LT3600 S6 LTE Operation

7/27/2019 LT3600 S6 LTE Operation

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SECTION 6

LTE OPERATION

LTE/SAE Engineering Overview

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II © Wray Castle Limited



Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.1

Cell Reselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.2

E-UTRA Radio Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.3

Measurements for RRC Connected Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.4

Measurement Gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.5

Timing Advance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.6

CQI Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.7

MIMO Options for LTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.8

EPS Initial Attach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.9

Default Bearer Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.10

EPC Support for Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.11

TAU (Tracking Area Update) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.12

Idle-mode Signalling Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.13

Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.14

IMS Functions in Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.15

Levels of Connectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.16

UE-Triggered Service Request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.17

Handling Additional Traffic Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.18

Dedicated Bearer Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.19

IMS Connection Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.20

Connected Mode Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.22

Intra-E-UTRAN Handover (X2-based) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.23

Inter-RAT HO Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.24

Inter-RAT Handover Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.25

CONTENTS

LTE Operation

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IV © Wray Castle Limited



At the end of this section you will be able to:

define the idle mode state for an LTE UE

identify all the key functions and procedures associated with idle mode operation

describe the cell reselection process

define the key radio measurements that are applicable to E-UTRA

describe how measurements are configured for the UE in connected mode

explain the need for and configuration of measurement gaps

describe timing advance adjustment for LTE

describe the process and application of CQI reporting

list and explain the MIMO options for LTE

discuss the reasoning behind the decision to make this a stage in the attach process

describe the activities related to IMS registration

discuss the roles played by various devices involved in EPS device selection outline the set of functions a UE will perform when in idle mode

describe the functions performed by the EPC in support of UEs in Idle Mode

outline the set of activities related to the TAU process

discuss the activities performed to allow UEs to be paged

outline the functions related to ISR (Idle-mode Signalling Reduction)

describe the actions performed by the MME and HSS in support of UE Reachability

discuss the use of the Service Request process and its relationship to the modify and create

bearer functions

list the stages through which the IMS connection establishment process must proceed

outline the processes used to establish CS services for EPS attached subscribers

describe how IMS signalling and media flows are routed and handled

outline the EPC’s support for charging

describe the functions that enable connected mode mobility management to operate

outline the processes employed to support various handover scenarios including intra-E-UTRAN

and inter-RAT HO

describe the procedures employed to detach a UE from the EPS

OBJECTIVES

LTE Operation

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VI © Wray Castle Limited



PLMN selectionand reselection

Cell selectionand reselection

Location

registration

Support for

manual CSG IDselection

Locationregistrationresponse

Servicerequests

Indicationto user Manual

mode

Automaticmode

AvailablePLMNs

SelectedPLMN

Registrationarea changes

Locationregistrationresponse

NAS

control

Radio measurements

CSG IDselected

Available CSGIDs to NAS

LT3600/v3.1 6.1© Wray Castle Limited

Idle Mode

Idle mode represents a state of operation for the UE where it has successfully performed the following:

PLMN selection, cell selection and location registration (by tracking area).

Once in idle mode, the UE will continue to reassess the suitability of its serving cell and, in some

circumstances, its serving network. In order to do this it will implement cell and PLMN reselection

procedures. A UE in idle mode will be monitoring its current serving cell in terms of radio performanceand signalling information. The radio performance measurements are done on the basis of a quality

measure. This is an assessment of radio signal strength and interference level, and it can be made for

both the serving cell and its neighbours. The aim will be to ensure that the UE is always served by the

cell most likely to give the most reliable service should information transfer of any kind be required.

The UE will also be monitoring two key types of signalling from the serving cell system information

messages and paging or notification messages. System information messages convey all the cell and

system parameters. The UE will record changes in these parameters that may affect the service level

provided by the cell, or access rights to the cell. Changes in these parameters could provoke a cell

reselection, or a PLMN reselection. Paging or notification messages will result in connection

establishment.

All of these procedures are performed through communication between the AS and the NAS. In general,

instructions are sent from the NAS to the AS; the AS then performs the requested procedure and returns

a result to the NAS.

If CSG (Closed Subscriber Group) is supported then these procedures are modified such that a cell’s

broadcast CSG ID forms another level of differentiation between cells. CSG is intended for use with

HeNBs (femtocells).

Further Reading: 3GPP TS 36.304:4.1

LTE Operation



Based on priority of RAT/Frequency layers

and thresholds

Based on priority of RAT/Frequency layers

and thresholds

Based on measurements,offsets, parameters and

mobility status

1 sec since last reselectionCell is suitable

I-RATInter-frequency

E-UTRAInter-frequency E-UTRA

Intra-frequency

Low

Medium

High

Measurement rules

Evaluation

Ranking

Reselection

LT3600/v3.16.2 © Wray Castle Limited

Cell Reselection

Cell reselection in LTE both reuses many principles that were are well established in legacy technologies

and introduces new strategies. A key addition for LTE is the use of RAT/frequency prioritization. Each

frequency layer that the UE may be required to measure, either E-UTRA or any other RAT, is assigned a

priority. The cell-specific priority information is conveyed to UEs via system information messages.

Additionally, UE-specific values can be supplied in dedicated signalling, in which case they take priority

over the system information values. Any indicated frequency layers that do not have a priority will not beconsidered by the UE for reselection.

In general, the measurement rules are used to reduce unnecessary neighbour cell measurements. The

UE always measures cells on a higher priority E-UTRA inter-frequency or I-RAT frequency. The UE will

only measure E-UTRA intra-frequency cells if the Srexlev value for the current selected cell falls below

an indicated threshold (Sintersearch). Similarly, the UE only measures E-UTRA inter-frequency or I-RAT

frequency cells on equal or lower priority layers if the Srexlev value for the current selected cell falls

below an indicated threshold (Snonintrasearch).

