-SCH
HARQ acknowledgements
Power control commands
L1/L2 signaling is transmitting in first 1-3
symbols of a subframe – control region
Size of control region may vary
dynamically – always whole number of
OFDM symbols (1,2,3)
Signaling – beginning of the subframe
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Reduces delay for scheduled mobiles
Improves power consumption for non-scheduled
mobiles
Three different PHY channel types
1.
2.
3.
PCFIC (PHY Control Format Indicator Channel)
PHICH (PHY – Hybrid ARQ Channel)
PDCCH (PHY Downlink Control Channel)
PCFICH
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PCFICH – PHY Channel Format Indicator Channel
Indicates to UE the size of the control region (1,2 or 3 OFDM symbols)
PCFICH value may be 1, 2 or 3 (0 is reserved for future use)
Decoding of PCFICH is essential for UE operation
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Encoded with 1/16 repetition code
Uses QPSK modulation
Mapped to the first symbol of each subframe
16 resource elements in 4 groups of 4 (RE Groups)
Location of the resource elements depends on cell identity
Processing of PCFICH
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Note: REGs of the PCFICH are spread in frequency
domain to achieve frequency diversity
PHICH
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PHICH = PHY Hybrid-ARQ Indicator Channel
HARQ acknowledgements for UL-SCH transmission
As many PHICH channels as the number of UEs in the cell
A set of PHICH channels is multiplexed on the same resource elements (8 normal
CP, 4 extended CP)
Transmitted in the first OFDM symbol of the subframe
Occupies 3 resource element groups (REGs) = 12 resource elements (RE)
PHICH response comes 4 sub-frames after PU-SCH
Processing of
PHICH
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PDCCH
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PDCCH = Physical Downlink Control Channel
Used for
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•
•
53
DL scheduling assignments
UL scheduling grants
Power control commands
PDCCH message occupies 1,2,4 or 8 Control Channel Elements (CCEs)
CCE = 9 Resource Element groups (REGs) = 36 Resource Elements (REs)
One PDCCH carrier one message with a specific Downlink Control Information (DCI)
Multiple UE-s scheduled simultaneously -> Multiple PDCCH transmissions in a subframe
PDCCH DCIs
•
•
54
PDCCH carrier Downlink Control Information (DCI)
Multiple DCI formats are defined based on type of information
DCI formats of PDCCH
Format
Purpose
Content
# of bits (FDD)
0
UL PUSCH grant
RB assignment, MCS, hopping flag, NDI, cyclic shift of DM-RS, CQI, …
44
1
DL PDSCH grant for single code
word
Resource allocation header, RB allocation, MCS, HARQ, HARQ PID,
…
55
1A
Compact DL PDSCH grant of single
code word
Similar to format 1, but with smaller flexibility
44
1A
RACH initiated by PDCCH order
Localized/distributed VRB assignment flag, preamble index, PRACH
message mask index
44
1B
Compact DL PDSCH grant with
pre-coding information
Similar to 1, but with distributed VRB flag, reduced RB allocation
flexibility, transmit PMI and pre-coding
49
1C
Very compact DL PDSCH grant
Reduced payload for improved coverage, always uses QPSK on
associated PDSCH, restricted RB assignment, No HARQ, …
31
1D
Compact DL PDSCH grant with
pre-coding information and
power offset
Same as 1, but with reduced RB allocation flexibility and addition of
distributed VRB transmission flag. Transmit PMI information for
pre-coding, DL power offset
49
2
MIMO DL grant
Same as 1, but for MIMO transmission
76
2A
Compact MIMO DL grant
Same as 1A, but for MIMO transmission
68
3
2-bit UL power control
TPC for 14 UEs plus 16 bit CRC
44
3A
1-bit UL power control
TPC for 28 UEs plus 16 bit CRC
44
PDSCH
•
•
•
•
•
DL-SCH = DL Shared channel
Used for user data coming from upper
layers (both signaling and payload)
Optimized for low latency and high data rate
Individual steps in the processing chain
operate on data blocks – enables parallel
processing
Many different adaptation modes
–
–
–
–
55
Modulation
Coding
Transport block size
Antenna mapping (TX diversity, beam forming,
spatial multiplexing)
Time/Frequency location of PBCH
and SS - FDD
•
PBCH = Physical Broadcast
Channel
–
•
SS = Synchronization Signal
–
–
–
56
Used for BCH transport channel
Note: PBCH and SS use innermost part of the
spectrum. This way the system acquisition is the same
regardless of deployed bandwidth
P-SS = Primary Synchronization
Signal
S-SS = Secondary
Synchronization Signal
SS are used only on Layer 1 – for
system acquisition and Layer 1
cell identity
Time/Frequency location of PBCH
and SS - TDD
•
PBCH = Physical Broadcast
Channel
–
•
SS = Synchronization Signal
–
–
–
57
Note: The position of the P-SS is different in TDD and
FDD. By acquiring P-SS, the UE already knows if the
system is FDD or TDD.
