Signalling Analysis of LTE

HUAWEI TECHNOLOGIES CO., LTD.

www.huawei.com

Huawei Confidential

Security Level: INTERNAL 04/11/2023

LTE TDDSignaling Procedure and Analysis

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential Page 2

Upon the completion of this course,

you will be able to:

Recognize the LTE signaling

procedures. Understand the typical signaling

procedures of network access,

paging, and handover. Analyze basic problems.

Objective


Chapter 1 Network Access Signaling Procedure

Chapter 2 Handover Signaling Procedure

Chapter 3 Paging Signaling Procedure

Contents



1 Cell Searching

2 PLMN and Cell Selection

3 Attach

Contents


Cell Searching

Page 5

Cell searching, known as downlink (DL) synchronization, is a process to

resolve primary synchronization signals (PSS) and secondary synchronization

signals (SSS) on DL.

Power on

Timeslot layer-1 timer

ID

PSS monitoring

SSS monitoring

Frame timer, cell ID, CP monitoring,

TDD/FDD

PBCH decoding

MIB/ SIB

PLMN and cell selection PLMN/cell

Idle state

RSRP/RSRQRS monitoring

Neighboring-cell monitoring

Initial synchronization


Cell Searching

Page 6

In FDD mode, PSSs are on the last OFDM symbol of slot 0 and slot 10, and

SSSs on the second OFDM symbol from the end of slot 0 and slot 10.

In TDD mode, PSSs are on the second OFDM symbol from the end of slot

2 and slot 12, and SSSs on the last OFDM symbol of slot 1 and slot 11.

PSSs are the same for transmission from the two slots. Whereas, SSSs are

different for transmission from the two slots. PSS resolution triggers a timer

of 5 ms, and SSS resolution triggers a timer of 10 ms.

The cell ID on the physical layer can be obtained by using PSS resolution,

the cell group ID (504 cells are divided into 168 groups) can be obtained by

using the SSS resolution, and the physical cell identifiers (PCIs) of the

current cell (each cell group has three cell PCIs) can be obtained by using

both PSS resolution and SSS resolution. PCIs of the current cell = Cell

group ID x 3 + Cell ID.



1 Cell Searching


3 Attachment

Contents


PLMN and Cell Selection

Page 8

After cell searching is complete, the UE obtains the PCIs of the current cell to resolve

the master information blocks (MIBs) and system information blocks (SIBs) in the

current cell. By performing MIB resolution, the UE obtains key information, including

DL synchronization and system bandwidth of the cell, detects physical dedicated

control channel (PDCCH) in the SIB time domain, and then obtains the System

Information Block Type1 (SIB1) information based on the PDCCH before resolving

other SIB information. E-UTRAN

MasterInformationBlock

UE

SystemInformationBlockType1

SystemInformation


PLMN and Cell Selection: MIB

Page 9

The transmission period for MIBs is 40 ms. In a period, the physical broadcast channel

(PBCH) is located in subframe 0 of each radio frame and occupies the first four

symbols of slot 2 within each subframe 0. PSSs and SSSs are the same in the

frequency domain, occupying 1.08 MHz in the center, that is, the 6 RBs in the middle

of the frequency domain.


PLMN and Cell Selection: SIB1

Page 10

The transmission period for

SIB1 is 80 ms. The first

transmission is performed in

the radio frame numbered a

multiple of 8 within a TTI, and

repeated transmission is

performed on other even-

numbered radio frames in the

TTI. Transmission is fixed on

subframe 5 in the

corresponding frame. The UE

detects the PDCCH on these

subframes. If there is a system

information radio network

temporary identity (SI-RNTI),

the UE receives SIB1 on the

physical dedicated sharing

channel (PDSCH).


PLMN and Cell Selection: SIBn

Page 11

The transmission period for SIBn

is a multiple of 80 ms. SIB2

contains radio resource–related

configuration. SIBs 3 to 5 contain

the cell reselection configuration

of the LTE system. SIBs 6 to 8

contain the reselection

configuration of the UTRA,

GERAN, and CDMA2000

systems. SIB 9 contains the HNB

configuration. SIB 10 contains

the primary earthquake and

tsunami warning system (ETWS)

notification, and SIB 11 contains

the secondary ETWS notification.


PLMN and Cell Selection

Page 12

SIB1 contains the public land mobile network (PLMN) table. The

access stratum (AS) of the UE transmits the resolved PLMN

table to the non-access stratum (NAS) of the UE. Then, the

NAS selects an appropriate PLMN.