Measurements are then evaluated for potential reselection. Again, the frequency/RAT priority level is

used along with system-defined threshold for this assessment. A UE will always reselect a cell on a

higher priority frequency if its value of Srxlex exceeds Threshx,high for longer than TreselectionRAT. It willonly select a cell on a lower priority frequency when the Srxlev of the serving cell falls below

Threshserving,low and Srxlev of the neighbour is above Threshx,low for TreselectionRAT and there is no other

alternative. For neighbour cells on intra-frequencies or on equal priority E-UTRA inter-frequencies, the

UE uses a ranking criterion ‘Rs’ for the serving cell and ‘Rn’ for the neighbour cell. Ranking is based on a

comparison of the respective Srxlev values with a hysteresis added to the serving cell value and an offset

added to the neighbour cell value. The UE will select the highest ranked cell if the condition is maintained

for TresectionRAT.

In addition to all of this, the UE will apply scaling to Treselection, hysteresis values and offset values

dependent on an assessment of its mobility state, which may be high, medium or low. This is based on

an analysis of resent reselection frequency.

Further Reading: 3GPP TS 36.304:5.2.4




(Reference Signal Received Power) (Received Signal Strength Indicator)

Total received power in RS OFDMsymbol periods including the serving

cell, all co-channel and adjacent channelinterference and thermal noise

Linear average power of the reference signalresource elements

The ratio of the reference signal power,calculated as N x RSRP, to the RSSI, where

N is the number of RBs in the RSSImeasurement bandwidth

(Reference Signal Received Quality)

RSRP RSSI

RSRQ

Serving cellServing cell


E-UTRA Radio Measurements

There are three key measurement values used in E-UTRA, the RSRP (Reference Signal Received

Power), the RSSI (Received Signal Strength Indicator) and the RSRQ (Reference Signal Received

Quality).

The standards define RSRP as:

‘The linear average over the power contributions of the resource elements that carry cell-specific

reference signals within the considered measurement frequency bandwidth’.

The standards define RSSI as:

‘The linear average of the total received power observed only in OFDM symbols containing

reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource

blocks by the UE from all sources, including serving and non-serving cells, adjacent channel

interference, thermal noise, etc.’

The standards define RSRQ as:

‘The ratio N xRSRP/(E-UTRA carrier RSSI), where N is the number of RBs of the E-UTRA carrier

RSSI measurement bandwidth’.

Note that the measurement of RSRP is based on reference signals from antenna port 0, but where

antenna port 1 can be received reliably, reference signals from that port may also be included.

Additionally, the values of RSRP and RSSI used to calculate RSRQ must have the same measurement

bandwidth.


LTE Operation



UE

RRC Connected

eNBServing cell

Gap configuration

Quantity configuration

Measurement identities

Reporting configuration

Measurement objects

Measurement Parameters

Neighbour

cells


Measurements for RRC Connected Mode

When the UE becomes RRC connected, the measurement and reporting process as well as mobility

decisions becomes the responsibility of the eNB. The required measurement and reporting settings are

signalled to the UE in the RRCConnectionReconfiguration message.

The measurement object defines what the UE is to measure. This is defined as a frequency and

measurement bandwidth; optionally it may also contain a list of cells. If it does contain a list of cells thenthey will be indicated as either white list or black list. The UE will measure any cells it detects but will not

report black list cells. Frequency- or cell-specific offsets will also be included in this field.

The reporting configuration sets what quantities the UE is to measure, what quantities the UE is to report

and under what circumstances a measurement report is to be set. Reporting may be set as either trigger-

based, periodic or triggered periodic. This field also defines the other contents of the measurement report

message.

Measurement identities provides a reference number such that some part of this identified measurement

can be modified or removed in future.

The Quantity configuration sets the filtering to be used on the measurements that are taken.

The gap configuration defines periods when the UE can take measurements of neighbour cells.





eNBServing cell

Transmission gap(6 ms)

Transmission gap repetitionperiod (N x 10 ms)

Neighbour cell Neighbour cellNeighbour cell


Measurement Gaps

When the UE is in RRC connected mode it will be engaged in data transfer in the uplink or downlink

directions or both. In order to simplify the design of the UE it is not required to be able to take neighbour

cell measurements and transfer data with the serving cell at the same time. This requires defined periods

where the UE is able to take neighbour cell measurements and is not required to communicate with the

serving cell.

Transmission gaps perform this function and are very similar in concept to compressed mode for UMTS.

The transmission gaps have a duration of 6 ms since this allows sufficient time to take measurements

and gain basic synchronization with most RATs in a single transmission gap. For GSM, however, 6 ms

remains a sufficient gap, but multiple transmission gaps are required to take measurements and

determine a cell’s BSIC (Base Station Identity Code).

The transmission gap period is variable, but will be a multiple of 10 ms.

The transmission gap pattern to be used by a UE is included in the measurement parameters.


LTE Operation



eNB measures propagationdelay from PRACH preamble

TA step size is 16Ts (0.52 μs)

Correction is included in theRAR as a value of steps in the

range 0 to 1282 (0 to 0.67 ms

TA adjustments are madeusing MAC controlmessages in the PDSCH

Correction is a value in therange 0 to 63 interpreted as+/– 31 steps (+/– 16 μs)


Timing Advance

In order to maintain orthogonality between uplink transmissions from multiples UEs in a cell, timing

adjustment must be applied to compensate for variations in propagation delay.