Used for BCH transport channel
P-SS = Primary Synchronization
Signal
S-SS = Secondary
Synchronization Signal
SS are used only on Layer 1 – for
system acquisition and Layer 1
cell identity
Synchronization Channel (SCH)
•
•
•
•
•
SCH – first channel acquired by UE
Based on SCH, UE determines eNode B PHY cell identity
504 possible PHY layer cell IDs
168 groups with 3 identities per group
SCH consist of 2 signals
–
–
•
•
•
PSS (Primary Synchronization Signal)
SSS (Secondary Synchronization Signal)
3 possible PSS sequences: NID(2) = 0,1, 2
168 possible SSS sequences: NID(1) = 0,1, …, 167
Cell ID: NIDcell = 3* NID(1) + NID(2)
For FDD (frame type 1)
• PSS is transmitted on OFDM symbol 7 in the first time slot of subframe 0 and 5
• SSS is transmitted on OFDM symbol 6 in the first time slot of subframe 0 and 5
For TDD (frame type 2)
• PSS is transmitted on OFDM symbol 3 in the first time slot of subframe 1 and 6
• SSS is transmitted on OFDM symbol 6 in the first time slot of subframe 0 and 5
58
PBCH
•
•
•
•
•
•
PBCH = PHY Broadcast Channel
PBCH provides PHY channel for static part
of Broadcast Control Channel (BCCH)
BCCH carriers RRC System Information
(SI) messages
SI messages carry System Information
Blocks (SIBs)
SI-M is a special SI that carrier Master
Information Block (MIB)
In LTE BCCH is split into two parts
–
–
59
Primary broadcast: Carriers MIB and provides UE
with fast access to vital system broadcast
information. Primary broadcast is mapped to PBCH
Dynamic broadcast: Carries all SIBs that contain
quasi-static information on system operating
parameters. Dynamic broadcast is mapped to
PDSCH
Mapping of the BCCH
information
PCH
•
•
•
PCH = Paging Channel
Transmitted over PDSCH (messages), PDCCH (paging indicator)
LTE support DRX (UE sleeps between paging occasions)
–
–
–
–
•
LTE defines DRX cycle
UE is assigned to P-RNTI (Paging – Radio Network Temporary Identifier)
P-RNTI is set on PDCCH
UE that finds set P-RNTI reads PCH on PDSCH to determine if it is being paged
DRX cycle compromise
–
–
Long cycle: good battery life, higher paging delay
Short cycle: faster paging response, shorter UE battery life
DRX and
paging
60
Mapping
of PCCH
Section review
1. Explain the main idea behind
OFDM?
2. How is OFDMA different from
FDMA?
3. What is the role of cyclic prefix
(CP) in OFDM?
4. What are DL reference signals?
5. How are cell specific reference
signals linked to cell’s physical
identity?
6. What is the role of PCFICH?
7. What is the role of PHICH?
8. What is the channel used for user
data and higher layer signaling?
61
9. What is SCH?
10. What portion of the time-frequency
resources is occupied by SCH?
11. What is the duration of LTE
frame?
12. How many subframe are in LTE
frame?
13. What is the time duration of one
LTE time slot?
