Electrical Engineering Department
Telecommunications System Department
Cognitive Radio Laboratory
Shahid Beheshti University
An Introduction to
Cellular Communications Signaling
LTE / LTE-A / 4G
Advanced Computer Networks
Lecturer : Dr. Abbaspour
Instructor : M. Naslcheraghi
Advisor : Dr. Ghorashi
Contents
1
LTE Architecture
2
LTE Physical Layer Overview
3
OFDMA Modulation
4
Hand-over Procedures
5
Conclusions And Research Aspects
LTE Architecture
EVOLVED NODE B (ENB) FUNCTIONS
• Radio resource management: radio bearer control, radio admission control,
connection mobility control, uplink/downlink scheduling
• IP header compression and ciphering of user data stream
• Mobility management entity (MME) selection
• Forwarding uplink data to serving gateway
• Paging
• Scheduling and transmission of broadcast information, originated from the
mobility management entity (MME) or operations and maintenance (O&M)
• Measurement and measurement reporting configuration for mobility and scheduling
• Scheduling and transmission of Earthquake and Tsunami Warning System
(ETWS) messages, originated from the MME
Mobility Management Entity (MME)
• Non-Access Stratum (NAS) signaling (attachment, bearer setup/deletion)
• NAS signaling security
• Signaling for mobility between 3GPP access networks (S3)
• Idle mode user equipment reachability
• Tracking Area list management
• PDN gateway and serving gateway selection
• MME selection for handoffs with MME change
• Roaming - S6a to home subscriber server (HSS)
• Authentication
• Bearer management functions including dedicated bearer establishment
• Support for Earthquake and Tsunami Warning System (ETWS)
message transmission
SERVING GATEWAY (S-GW)
• Local mobility anchor for inter-evolved Node B (eNB) handover
• Mobility anchor for inter-third-generation partnership project
(3GPP) mobility
• Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) idle-mode
downlink packet buffering and initiation of network triggered service
request procedure
• Lawful intercept
• Packet routing/forwarding
• Transport level packet marking (uplinking and downlinking)
• Accounting on user and Quality of Service (QoS) class identifier granularity
for inter-operator charging
• Uplink and downlink charging per user equipment, packet data node
(PDN), and QoS class identifier (for roaming with home routed traffic)
HOME SUBSCRIBER SERVER (HSS)
• Storage of subscriber data
• Enhanced Presence Service (EPS) QoS subscriber profile
• Roaming restrictions list
• Accessible Access Point Names (APNs)
• Address of current serving mobility management entity (MME)
• Current Tracking Area (TA) of user equipment (UE)
• Authentication vectors and security keys per UE
• Provide subscriber policies using Sp interface to PCRF
PDN GATEWAY (P-GW)
• PDN gateway
• Per-user packet filtering
• Lawful intercept
• User equipment (UE) IP address allocation
• Transport level packet marking for downlinking
• Uplink/downlink service level charging, gating, and rate enforcement
• Downlink rate enforcement based on aggregate maximum bit rate
(AMBR)
LTE Physical Layer Overview
LTE Requirements
 Peak bit (not data) rate
– 100 Mbps DL/ 50 Mbps UL within 20 MHz bandwidth (i.e., SISO)
 Up to 200 active users in a cell (5 MHz)
 Less than 5 ms user-plane latency condition (i.e., single user with single data
stream
 Mobility
– Optimized for 0 ~ 15 km/h
– 15 ~ 120 km/h supported with high performance
– Supported up to 350 km/h or even up to 500 km/h
 Enhanced multimedia broadcast multicast service (E-MBMS)
 Spectrum flexibility: 1.25 ~ 20 MHz
 Enhanced support for end-to-end QoS & QoE
LTE Physical Layer Overview
LTE Enabling Technologies
 OFDM (Orthogonal Frequency Division Multiplexing) for Down Link
• Frequency domain equalization
• SC-FDMA (Single Carrier FDMA) for Up Link
 Utilizes single carrier modulation and orthogonal frequency Multiplexing using
 DFT-spreading in the transmitter and frequency domain equalization in the
receiver
 A salient advantage of SC-FDMA over OFDM/OFDMA is low PAPR.