After selecting a PLMN, the UE selects an appropriate cell of

the PLMN to camp on a cell with the strongest signal.



1 Cell Searching


3 Attach

Contents


Attach: Attach

Page 14

UE eNB MMERRC connection Setup

RequestRRC connection Setup

RRC connection Setup Complete

Initial UE message

DL NAS Transfer

UL NAS Transfer

UL NAS Transfer

DL NAS Transfer

UL Information Transfer

DL Information Transfer


DL Information Transfer

Initial Context Setup RequestSecurity Mode Command

Security Mode Complete

UE Capability Enquiry

UE Capability Information UE Capability Information Indication

RRC connection Reconfiguration

RRC connection Reconfiguration Complete Initial Context Setup Response

UL NAS Transfer


Authentication

NAS Encryption

Radio Interface Encryption

UE capability reporting

NAS message: Attach Complete, Active Default

EPS Bearer Context

NAS message: Attach Accept, Active Default EPS

Bearer Context Request

NAS message: :Attach Request, PDN

Connectivity Request


Attach: RRC Connection Setup

Page 15


Attach: RRC Connection Setup Request

Page 16


Attach: RRC Connection Setup

Page 17


Attach: RRC Connection Setup Complete

Page 18

The UL dedicated control channel (DCCH) carries the UE registration information and NAS messages (Initial Direct Transfer messages).


Attach: Initial UE Message (I)

Page 19


Attach: Initial UE Message (II)

Page 20


Attach: DL/UL NAS/Information Transfer

Page 21

Uu tracing

S1 tracing


Attach: Initial Context Setup Request (I)

Page 22


Attach: Initial Context Setup Request (II)

Page 23

Message resolution


Attach: Initial Context Setup Request (III)

Page 24

Message resolution (continued)


Attach: RRC Security Mode Command/Complete

Page 25


Attach: UE Capability Enquiry/Information

Page 26

UE eNBUE Capabi l i ty Enqui ry

UE Capabi l i ty Infomation


Attach: RRC Connection Reconfiguration

Page 27

Used to establish signaling radio bearer (SRB) 2 and data radio bearer (DRB) 1.


Attach: Initial UE Context Setup Response

Page 28


Attach: Signaling over the Uu and S1 Interfaces

Page 29

Signaling over the Uu interface

Signaling over the S1 interface


Attach: IMSI-Based Entire-Network Signaling Tracing (I)

Page 30

IMSI Signaling Tracing for the Entire Ne


Attach: IMSI-Based Entire-Network Signaling Tracing (II)

Page 31


Attach: IMSI-Based Entire-Network Signaling Tracing (III)

Page 32



1 Introduction to a Handover

2 Intra-eNodeB Handover Between Cells

3 Inter-eNodeB Handover Between Cells

Contents


Introduction to Handovers Definition

A handover is implemented when a UE in the connected state

moves to a different cell to perform the UE context update. Objective

Coverage-based handover: to achieve continuity of services for

a moving UE Load-based handover: load-triggered handover; to ensure the

optimal performance of the whole system


Introduction to Handovers

Page 35

Three types of handovers in the LTE system


Introduction to Handovers: LTE System

Page 36

Handover is implemented in three phases: handover measurement, handover decision, and handover execution.

The whole handover procedure is performed based on UE-assisted network control. The eNodeB delivers measurement configuration to the UE, the UE reports the measurement results, and then the eNodeB performs handover decision, resource preparation, handover execution, and release of original resources.

In the handover measurement phase, the UE performs measurements according to the measurement configuration delivered by the eNodeB and reports measurement results to the eNodeB.

In the handover decision phase, the eNodeB checks the measurement results reported by the UE and determines whether to initiate a handover.

In the handover execution phase, the eNodeB controls the procedure of UE handover to the target cell based on the decision, to complete the handover.


Introduction to Handovers: Measurement Control

Page 37


Introduction to Handovers: Measurement Report

Page 38


Introduction to Handovers: Handover Command

Page 39






Contents


Intra-eNodeB Handover Between Cells

Page 41

Intra-eNodeB handover between cells is divided into intra-frequency handover and inter-frequency handover.

In the handover, only resources on the Uu interface are updated. The eNodeBs use internal messages to apply for and release resources of the source cell and the target cell.

Inter-eNodeB data transfer is unnecessary, so are the random access procedure of the UE and the signaling interaction with the evolved packet core (EPC).