Initial timing advance is calculated at the eNB from a UE’s preamble transmission on the PRACH. The

timing advance correction is given as an 11-bit value although the range is limited to 0–1282 timing

advance steps. Granularity is in steps of 16Ts (0.52 μs) so timing advance can be varied between 0 and0.67 ms. One timing advance step corresponds to a distance change of c.78 m and is significantly

smaller than the normal CP. The maximum timing advance value corresponds to a range of c.100 km.

The maximum specified speed for a UE relative to an eNB is 500 km/h (139 m/s), which would require

slightly more than one timing advance change every two seconds. Consideration also needs to be given

to the possibility of more extreme changes in the multipath characteristics of a channel, for example the

sudden appearance or disappearance of a strong reflected path from a distant object or delay through a

repeater. However, these are extreme examples and, in any case, timing advance update commands

can indicate up to +/– 16 μs in a single step. Thus the rate at which timing advance commands need to

be sent in practice is typically much less than one every two seconds.

Timing update commands are transmitted to UEs as MAC control messages and as such are included inMAC PDUs carrying data for the UE on the PDSCH. The command itself is a six-bit value giving a

number range from 0–63. Values less than 31 will reduce timing advance and values greater than 31 will

increase timing advance.





Efficiency

(bits/symbol)

0 No TX ... ...

1 QPSK 0.076 0.1523

2 QPSK 0.12 0.2344

3 QPSK 0.19 0.377

4 QPSK 0.3 0.6016

5 QPSK 0.44 0.877

6 QPSK 0.59 1.1758

7 16QAM 0.37 1.47668 16QAM 0.48 1.9141

9 16QAM 0.6 2.4063

10 64QAM 0.45 2.7305

11 64QAM 0.55 3.3223

12 64QAM 0.65 3.9023

13 64QAM 0.75 4.5234

14 64QAM 0.85 5.1152

15 64QAM 0.93 5.5547

CQI Index ModulationApprox. code

rate

Downlink channel

adaptation based on

UE CQI reporting

Uplink channel adaptation based on

eNB measurements of UL data

transmissions and SRS if requested


CQI Reporting

Link adaptation is a crucial part of the LTE air interface and involves the variation of modulation and

coding schemes to maximize throughput on the air interface.

Link adaptation for scheduled uplink resources can be can be calculated by the eNB from a number of

different inputs based on measurements of a UE’s uplink transmissions. Additionally the eNB may

request that UEs transmit sounding reference signals, the measurement results of which can also beused for link adaptation.

For downlink transmissions the eNB needs information about the success or otherwise of the UE in

receiving its downlink transmissions. The UE assesses the quality of the downlink signal through

measurements of the received signal and consideration of the error correction scheme. It then calculates

the maximum modulation and coding scheme that it estimates will maintain an error rate better than 10%.

This is indicated to the eNB as a CQI (Channel Quality Indicator) value. The table in the diagram (taken

from the 3GPP standards) shows how the CQI values are interpreted as modulation and coding

schemes. The table is also useful for estimating the likely physical layer throughput in a given radio

configuration.


LTE Operation



Transmit Diversity Beamforming

Closed loop with PMI feedback

SU-MIMO (ranks up to 4)MU-MIMO (virtual MIMO)


MIMO Options for LTE

In its first release, LTE is specified with several options for SU-MIMO implementation and a more limited

option for MU-MIMO operation. The specification include descriptions of operation up to rank 4 (4x4

MIMO).

The simplest option is not MIMO, as such, but uses the multi antenna array at an eNB to provide transmit

diversity. The standards allow configuration with up to four antennas at the base station. It is likely thatcross-polar antennas would be used as part of the antenna array, so a two-antenna array could be

implemented using a single cross-polar panel, with a four-antenna array requiring two cross-polar panels.

Transmit diversity involves the transmission of a single data stream to a single UE, but makes use of the

spatial diversity offered by the antenna array. This can increase channel throughput or increase cell

range.

There are also two beamforming options available. These are based on the use of a single layer with rank

one pre-coding but make use of a multi antenna array for beamforming to a single UE. The two options for

this are a closed loop mode, which involves feedback of PMI (Pre-coding Matrix Indicators) from the UE,

and an open loop mode, which involves the transmission of UE-specific reference signals and the eNB

basing the pre-coding for beamforming on uplink measurements.

Full SU-MIMO configurations are available in LTE in the downlink direction with ranks up to four. However,

a maximum of two data streams is used, even when four antenna ports are available. In SU-MIMO the UE

can be configure to provide PMI feedback as well as RI (Rank Indicators), which indicates the rank that

the UE calculates will give the best performance.

In the first release of the LTE specification there is only a limited implementation of MU-MIMO specified. It

is applicable in the uplink direction and allows two UEs to use the same time frequency resource within

one cell.

Further Reading: 3GPP TS 36.211:6.3.3, 6.3.4, 36.213:7.1




UE eNB MME S-GW PDN-GW HSSEIR

1. AttachRequest

2. AttachRequest

3. AKA/Security

Optional Stage

4. IdentityRequest/Response

4. ME Identity Check

4. CipheredOptions Request

4. CipheredOptions Response

5. Update Location

5. Insert Subscriber Data

5. Insert Subscriber Data Ack

5. Update Location Ack


EPS Initial Attach

The UE’s objective when performing an attach is to register the subscriber’s identity and location with the

network to enable services to be accessed. During the attach procedure the UE will be assigned a default

EPS bearer to enable always-on connectivity with a PDN. The UE may be provided with details of a local

P-CSCF to enable it to register with the IMS.