DFTS-OFDM
•
•
•
•
DFTS-OFDM = DFT Spread OFDM
Also known as s Single Carrier FDMA (SC-FDMA)
Used on RL of LTE
Advantages:
–
–
–
•
Lower PAPR than OFDM (4dB for QPSK and 2dB for 16-QAM)
Orthogonality between the users in the same cell
Low complexity TX/RX due to DFT/FFT
Disadvantage:
–
–
Needs an equalizer at the Node B RX
Need for some synchronization in time domain
Outline of the
DFTS-OFDM
62
Note: In DFTS-OFDM, M < N
DFTS-OFDM TX/RX chain
Note: the TX/RX of DFTS-OFDM is almost the same as OFDM. The DFT precoding / decoding and equalization are done in software
63
Uplink user multiplexing
•
Two ways of mapping the output of the DFT
– Consecutive carriers: Localized DTFS-OFDM
– Distributed carriers: Distributed DTFS-OFDM
•
Distributed OFDM has benefit of frequency diversity
Note 1: Mapping between
output of the OFDM and
carriers is performed by
MAC scheduler
Note 2: Spectrum bandwidth
may be allocated in dynamic
fashion
Localized DFTS-OFDM
64
Distributed DFTS-OFDM
Uplink frame format
Need for two different CP:
1. To accommodate environments
with large channel dispersion
2. To accommodate MBSFN (MultiCast Broadcast Single Frequency
Network) transmission
Note: UL and DL frame formats
are identical
TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for
other six symbols
TCP-e: 512 Ts (16.7 us) for all symbols
65
PHY channels supporting UL TX
•
•
•
•
•
PRACH – initial random access and
UL timing alignment
PUSCH – channel used for
transmission of user data and upper
layer signaling
PUCCH – uplink control channel
used for scheduling requests for
synchronized UEs
PDCCH – uplink scheduling grants
PHICH – HARQ feedback channel
supporting UL transmission
66
Uplink reference signals (1)
•
•
Used for uplink channel estimation
Two types of sequences
–
–
•
Data demodulation Reference Signal (DM-RS)
Sounding Reference Signal (SRS)
DM-RS
–
–
–
–
Sent on each slot transmission to help
demodulate data
Occupies center part of the slot transmission
(symbols 4) in both transmission slots
Use same bandwidth as the UL data (multiples
of 12 carrier RBs)
Properties of DM-RS sequences
•
•
67
Small power variations in frequency domain
Small power variations in time domain
Uplink reference signals (2)
•
SRS
–
–
–
–
–
Allow network to estimate channel quality
across entire band
Used by MAC scheduler to perform
frequency dependent scheduling
Optional implementation
UE can be configured to send SRS
sequence at time intervals from 2ms to
160ms
Two modes of operation
•
•
68
Wideband SRS – UE send the sequence across
the entire spectrum
Hopping SRS – UE sends narrowband
sequence that hops across different parts of the
spectrum
PUSCH
•
•
•
•
•
•
•
•
PUSCH = PHY Shared channel
PUSCH carries UL-SCH (user data/higher layer
signaling)
During data transmission L1/L2 signaling also
mapped o PUSCH – preserve single carrier TX
Resources allocated to the UE on per subframe
basis
Allocation is done in PRB (12 carriers by 1 ms)
Modulation used may be QPSK, 16-QAM or
64-QAM (optional)
Allocated PRBs may be hopped from subframe
to subframe
Two modes of hopping
–
–
•
Intra subframe and inter subframe
Only inter subframe
Hopping may be on the basis of explicit grants
from Node B or following predefined cellspecific mirroring patterns
69
Example: 2 UE’s, 10MHz (50 RB)
Note: Frequency hopping provides
frequency diversity and interference
averaging for the UL transmission
PUCCH
•
•
PUCCH = PHY Uplink Control Channel
Used for L1/L2 signaling
–
–
–
•
•
Used only when there is no scheduled
PUSCH transmission (single carrier TX)
Uses PRBs at the very end of the
allocated channel bandwidth
–
–
•
•
Scheduling request
ACK/NACK/DTX for DL-SCH transmission
Feedback on DL channel quality (CQI/PMI/RI)
Increases frequency diversity
Allows scheduling of larger resource “chunks” for
uplink transmission
Number of PRBs is configured by the
network in a semi-static manner
Bandwidth of a single resource block in a
subframe is shared by several UE’s
–
–
70
Economical use of allocated resources
Reduces signaling overhead
Note: PUCCH performs frequency