Intra-eNodeB Handover Between Cells

Legend

UE Source Cell Target Cell Serving Gateway

Detach from old cell and synchronize to new cell

Buffer packets from MME

L3 signalling

L1/L2 signalling

User Data

Handove

r C

om

ple

tion

Ha

ndove

r E

xecu

tion

Han

dove

r P

repara

tion

MME/MMEs

Area Restriction Provided

1. Measurement Control

packet data packet data

UL allocation

2. Measurement Reports

DL allocation

4. Handover Command

5. Synchronization

6. UL allocation + TA for UE

7. Handover Confirm

eNB

packet data

3. HO decision

packet data

Flush DL buffer, continue delivering in transit packets

packet datapacket data


Measurement Reports

Handover Command

Handover Confirm

The UE reports the Measurement Report message in the source cell.

The eNodeB delivers a Handover Command message after completing cell admission control and radio resource allocation in the target cell.

The UE accesses the target cell.

After the handover is complete, resources of the source cell are released.


Intra-eNodeB Handover Between CellsUu tracing on the UE side

Uu tracing on the eNodeB side






Contents


Inter-eNodeB Handover over the X2 Interface

Page 45

If the target cell and source cell belong to two different eNodeBs over two different X2 links, an inter-eNodeB handover needs to be triggered over the X2 interface, and event A3 or A4 is reported. The prerequisite is that the X2 relationship has been configured between the two eNodeBs.

After receiving the measurement report from the UE and deciding a UE handover to the target eNodeB, the source eNodeB applies to the target eNodeB over the X2 interface for resources for the purpose of resource preparation in the target cell, and then informs the UE to trigger a handover to the target cell through the reconfiguration message over the radio interface. After the handover is successful, the target eNodeB informs the source eNodeB to release radio resources of the source cell. In addition, the source eNodeB transfers non-transmitted data to the target eNodeB and updates the node relationship on the user plane and control plane.

Handover


Inter-eNodeB Handover over the X2 Interface (I)

Legend


UL allocation


3. HO decision

4. Handover Request

5. Admission Control

6. Handover Request Ack

7. Handover Command

DL allocation

Data Forwarding

UE Source eNB Target eNB Serving Gateway


Deliver buffered and in transit packets to target eNB

Buffer packets from Source eNB

9. Synchronisation


L3 signalling

L1/L2 signalling

User Data


Han

dove

r E

xecu

tion

Han

dove

r P

repa

ratio

n

MME

0. Area Restriction Provided

SN Status Transfer8.

The UE has accessed to the cell and performs services.

The source eNodeB delivers measurement control information and notifies the UE of neighboring-cell measurement.

The UE returns the measurement report after detecting a cell that meets handover conditions.

The source eNodeB decides a handover based on the handover algorithm and current state.

Handover preparation: The source eNodeB sends a Handover Request message to the target eNodeB. The Handover Request message contains the service information and other access layer information (encryption, integrity, and measurement information) of the current UE.

The target eNodeB returns the admission result and radio resource configuration information to the source eNodeB, completing the handover preparation.

After receiving the Handover Request message from the source eNodeB, the target eNodeB performs admission control based on the contained service information and radio resource configuration.

The source eNodeB sends the container delivered by the target eNodeB to the UE through the radio message to inform the UE to trigger a handover.

Sequence number (SN) information is used to transfer the SN status and hyper frame number (HFN) information of the UE from the source eNodeB to the target eNodeB for the purpose of data retransmission and encryption integrity protection.

Data forwarding. For details, see subsequent description in this document.


Inter-eNodeB Handover over the X2 Interface (II)

After the UE complete the random access procedure to access the target eNodeB, the UE sends a Handover Confirm message to the target eNodeB.

After receiving the Handover Confirm message from the UE, the target eNodeB initiates a path switch to the MME to complete the user data transmission.

After completing the user data transmission, the MME sends a Path Switch Request Ack message.

After the path switch procedure, the target eNodeB informs the source eNodeB to release related resources, completing the whole handover procedure.