A simplified view of the attach process – assuming that it is an initial attach with stored details from arecent previous context for a UE using its H-PLMN (Home PLMN) and accessing via the Home E-UTRAN

– is shown, and the stages of the process are described below.

Once a suitable cell has been selected the UE employs the Random Access procedure to request an

RRC connection with the chosen eNB. With that in place an Attach Request message (1) can be

transmitted. If the UE has previously been registered with the PLMN, it may include a previously assigned

GUTI in the message, otherwise the Attach Request message contains the subscriber’s IMSI and some

other parameters.

On receipt of the Attach Request the eNB either derives the identity of the previously used MME from the

supplied GUTI or selects an MME from the pool available and forwards the message (2).

The MME contacts the HSS indicated by the subscriber’s IMSI and in response receives the relevant

elements of the ‘quintuplet’ that allows the EPS-AKA process to take place (3).

Optionally, at this point the MME may be required to check the identity and status of the UE via the EIR

(4) using the ME Identity Check process. Ciphering may then be invoked over the air interface.

Once the AKA procedures have successfully concluded the MME transmits an Update Location message

to the HSS and receives the Insert Subscriber Data message in response containing the user’s service

profile (5). An Insert Subscriber Data Ack from the MME is followed by an Update Location Ack from the

HSS. The UE is now Attached to the EPC.


LTE Operation



UE eNB MME S-GW PDN-GW HSSPCRF

6. Create DefaultBearer Request

7. PCC

Lookup

Optional Stage

8. Create DefaultBearer Response

9. Initial Context SetupRequest/ Attach Accept

10. RRC Conn

Reconfig

11. RRC Conn

Reconfig

Complete 12. Initial Context

Setup Response13. Direct

Transfer 14. Attach

CompleteData flow


Default Bearer Establishment

A default bearer must then be established and the MME selects the S-GW that will handle and a PDN-GW

that supports the requested APN. The MME issues a Create Default Bearer Request to the selected S-GW,

which assigns a GTP TEID to the EPS bearer and passes the request to the indicated PDN-GW (6).

If the network employs dynamic PCC the PDN-GW will query a PRCF for bearer parameters, otherwise the

bearer will be established using local QoS parameters stored in the PDN-GW (7).

A Create Default Bearer Response message passes from the PDN-GW to the S-GW, which contains

relevant parameters such as the EPS bearer’s IP address and possibly the IP address or DNS name of a

local IMS P-CSCF. The S-GW creates the bearer as specified and passes the Create Default Bearer

Response message to the MME (8). The details that define the S1-U service will also have been defined

during this stage.

The MME sends an Initial Context Setup Request/Attach Accept message, which contains the assigned

parameters for the EPS bearer context, to the eNB (9). That element in turn sends an RRC Connection

Reconfiguration message to the UE (10) to inform it of the bearer details and the changed air interface

parameters.

The UE returns an RRC Connection Reconfiguration Complete message (11) to verify that the radio bearer,

which was initially established just to carry the attach message, has been reconfigured to support the new

parameters. The eNB forwards an Attach Complete message to the MME (12).

The UE then sends a Direct Transfer message to the eNB (13), which confirms the details of the EPS

Bearer. Finally, the eNB sends an Attach Complete message to the MME to confirm that both the Attach

and the Default EPS Bearer processes have completed successfully.

Uplink and downlink data can now flow if required.





ECM StateEPS Bearer ID

QoS

TA 9

TA 12

MME Pool A

UE TA List UE Default Bearer

Context – Inactive

TA 9TA 12


EPC Support for Idle Mode

The MME currently serving each UE is responsible for ensuring its ‘reachability’. It achieves this by

monitoring the current TA in which the terminal is located.

The EPS allows a cell to be a member of more than one TA. This allows a UE to roam within a set of

contiguous TAs without being required to perform a TAU, which reduces the amount of location-related

signalling that is required, although it may conversely increase the amount of paging required per UEconnection request.

The MME reflects this extended mobility by maintaining a TA list for each registered UE within which the

list shows the set of TAs the UE is currently registered.

During a TAU, and periodically in the event that a TAU does not occur within a set time-frame, the MME

is responsible for reauthenticating each registered UE and for reissuing the M-TMSI used to

confidentially identify it.

When a UE drops into the ECM-IDLE state its existing default bearer can be ‘parked’ and any dedicated

bearers can either be parked or released. To support this, the MME stores details of the UE’s current

‘bearer contexts’ ready to reactivate them in the event of a UE or network-triggered Service Request.

A TAU may result in the need to change the S-GW assigned to handle an idle UE’s bearer contexts or of

the MME with which the UE is registered, if the reselected cell is associated with a different S-GW

Service Area or MME Pool.

If ISR (Idle-mode Signalling Reduction) is active for a UE, the MME may be required to pass location

updates and other pertinent information to the SGSN with which the UE is co-registered.


LTE Operation



UE eNB MME HSS

TAU trigger event

1. TAU Request

2. TAU Request+ TAI and ECGI

3. Authentication/Security

Optional Stage

4. TAU Accept

5. TAU Complete


TAU (Tracking Area Update)

A TAU takes place between a UE and the MME with which it is registered and is triggered by the UE

detecting a change in TAI after a cell reselection. A TAU is also be used as part of the Initial Attach

process and may additionally be triggered by events such as the expiry of the periodic TAU timer or as

part of MME load balancing or rebalancing.