hopping between two slots of a
subframe
PUCCH formats
PUCCH format
Modulation
Purpose
Bits/subframe
1
On/off keying
Scheduling requests
N/A
1a
BPSK
ACK/NACK for SIMO
1
1b
QPSK
ACK/NACK for MIMO
2
2
QPSK
CQI/PMI/RI
20
2a
QPSK+BPSK
CQI/PMI/RI+ACK/NACK for
SIMO
21
2b
QPSK+QPSK
CQI/PMI/RI+ACK/NACK for
MIMO
22
Note 1: There are 2 formats: Format 1 (1, 1a and 1b) and Format 2 (2, 2a and 2b)
Note 2: PUCCH power offset depends on the PUCCH format
71
PUCCH – Format 1
•
•
Small in size (1 or 2 bits)
Used for
–
–
•
DL HARQ ACK/NACK for MIMO/SIMO
Scheduling request
•
By using different cyclic shifts and
different covers sequences, multiple
users may be multiplexed on the same
PUCCH resource
Typically there are 6 shifts and 3 cover
sequences – 18 UE’s per PUCHH
resource
Note: Format 1 is
repeated in two
corresponding slots
in the subframe
72
PUCCH – Format 2
•
Larger in size (20, 21 or 22 bits)
−
−
•
Used for
–
–
–
•
•
10 bits for CQI report
2 bits for ACK/NACK
DL HARQ ACK/NACK for MIMO/SIMO
Scheduling request
CQI/PMI and RI information
By using different cyclic shifts of
the CAZAC sequence multiple
UE’s may be multiplexed on one
PUCCH resource
Format 1 and 2 share the same
basic format
Note: for Format 2, both CQI report
and ACK/NACK information are sent
73
Processing of CQI
report
PRACH
•
•
•
•
•
•
•
PRACH = PHY Random Access Channel
Physical channel used in support of random
access
In LTE initial access is handled only on PHY, all
the signaling is sent through UL-SCH (PUSCH)
PRACH carries one of 64 preambles
Available preambles are signaled in SIB-2
UE selects a preamble based on the amount of
data it needs to send on UL-SCH (this way Node
B knows how to reserve resources)
PRACH preamble is sent over PRACH time
frequency resource
–
–
–
–
74
Occupies middle 1.08MHz of spectrum
Same spectrum regardless of total LTE bandwidth
PRACH access subframe may occur every 1, 2, 5, 10 or
20 ms (20 ms – optional, only in synchronized networks)
Subframe allowed for access – signaled on SIB-2,
paremeter PRACH_Configuration index
UL time frequency resources
for PRACH
Section review
1. Why is OFDM not suitable for UL
transmission?
2. What is PAPR?
3. What is DFTS-OFDM?
4. What are two types of UL
reference signals?
5. Why is there need for sounding
reference signals?
6. How often can a mobile
configured to send SRS signals?
7. What is PUSCH?
8. What is PUCCH?
9. What are PUCCH formats?
75
10. What information is carried on
PUCCH?
11. What is PRACH?
12. How does UE learn what
preamble sequences are available
for PRACH?
Part 3
MULTIPLE ANTENNA TECHNIQUES
76
Multi antenna configuration
•
•
LTE uses of multiple antennas at
both communication ends
LTE standard requires support for
–
–
•
TX diversity
Beam forming or SDMA
Spatial multiplexing
Uplink MIMO
–
77
Reception/transmission diversity
Beam forming
Spatial multiplexing (MIMO antenna
processing)
Downlink MIMO
–
–
–
•
4 antennas at the eNodeB
2 antennas at the UE
Multiple antennas may be used in
three principle ways
–
–
–
•
Downlink MIMO
Multi user MIMO (SDMA)
Uplink MIMO
Note: UL MU MIMO
avoids use of multiple
PAs at the UE
DL transmit diversity
•
Two implementations
–
–
•
CDD
–
–
–
•
Cyclic Delay Diversity (CDD)
Space-Time Transmit Diversity (STTD)
Multiple antenna elements are used to
introduce additional versions of the
signal that are cyclically delayed
UE perceives these signals as
additional multi-paths
Assuming low correlations between TX
antennas –created “multi-paths” fade
independently – source of diversity
CDD TX diversity
STTD
–
–
–
Uses Space-Frequency Block Codes
Special encoding (SFBC) makes the
channel matrix unitary (full rank)
Reference symbols are used to
estimate and invert channel matrix
SFBC TX diversity
78
TX Diversity - CDD
•
•
•
OFDM is robust with respect
to multi-path propagation
(within CP interval)
CDD simulates multi-path
propagation
No modification in RX signal
processing – UE ‘sees’ single
antenna transmission in
dispersive environment
Note: Extension of CDD to more
than 2 antennas is straightforward.
Each antenna has its own cyclic
delay.