 Efficient transmitter and improved cell-edge performance
 MIMO (Multi-Input Multi-Output)
• e.g., Open loop, Close loop, Diversity, Spatial multiplexing
 Multicarrier channel-dependent resource scheduling
 Fractional frequency reuse
 Active interference avoidance and coordination
LTE Physical Layer Overview
 LTE Key Features
 Multiple access scheme
• DL: OFDMA with CP (Cyclic Prefix)
• UL: Single Carrier FDMA (SC-FDMA) with CP
 Adaptive modulation and coding
• DL/UL modulations: QPSK, 16QAM, and 64QAM
 Convolutional code and Rel-6 turbo code
 Advanced MIMO spatial multiplexing techniques
• (2 or 4)x(2 or 4) downlink and uplink supported
• Multi-user MIMO also supported
 Support for both FDD and TDD
 H-ARQ, mobility support, rate control, security, and etc...
Orthogonal Frequency Division Multiple Access
Orthogonal Frequency Division Multiple Access
Orthogonal Frequency Division Multiple Access
Some Mathematics Decisions That
Should be solved in One RB
One typical resource allocation
optimization problem….
One typical Spectrum Sharing
optimization problem….
Some Mathematics Decisions That
Should be solved in One RB
Another example of calculations of interference at subcarrier
Correlation between signals……
Orthogonal Frequency Division Multiple Access
OFDMA Time-Freq Multiplexing
LTE Physical Layer Overview
Physical Channel
Structure
Downlink
– PBCH: Transmit Broadcast channel
– PCFICH: Indicate PDCCH symbol
– PDCCH: Assign PDSCH/PUSCH
– PHICH: Indicate HARQ-ACK for UL
– PDSCH: Transmit Data
– PMCH: Transmit Multicast channel
– Synchronization Signal: UE synchronization
LTE Physical Layer Overview
Physical Channel
Structure
Uplink
– PUCCH: Transmit ACK/NACK, CQI, SR
– PUSCH: Transmit Data
– PRACH: Transmit Random Access Preamble
– SRS: For UL CQI measurement
LTE Physical Layer Overview
• Basic Procedure Between eNodeB and UE
The procedure for synchronization and obtaining of system Info
MIB : system frame number, DL bandwidth, PHICH information are included
SIB : Cell specific information are included for system operation except MIB
information
SIB1: cell access configuration, frequency band indicator, scheduling
information for system and other SIBs and systemInfoValueTag
SIB2: radio configuration information are included (PUCCH, PUSCH, SRS etc)
GTP and One Handover
Procedure in Detail
GTP Versions and GPRS Interfaces Overview
Main Purpose: The General Packet Radio Service (GPRS) tunneling protocol
(GTP) is used to tunnel GTP packets through 3G and 4G networks.
A Mobile Next Broadband Gateway configured as a gateway GPRS support node
(GGSN), Packet Data Network Gateway (P-GW), or GGSN/P-GW automatically selects
the appropriate GTP version based on the capabilities of the serving GPRS support node
(SGSN) or Serving Gateway (S-GW) to which it is connected.
GTP Versions and GPRS Interfaces Overview
GTP Versions and GPRS Interfaces Overview
GTP-C
GTP-C is used within the GPRS core network for signaling between gateway GPRS
support nodes (GGSN) and serving GPRS support nodes (SGSN). This allows the SGSN to
activate a session on a user's behalf (PDP context activation), to deactivate the same
session, to adjust quality of service parameters, or to update a session for a subscriber who
has just arrived from another SGSN.
Why is GTP used in LTE?
• It provides mobility. When UE is mobile, the IP address remains same and
packets are still forwarded since tunneling is provided between PGW and eNB
via SGW
• Multiple tunnels (bearers) can be used by same UE to obtain different
network QoS
• Main IP remains hidden so it provides security as well
• Creation, deletion and modification of tunnels in case of GTP-C
GTP Interfaces in LTE
In simple LTE network implementation, GTP-v2 is used on S5 and S11 interfaces and
GTPv1 is used on S1-U, S5, X2-U interfaces (as shown below). In inter-RAT and
inter PLMN connectivity, S3, S4, S8, S10, S12 and S16 interfaces also utilize GTP
protocols:
How GTP-U Works ?
GTP-C Signaling
Handover Procedure
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
Handover Procedure



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Handover procedure in LTE can be divided into
three phases:
Handover preparation
handover execution
handover completion
LTE Physical Layer Overview
Handover Measurements
Handover decisions based on the downlink channel
measurements which consist of :
1. Reference Signal Received Power (RSRP)
2. Reference Signal Received Quality (RSRQ)
LTE Physical Layer Overview
RSRP
important item UE has to
measure for cell selection,
reselection and handover.
one downlink radio frame. The
red part is the resource elements
in which reference signal is
being transmitted. RSRP is the
linear average of all the red part
power.