In the example message flow it is assumed that the UE is connected to its HPLMN and that an S-GWchange and MME relocation are not required.

After detecting a change in TAI, the UE transmits a TAU Request message to the eNB (1). The TAU

Request contains data such as the old GUTI, old TAI, EPS bearer status and a NAS MAC (Message

Authentication Code) for integrity protection purposes.

The eNB forwards the TAU Request (plus the new TAI and ECGI) to the MME indicated by the supplied

GUTI (2). If the MME indicated by the GUTI is not associated with the new eNB, an MME relocation will

be triggered and the base station will select a new MME to pass the TAU Request to.

If the integrity check of the MAC carried in the TAU Request is successful, the MME may elect not to

reauthenticate the UE. If the MME is configured to always reauthenticate, or if the integrity check fails,then the EPS-AKA process must be followed and a new GUTI (which includes the new M-TMSI) will be

issued (3).

Once the MME is satisfied that the UE/USIM is authentic and assuming that the UE is allowed to roam in

the new TA, it transmits a TAU Accept message to the eNB, which relays it to the UE (4). The TAU

Accept message contains the new GUTI, if one was assigned, plus the current TA List associated with

the UE. The TA List enables the UE to determine the set of TAs within which it can roam without being

required to perform another TAU. The UE responds with a TAU Complete message (5), which finishes

the process.





S-GW

MME

GERAN/UTRAN

SGSN

PDN-GW

E-UTRAN

TA

RA

S3UE can reselect to any

registered RAT without

sending an update

UE and Bearer

Contexts stored

UE and Bearer

Contexts stored

UE paged across

all registered

areas


Idle-mode Signalling Reduction

ISR is designed, as the name suggests, to reduce the amount of UE-network and MME-SGSN signalling

required to manage idle mode terminals. ISR is a feature of the S3 and S16 interfaces and is not

available to legacy SGSNs that do not support them.

When an Idle UE activates (or is instructed to activate) ISR, copies of UE Context and Bearer Contexts

are stored in both an MME (for E-UTRAN access) and SGSN (for GERAN/UTRAN access). The UE isable to reselect freely between registered RATs without transmitting location updates, unless a change in

RAI or TAI is detected. Any location updates that are sent need only be transmitted via the RAT currently

in use; the receiving core network element will forward the update to its peer over the S3 interface.

The MME and SGSN both store copies of the UE’s bearer contexts and will both page for the UE. When

the UE needs to move to connected mode, whether in response to a page or to a user-initiated event, it

can do so by sending a Service Request via whichever RAT it is currently camped on. The receiving

device will then instruct the S-GW to re-establish the parked bearers.

A UE with ISR activated maintains details of the RAT and therefore the RAT-specific temporary identifier

that is in use using the TIN (Temporary Identity used in Next update) parameter.

The TIN can be set to P-TMSI (for GERAN/UTRAN access), GUTI (for E-UTRAN access) or RAT-related

TMSI. This last option means that the UE will use the P-TMSI or GUTI depending upon which RAT is

currently in use.

A UE will deactivate ISR if it loses contact with one of the registered access networks. For example, a UE

might be within the coverage of both an E-UTRAN and a GERAN cell when ISR is activated but may

roam out of coverage of the E-UTRAN cell; in such circumstances it would revert to being attached to just

an SGSN.

Further Reading: 3GPP TS 23.401:Annex J

LTE Operation



S1 Pagingmessages

TA 9

TA 12

UE TA List

TA 9TA 12


Paging

The main purpose of the TAU process is to ensure that the MME knows roughly where each UE is in the

event that there is inbound traffic to deliver. Paging will usually be triggered by the receipt of an S-GW

Downlink Data Notification at the MME, indicating that data has arrived at the S-GW on the S5/S8 portion

of a parked EPS Bearer.

If it becomes necessary to contact an idle UE (that is, a UE that has entered the ECM-IDLE state), theMME will employ the paging process.

With no equivalent node to the RNC, EPS paging is managed directly between the MME and eNBs.

When a Paging message is to be sent, the MME checks the current TA list stored for the target UE and

inserts the paging data into the S1 paging messages sent to all eNBs in the indicated TAs.

Each eNB inserts the UE’s NAS paging ID (IMSI or S-TMSI can be used) into the appropriate repetitions

of its PCH. Paging groups may be established to reduce the number of repetitions of the PCH that each

UE is required to monitor; the operation of the paging reduction scheme is controlled via cell-specific

DRX (Discontinuous Reception) functions.

When a UE receives its paging ID on the PCH it initiates the service request process, which ensures that

any ‘parked’ EPS bearers are reactivated ready to carry traffic.

If a UE has ISR activated the paging notification will be forwarded to the peer core network node; either

MME or SGSN.

Further Reading: 3GPP TS 23.401:5.3.4; 36.300




S-CSCF

UE EPS

Re-registration causes:

Change of IP Address

Change of PDN-GW

Expiry of Registration Timer


IMS Functions in Idle Mode

There is no specific equivalent of idle mode in the IMS – a UE is either registered or deregistered.

The main function that an idle mode UE performs in relation to the IMS is to perform periodic re-

registration. The periodicity of the re-registration is determined by the registration expiry value included in

the initial Registration message and the process ensures that the S-CSCF is kept informed of the

reachability of each registered UE.

Re-registration is also required if the UE’s IP address changes – either as a result of a change of PDN-GW

or as part of a network’s DHCP IP address allocation processes (which may seek to reduce the

possibilities of fraud or connection hijack by periodically refreshing the IP addresses assigned to

terminals).