79
Processing in case of 2 antenna CDD TX diversity
TX Diversity – 2 TX SFBC
•
Data sent to different antenna are encoded using SFBC
–
–
2 symbols at the time for 2 antennas TX diversity
Open loop
SFBC in case of 2 TX
diversity
 r2 n   h1
r   h*
 2 n 1   2
sˆn 
80
 h2   a2 n 
 Hs n

*  *
h1  a2 n 1 
1
h1  h2
2
2
 h1*
 *
 h2
h2   r2 n 


h1  r2 n 1 
Note 1: UE needs to have good estimate of
the channel – estimate obtained using PHY
reference sequences
TX Diversity – 4 TX SFBC
•
Data sent to different antenna are encoded using SFBC
–
–
–
4 symbols at the time for 4 antennas TX diversity
TX diversity operates on a resource element group (REG)
Open loop
SFBC in case of 4 TX
diversity
Note 1: 4 TX SFBC diversity may be seen as
two 2 TX SFBC diversity transmissions
multiplexed in time
81
Spatial multiplexing
•
Capacity benefit of SM MIMO
Basic idea: fading channel
provides uncorrelated parallel
paths for data transmission
 N S
C
 N L log 2 1  R  
BW
 NL N 
N L  min NT , N R 
Example: 2 by 2
h  s  n 
h
r   11 12   1    1 
h21 h22  s2  n2 
82
 sˆ1 
ˆ 1r   s1   Hˆ 1n

Wr

H
sˆ 
s 
 2
 2
Spectral efficiency (bps/Hz)
NT
NR
- number of TX antennas
- number of RX antennas
12.00
10.00
8.00
6.00
C/W (1,1)
C/W (1,2)
4.00
C/W (2,2)
2.00
0.00
0
5
10
S/N (dB)
15
Spatial multiplexing in LTE
•
Two types
– Open loop (used high speed scenarios)
•
Large delay Cyclic Delay Diversity (CDD)
– Closed loop (used in low speed scenarios)
•
Mobile provides channel feedback to eNode B
Feedback
Closed loop spatial multiplexing
Open loop spatial multiplexing
PMI (Pre-coded
matrix indicator)
PMI feedback from UE based on instantaneous
channel state
No feedback from UE. Fixed pre-coding at eNode
B implementing cyclic delay diversity (CDD)
CQI (Channel quality
indicator)
Separate CQI for each code word
Aggregate CQI (one value)
RI (Rank indicator)
Based on the rank of estimated channel matrix
(indicates number of spatial channels)
Based on the rank of estimated channel matrix
when SFBCs are used
83
Closed loop
spatial
multiplexing
Code word – layer mapping
•
•
LTE uses either 1 or 2 code words
Code words are mapped onto layers
–
–
•
Number of modulation symbols in
each layer is the same
–
•
1 layer for 1 codeword
2, 3 or 4 layers for 2 code words
Mapping between code-words and
layers
Accomplished through numerous transportblock formats and sizes
Through a pre-coding matrix the
layers are mapped onto the antennas
–
–
–
84
There is a set of pre-defined pre-coded
matrices
Through PMI, UE recommends to eNodeB
which pre-coded matrix to use
eNodeB may not follow UE’s
recommendation – informs UE about precoding matrix through explicit signaling
Note: layers are mapped to
antennas one symbol at the time
Antenna configurations
85
Transmission
modes
Description
Comments
1
Single antenna (Port 0)
Used for SISO and SIMO transmission
2
Transmit diversity
Used in low SNR and high mobility
3
Open loop spatial multiplexing
(large delay CDD)
Beneficial in high SNR and rich
multipath environment
4
Closed loop spatial multiplexing
(Rank 2, 3 or4)
Beneficial in high SNR and rich
multipath environment
5
Multi-user MIMO
Beneficial in high SNR environment for
interference reduction
6
Closed loop Rank = 1
Beneficial in low SNR environments
7
Single antenna port (Port 5)
Used for beam forming of antenna
arrays
SIMO/MIMO mode selection
Note: Detection of the environment
type and best use of MIMO/SIMO is
one of the tasks for scheduler –
major differentiating factor between
different equipment vendors
86
Section review
1.
2.
3.
4.
5.
6.
7.
What is MIMO?
What is receive diversity?
What is transmit diversity?
What is beam forming?
What is SDMA?
What is spatial multiplexing?
How much is capacity of link
increased using spatial
multiplexing?
8. What is CQI?
9. What is RI?
10. How is RI used by the scheduler?
87
11. What is the main idea behind
SFBC?
12. What is CDD?
13. Explain the main idea behind
CDD?