LTE Physical Layer Overview
RSRP
• UE usually measures RSRP or RSRQ based on the direction (RRC
message) from the network and report the value. When it report this value,
it does use the real RSRP value.
• It sends a non-negative value ranging from 0 to 97 and each of these values
are mapped to a specific range of real RSRP value as shown in the
following table from.
Intra-LTE (Intra-MME/SGW) Handover Using
the S1 Interface
Intra-LTE (Intra-MME/SGW) Handover
Using the X2 Interface
Inter-MME Handover Using the S1 Interface
(Without Changing S-GW)
LTE X2 Handover Sequence Diagram
LTE Architecture
LTE X2 Handover Sequence Diagram
Source eNodeB 
Target eNodeB
X2AP Handover Request
• eNodeB decides to initiate an X2
handover based on:
• UE reported RRC downlink signal quality
measurements
• Uplink signal quality measured at the
eNodeB
• eNodeB picks the target cell id for
the handover.
• X2 handover is initiated if and If
the target cell is served by the
same MME as the current cell
• The message includes UE context
information that identifies the UE
on the S1AP interface.
• Security parameters are also included in the message
• Information about the radio
bearers is included in the message.
The per RAB information includes
• QoS parameters
• GTP Tunnel Information
• The message also includes RRC
context information
LTE X2 Handover Sequence Diagram
Target eNodeB 
Source eNodeB
X2AP Handover Request
Acknowledge
• The target eNodeB receives
performs admission control on
receipt of the Handover Request.
• The target eNodeB responds with
X2AP Handover Request
Acknowledge.
• Information about the accepted
RABs is included in the message.
• The Uplink and Downlink GTP Tunnel
information is included for each RAB.
• The tunnel assignments are made at the
target to transport traffic during the
handover.
• A Handover Command message
sent via a transparent container.
• The source eNodeB send this message to
the UE.
LTE X2 Handover Sequence Diagram
Source eNodeB 
Target eNodeB
X2AP SN Transfer Status
• The source eNodeB now sends the
SN Transfer Status
• The following fields are present for
each RAB
• The uplink PDCP sequence number
• Uplink Hyper Frame Number
• The downlink PDCP sequence number
• Downlink Hyper Frame Number
• These fields are needed for
continuing ciphering and integrity
protection after the handover.
LTE X2 Handover Sequence Diagram
Target eNodeB  MME
S1AP Path Switch
Request
• The target eNodeB requests
switching of the S1-U GTP tunnel
towards the target eNodeB.
• The MME identifies the UE with
the “eNB to UE S1AP ID”
• The message includes the new cell
ID and the tracking area ID
• Security capabilities of the target
eNodeB are also included.
LTE X2 Handover Sequence Diagram
MME  SGW
Modify Bearer Request
• The MME requests the SGW to
switch the path to the target
eNodeB.
The S1-U TEID received from the
target eNodeB is passed to the
SGW.
LTE X2 Handover Sequence Diagram
SGW  MME
Modify Bearer Response
•
SGW updates the bearer and responds back
LTE X2 Handover Sequence Diagram
S1AP: MME Target eNodeB
S1AP Path Switch
Acknowledge
• The target eNodeB requests
switching of the S1-U GTP tunnel
towards the target eNodeB.
• The MME identifies the UE with
the “eNB to UE S1AP ID”
• The message includes the new cell
id and the tracking area id
• Security capabilities of the target
eNodeB are also included.
LTE X2 Handover Sequence Diagram
Target eNodeB 
Source eNodeB
X2AP UE Context Release
• Sent when the target eNodeB has
successfully completed the path
switching and radio signaling for
the handover.
LTE X2 Handover Sequence Diagram
UE-NAS  MME-NAS
Tracking Area Update
Request
MME-NAS  UE-NAS
Tracking Area Update
Accept
• Sent if the just completed
• Sent if the just completed
handover resulted in a Tracking
handover resulted in a Tracking Area Update
Area Update
Conclusions and Research Aspects
The main criteria for designing handovers are:
 Minimize the number of handover failures.
 Minimize the number of unnecessary handovers.
 Minimize the absolute number of initiated handovers.
 Minimize handover delay.
 Maximize the total time the user being connected to the best
cell.
 Minimize the impact of handover on system and
service
performance.
Thank You
for Your Attention