An additional trigger for re-registration would be if the UE or IMS capabilities changed, for example if the

client supporting a new IMS application was loaded to the terminal.


LTE Operation



PDN–GWS–GWeNBUE

IMS

Internet

Inactive Default

EPS Bearer

Inactive Dedicated

EPS Bearer

Reactivate ‘parked’

EPS Bearers

ServiceRequest

Modify/CreateDedicated

Bearer

To carry new SDFs


Levels of Connectivity

User connectivity in a combined EPS/IMS network requires two levels of connection to be established:

firstly, the radio and EPS bearers that will carry traffic through the E-UTRAN and EPC, and secondly the

IMS SIP and media connections that will carry call-related signalling and end-to-end user traffic.

A UE’s default bearer may be an operator’s first choice for carrying application traffic, but if the QoS

demanded by a new service data flow is incompatible with that of the default bearer, then the PDN-GW/PCRF may decide that an additional dedicated bearer is established.

When a UE enters idle mode the physical S1 and radio resources assigned to the default EPS bearer will

be released and the bearer context details will be stored. Any existing dedicated bearers may be

released or stored also.

When the UE moves from ECM-IDLE to ECM-CONNECTED the stored bearer contexts will be

reactivated using the Service Request procedure.






Trigger event

1. NAS Service

Request2. NAS Service

Request3. Authentication/Security

Optional Stage

4.S1 AP Initial Context

Setup Request5. Radio Bearer

Establishment

6. Uplink Data Flow

7. S1 AP Initial Context

Setup Complete

8. Update Bearer

Request

9. Update Bearer

Request/Response

10. Update Bearer

Response

11. Downlink Data Flow


UE-Triggered Service Request

A UE will trigger a Service Request to reactivate its parked bearer contexts in response to a command

from an application client, the terminal management software or the user interface. A response to a

network initiated paging message will also trigger a Service Request.

The process begins with the transmission of a NAS: Service Request either following the random access

procedure or carried in scheduled uplink capacity. The NAS: Service Request contains the UE’s currentS-TMSI and the service type (data or paging response). The request is initially forwarded to the eNB

encapsulated in an RRC message (1).

Direct Transfer NAS messages were transparent to the UMTS Node B and were only accessible to the

RNC. In the E-UTRAN, NAS messages are switched from the RRC bearer used on the air interface to an

S1AP bearer for forwarding to the MME (2) and in some cases are interpreted by the eNB.

Depending upon configuration, the MME may initiate a reauthentication of the UE/USIM before

processing the Service Request (3).

The MME sends the eNB an S1AP: Initial Context Setup Request, which issues the commands that re-

establish physical resources for the stored bearer contexts on the S1 interface between the UE and theS-GW (4). The eNB allocates radio resources (5) on the air interface and informs the UE. Uplink traffic is

then able to flow (6). The eNB confirms these actions with an S1AP: Initial Context Setup Complete

message (7).

The MME instructs the S-GW to establish its end of the S1-U tunnels using the Update Bearer Request

message (8). If the PDN-GW has requested updates regarding the UE’s location, the S-GW will pass this

on in an Update Bearer Request (9). After the PDN-GW and S-GW return Update Bearer Responses,

data can begin to flow on the downlink (9, 10 and 11).


LTE Operation




Trigger event

1. Request Bearer Resource

Modification

2. Request Bearer Resource

Modification

3. PCC Lookup

Optional Stage

4. Existing TFT modified, new TFT or Bearer activated or existing TFT or Bearer deactivated


Handling Additional Traffic Flows

If a UE determines that there is a requirement to establish a traffic flow aggregate (which may contain

one or more SDFs) to a new AF (Application Function) destination – in response to a user interface

request, for example – it will transmit a Request Bearer Resource Modification to the MME. If the UE had

been in Idle Mode when it made this determination it will first send a Service Request to reactivate the

existing bearers.

The MME forwards the request to the S-GW currently dealing with the UE’s EPS Bearer(s), which in turn

forwards it to the appropriate PDN-GW. If dynamic PCC is in use, the PDN-GW interacts with the PCRF

to determine how best to deal with the request: if static PCC is in use then the PDN-GW makes the

determination itself.

The Modification request includes the required QoS, the EPS Bearer ID and a TAD (Traffic Aggregate

Descriptor), which describes the modification function to be performed (add, modify or delete) and the

SDF 5-tuple details that enable the PCRF to build a packet filter for the flow. The PCC function will

evaluate the request and either accept or reject it. Accepted requests result in new or updated packet

filters.

In the case of a new traffic flow that is to be added to an existing bearer, the PCC function will add anadditional packet filter to the TFT (Traffic Flow Template) related to the bearer over which the flow will

travel. If the addition of the new flow alters the bearers QoS requirements the adjustment will be

communicated to other elements using the Update Bearer Request process.

In addition to UE-initiated Bearer Modification the EPC also supports PDN-GW-initiated Bearer

Modification; HSS-initiated Bearer QoS Modification and MME and PDN-GW initiated Bearer

Deactivation.






1. Trigger event

2. Request Bearer

Resource Modification 3. Request Bearer

Resource Modification

4. PCC Lookup

Optional Stage

5. Create Dedicated

Bearer Request

6. Bearer Setup Request/

Session Management Request7. RRC Conn

Reconfig

7. RRC Conn

Reconfig

Complete8. Bearer Setup

Response

9. Direct Transfer

10. Session Management

Response11. Create Dedicated

Bearer Response 12. IP-CAN Session

Modified

Data Flow


Dedicated Bearer Creation

If PCC decides that a new traffic flow is incompatible with any of the UE’s existing bearers it may decide

that a new Dedicated Bearer is required, in which case it will instruct the PDN-GW to issue a Create

Dedicated Bearer Request.

