Lte signaling

An Introduction to

Cellular Communications Signaling

LTE / LTE-A / 4G

Electrical Engineering Department

Telecommunications System Department

Cognitive Radio Laboratory

Shahid Beheshti University

Instructor : M. Naslcheraghi

Advisor : Dr. Ghorashi

Advanced Computer Networks

Lecturer : Dr. Abbaspour

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Contents

LTE Architecture

LTE Physical Layer Overview

OFDMA Modulation

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

• 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)

SERVING GATEWAY (S-GW)

• 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

HOME SUBSCRIBER SERVER (HSS)

• 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)

PDN GATEWAY (P-GW)


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 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 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

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……

OFDMA Time-Freq Multiplexing

Orthogonal Frequency Division Multiple Access


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


Physical Channel

Structure

Uplink

– PUCCH: Transmit ACK/NACK, CQI, SR

– PUSCH: Transmit Data

– PRACH: Transmit Random Access Preamble

– SRS: For UL CQI measurement


• 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-CGTP-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.

https://en.wikipedia.org/wiki/GPRS_core_network

https://en.wikipedia.org/wiki/GPRS_core_network#Gateway_GPRS_support_node_.28GGSN.29

https://en.wikipedia.org/wiki/GPRS_core_network#Serving_GPRS_support_node_.28SGSN.29

https://en.wikipedia.org/wiki/GPRS_core_network#PDP_context

https://en.wikipedia.org/wiki/Quality_of_service

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 in LTE can be divided into

three phases:

Handover preparation

handover execution

handover completion

Handover Procedure


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)


RSRPimportant 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.


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


Source eNodeBTarget eNodeBX2AP Handover Request

• eNodeB decides to initiate an X2handover based on:• UE reported RRC downlink signal quality measurements• Uplink signal quality measured at the eNodeB

• eNodeB picks the target cell id forthe handover.• X2 handover is initiated if and Ifthe target cell is served by thesame MME as the current cell

• The message includes UE contextinformation that identifies the UEon the S1AP interface.• Security parameters are also included in the message

• Information about the radiobearers is included in the message.The per RAB information includes• QoS parameters• GTP Tunnel Information

• The message also includes RRCcontext information


• 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 messagesent via a transparent container.• The source eNodeB send this message to the UE.

Target eNodeBSource eNodeBX2AP Handover RequestAcknowledge• The target eNodeB receivesperforms admission control onreceipt of the Handover Request.• The target eNodeB responds withX2AP Handover RequestAcknowledge.• Information about the acceptedRABs is included in the message.


Source eNodeBTarget eNodeBX2AP SN Transfer Status• The source eNodeB now sends theSN Transfer Status• The following fields are present foreach RAB• The uplink PDCP sequence number• Uplink Hyper Frame Number• The downlink PDCP sequence number• Downlink Hyper Frame Number

• These fields are needed forcontinuing ciphering and integrityprotection after the handover.


Target eNodeB MMES1AP 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.


• The MME requests the SGW to

switch the path to the target

eNodeB.

MME SGWModify Bearer Request

The S1-U TEID received from the

target eNodeB is passed to the

SGW.


• SGW updates the bearer and responds back

SGW MMEModify Bearer Response


S1AP: MME Target eNodeBS1AP Path SwitchAcknowledge

• 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.


Target eNodeBSource eNodeBX2AP UE Context Release• Sent when the target eNodeB hassuccessfully completed the pathswitching and radio signaling for

the handover.


UE-NAS MME-NASTracking Area UpdateRequest• Sent if the just completed

handover resulted in a Tracking Area Update

MME-NAS UE-NASTracking Area UpdateAccept• Sent if the just completedhandover resulted in a Tracking

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

Lte signaling

Engineering

Transcript of Lte signaling