11. Handover Confirm

17.Release Resource

12. Path Switch Request


packet data

Data Forwarding

Flush DL buffer,Contiue delivering transmited

packets

packet data

16.Path Switch Request Ack

18.Release Resources

Han

dove

r Com

plet

ion

MME

13.User Plane update request

15.User Plane update response

14.Switch DL pathEnd Marker

End Marker


Signaling Procedure for the Inter-eNodeB Handover over the X2 Interface (I)

Uu tracing on the UE side

Uu tracing on the source eNodeB side

X2 tracing on the source eNodeB side (same as X2 tracing on the target eNodeB side)


Signaling Procedure for the Inter-eNodeB Handover over the X2 Interface (II) Uu tracing on the target eNodeB side

S1 tracing on the target eNodeB side


Inter-eNodeB Handover over the S1 Interface

Page 50

Inter-eNodeB handover over the S1 interface is similar to that over the X2 interface. The only difference lies in the fact that there is no X2 link between two eNodeBs. If no X2 link is configured, an inter-eNodeB handover is performed over the S1 interface. If X2 and S1 links are configured simultaneously, an inter-eNodeB handover is preferentially performed over the X2 interface.

If a handover uses the S1 interface, the Handover Request and Handover Acknowledge messages and data are transmitted over the S1 interface.


Inter-eNodeB Handover over the S1 Interface (I)

Legend

3. HO decision

7. Admission Control



Deliver buffered and in transit packets to target eNB

Buffer packets from MME

L3 signalling

L1/L2 signalling

User Data

Hand

over

Exe

cutio

nHa

ndov

er P

repa

ratio

n

MME/MMEs

Area Restriction Provided

5. Handover Request

7. Handover Request Acknologe



UL allocation


4. Handover Reauired

8. Handover Command

DL allocation

9. Handover Command

10. eNB SN Status Transfer

Data Forwarding

12. Synchronization


11. MME SN Status Transfer

Data Forwarding

The source eNodeB sends a Handover Request message to the MME.

The MME transfers the Handover Request message to the corresponding target eNodeB for handover preparation.

After completing admission control and radio resource configuration, the target eNodeB returns a Handover Ack message to the MME.

The MME transfers the Handover ACK message to the source eNodeB.

Similar to the handover over the X2 interface, the source target send the SN information to the MME, which then transfers the SN information to the target eNodeB.



Han

dove

r C

ompl

etio

n

MME/MMEs

15. Handover Notify

14. Handover Confirm

packet data

16. UE Context Release Command

16. UE Context Release Completed

Inter-eNodeB Handover over the S1 Interface (II)

The UE accesses the target eNodeB.

The target eNodeB notifies the MME of the handover completion.

The MME notifies the source eNodeB of resource release.

The source eNodeB returns a resource release completion message.


Signaling Procedure for the Inter-eNodeB Handover over the S1 Interface (I) Uu tracing on the UE side

Uu tracing on the source eNodeB side

S1 tracing on the source eNodeB side


Signaling Procedure for the Inter-eNodeB Handover over the S1 Interface (II)

Uu tracing on the target eNodeB side

S1 tracing on the target eNodeB side


Chapter 3 Paging Signaling Procedure

1 Introduction to Paging

2 Paging Signaling analysis

Contents


Introduction to Paging

Page 56

Paging is triggered in the following three situations:

The UE is in the idle state and the network needs to send data to the UE.

The network notifies the UE of an SIB update.

The network notifies the UE of an available ETWS notification.


Introduction to the Paging Procedure (1)

Page 57

The UE is in the idle state and the network needs to send data to the UE.

UE eNB SAES1_Pagi ng

RRC_Pagi ng

RRC_CONN_REQ

RRC_CONN_SETUP

RRC_CONN_SETUP_CMPS1AP_I NI TI AL_UE_MSG

S1AP_I NI TI AL_CONTEXT_SETUP_REQRRC_SECUR_MODE_CMD

RRC_CONN_RECFG

RRC_SECUR_MODE_CMP

RRC_CONN_RECFG_CMPS1AP_I NI TI AL_CONTEXT_SETUP_RSP


Introduction to the Paging Procedure (2)

Page 58

The network notifies the UE of an SIB update or an ETWS notification.

If a UE receives a paging message of SIB update, the UE must resolve the SIB again and use the network configuration parameters contained in newly received SIBs.

If the UE receives a paging message containing an ETWS notification, the UE must resolve the SIB1 immediately. If the SIB1 contains the SIB10 or SIB11 scheduling, the UE must resolve SIB10 or SIB11.

UE eNB SAES1_Pagi ng

RRC_Pagi ng


3 to 5 key points upon the completion of this course

Processes of cell searching, PLMN selection, and cell selection for a UE just powered on

UE attach procedure and different types of attach procedures

LTE handover procedure and several common handover types in the LTE system

LTE paging procedure

Summary

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Signalling Analysis of LTE

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