The stages of this process are outlined in the diagram.


LTE Operation



UEHome

P-CSCF

HomeS-CSCF Destination IMS/UEHome

PDN-GW

Optional Stage

Trigger event

1. Invite

2. Invite3. Invite

4. Session Progress

Edit4. Session Progress

Edit4. Session Progress

5. Provisional

Acknowledgement5. PRACK

5. PRACK

Resource

Allocation

6. 200 OK

6. 200 OK

6. 200 OK


IMS Connection Establishment

IMS connection establishment is the responsibility of SIP. The EPS default bearer is established to a

home network PDN-GW and maintained mainly to provide a path for SIP messaging between a UE and

its serving I-CSCF.

Consider an example SIP flow between a roaming UE and its home S-CSCF during which a media

session to a distant IMS-connected UE is initiated. Not all network elements involved in the processhave been shown.

In response to an instruction received via the user interface, the originating UE initiates the session by

transmitting a Service Request to reactivate its bearers followed by a SIP Invite message to the current

I-CSCF (1). The Invite message contains an SDP payload, which describes the type of connection the

originating UE wishes to establish with the destination UE.

The I-CSCF passes the message on to the assigned S-CSCF for authorization (2). The S-CSCF

discovers the called party’s home network and passes the Invite to an I-CSCF in that network for

forwarding to the S-CSCF and the destination UE (3).

Once discovered, the destination UE inspects the SDP payload and determines if it can support thetype of service and QoS parameters specified. A Session Progress message is returned to the

originating UE containing the IP address of the distant terminal and a response to the SDP parameters

(4).

Each CSCF in the return path is able to approve or edit the SDP response so that the eventual media

session’s parameters match the capabilities of the busiest link in the chain.

The originating UE returns a PRACK (Provisional Acknowledgement), which confirms the parameters of

the media session (5). This triggers the reservation of resources for the distant UE, which it confirms

with a 200 OK message (6).





UEHome

P-CSCFHome

S-CSCF Destination IMS/UEHomePDN-GW

VisitedMME

Optional Stage

7. Resource Bearer

Resource Modification

8. Update8. Update

8. Update

9. Ringing

9. Ringing9. Ringing

Exchange of PRACK & 200 OK

Answer

11. 200 OK11. 200 OK

Resources

Committed

11. 200 OK

Media Flow


IMS Connection Establishment (continued)

Once confirmed, the originating UE may issue a Request Bearer Resource Modification to the MME to

trigger a modification of the default bearer, or possibly the establishment of a new dedicated bearer (7).

An Update message is sent to the distant network to confirm that a bearer with the required QoS is

reserved (8).

At this point the distant UE informs its user of the requested session and returns the Ringing indication to

the originating end (9).

When the called party answers, the distant UE sends a 200 OK message (11) (which, technically, is

issued in response to the original Invite message); the I-CSCFs instruct the PDN-GWs to release the

resources previously reserved for the session and data begins flowing directly between the UEs without

travelling through the IMS.

The mobile-terminated scenario follows the same procedure as mobile-originated procedure except it

begins with a SIP Invite message being sent to the terminating UE, which may result in the UE being

paged and a network-triggered Service Request to reactivate its default bearer from idle mode.

From that point onwards it can exchange SIP messages with the originating UE via its current

I-CSCF.


LTE Operation



Intra-E-UTRAN Inter-S-GW Inter-MMEInter-system Non-3GPP


Connected Mode Procedures

Mobility management functions related to UEs in the ECM-CONNECTED state with active EPS Bearers

and service data flows can be summarized as follows:

intra-E-UTRAN handover

intra-E-UTRAN handover with S-GW relocation

intra-E-UTRAN handover with MME relocationInter-RAT handover

E-UTRAN-Non-3GPP Access Handover





UE SourceeNB

MME S-GW PDN-GWTargeteNB

1. Uplink and downlink data flow

Optional Stage

2. Handover Preparation

2. Handover Execution including new S1 tunnel creation

3. X2 Direct Forwarding

3. Uplink Data

4. Path Switch

Request 5. User Plane

Update Request

6. UE locationinformation updated

7. User Plane

Update Response

8. Downlink Data

9a. End Markers

9b. End Markers forwarded,

X2 data forwarding ends 10. Path Switch

Request Ack11. Release

Resources


Intra-E-UTRAN Handover (X2-based)

In this scenario a UE with active EPS bearers initiates the inter-eNB cell handover procedure when both

source and target eNBs are associated with the serving MME and S-GW and an X2 path can be

established between them.

At the start of the process the UE is connected to the E-UTRAN and is taking neighbour cell

measurements (1). Traffic may or may not be flowing over the connected EPS Bearers. A neighbour cellachieves the criteria necessary for the UE to initiate handover, which is effected by the source and target

eNBs without core network involvement beyond the establishment of S1 resources towards the target

eNB (2).

Once the handover is complete the source eNB forwards any further downlink traffic received for the UE

to the target eNB either directly via the X2 interface (3); this is termed Direct Forwarding. Uplink traffic

travels from the target eNB to the S-GW via the newly established tunnel. The target eNB sends a Path

Switch Request to the MME informing it of the handover (4).

The MME determines that the existing S-GW is still capable of serving the UE and instructs it to switch

the Downlink user plane over to the tunnel created towards the new cell using a User Plane Update

Request message (5). If the PDN-GW has indicated that needs to be kept informed of the UE’s location(for variable charging purposes), the S-GW informs it using an Update Bearer Request (6).

The S-GW realigns the tunnel carrying the EPS bearer(s) to point to the target eNB and sends

confirmation to the MME that the path has been switched successfully (7); both uplink and downlink

traffic now travels over the new S1 tunnels (8). The S-GW sends ‘end marker’ packets to the source eNB

to confirm that the path has been switched, which are forwarded to the target eNB to indicate that X2

forwarding will cease (9).

The MME confirms the procedure to the new eNB with the Path Switch Request Ack message (10) and it

in turn confirms the handover to the old eNB using the Release Resource message (11).


LTE Operation



UE SourceeNB

SourceS-GW

PDN-GWTargetRNC

TargetMMESGSN

Data Flow

Optional Stage

1. Handover decision

2. Handover Required

3. Forward Relocation

Required

4. Create Bearer Request/Response

5. Relocation Request

6. Relocation Request Ack

7. Forward Relocation

Response

8. Create Bearer Request/Response


Inter-RAT HO Preparation

The example provided below is for handover of active traffic connections between a home-network

E-UTRAN cell and a home-network 3G UTRAN cell without an S-GW change and with no S12 Direct

Tunnel support.

Following a UE’s decision to request a handover (1), but before that handover can be initiated, a certain

amount of preparation must take place.

The source eNB determines that the indicated handover candidate is an inter-RAT neighbour and

informs the source MME using the Handover Required S1AP message (2).

The source MME selects a target SGSN and issues a Forward Relocation Request via the S3 interface

(3).

If the target SGSN has an S4 interface to the existing S-GW it issues a Create Bearer Request to that

S-GW (4). If the target SGSN has no S4 interface to the serving S-GW (if they are in different PLMNs for

example) then the target SGSN must select a local target S-GW, establish a bearer to it and then initiate

an S-GW Relocation between the source and target nodes. Inter-PLMN traffic travels from visited S-GW

to home PDN-GW, not visited SGSN to home PDN-GW.

The target SGSN instructs the target BSC/RNC to reserve radio resources for the UE in the target cell

using the Relocation Request message (5), and the target RNC responds with Relocation Request Ack

once this is complete (6).

Once the GERAN/UTRAN RAB and PDP context are in place, the target SGSN sends the Forward

Relocation Response to the source MME (7).





UE SourceeNB

SourceS-GW

PDN-GWTargetRNC

TargetMMESGSN

Data Flow

Optional Stage

9. Handover Command

10a . HO from E-UTRAN

Command

Release and reconnect

10b. HO to UTRAN

Complete11a. Downlink Data via Direct Forwarding (eNB-RNC)

Direct Forwarding

11b. Downlink Data via Indirect Forwarding

Indirect Forwarding

11a/b. Downlink Data

11c. Uplink Data

12. Relocation Complete

13. Forward Relocation Complete


Inter-RAT Handover Execution

The UE remains connected to the source E-UTRAN cell during the preparation phase, but once

alternative resources are in place the source MME issues a Handover Command to the source eNB (9),

which in turn sends a HO from E-UTRAN Command to the UE (10a). This message encapsulates a

‘transparent container’ that travels between the target RNC and the UE, which contains details of the

resources that have been reserved for the UE in the target cell.

The UE releases its E-UTRAN resources and performs the access activities required to establish

connectivity in the target UTRAN cell and sends the Handover to UTRAN Complete message (10b) in the

new cell to confirm the connection.

As the tunnel from the PDN-GW has not yet been realigned, Downlink packet traffic destined for the UE

is still being sent to the source eNB and must be forwarded to the target RNC. Direct forwarding between

the source eNB and target RNC (11a) uses an unnamed forwarding interface. Indirect Forwarding travels

between source eNB, source S-GW, target SGSN and target UTRAN (11b). Once the handover is

complete, the UE can send traffic on the uplink via the PDP Context that has been created towards the

SGSN, from where it will be forwarded to the S-GW and on to the PDN-GW (11c).

Once the UE has successfully connected to the UTRAN cell the target RNC sends a RelocationComplete message (12) to the target SGSN, which in turn informs the source MME using the Forward

Relocation Complete message (13).

Further Reading: 3GPP TS23.401:5.5.2

LTE Operation



UE SourceeNB

SourceS-GW

PDN-GWTargetRNC

TargetMMESGSN

14. Update Bearer Request/Response

Optional Stage

15. Data Flow

16. Routing Area Update

17. Release Resources 17. Delete Bearer

Request/Response

18. Delete Bearer

Request/Response(if Indirect

Forwarding used)

Direct Forwarding

Indirect Forwarding

Inter-RAT HO Execution (continued)

The target SGSN issues an Update Bearer Request to the S-GW (14), which initiates the path switch.

Any indirect forwarding will cease and downlink traffic will travel from the S-GW directly to the target

SGSN (15). If the relocation involved a change in S-GW the PDN-GW would also need to be informed so

that it could realign its end of the EPS bearer tunnel.

The UE performs a RAU (Routing Area Update) (16) and the target SGSN may decide to instruct the UEto reauthenticate.

The MME started a timer when the handover initiated; when it expires it instructs the source S-GW and

source eNB to release any resources and contexts stored for the UE (17).

If indirect forwarding was employed, the source S-GW and SGSN will release any tunnel resources that

were created (18).

Traffic is now flowing to the UE from the PDN-GW, via an S-GW to an SGSN and onwards via the

UTRAN.


Documents

LT3600 S6 LTE Operation