lte location management and mobility management

Irfan Ali 1

Power Management and

Mobility Management in LTE

Irfan Ali

October 2014

Irfan Ali 2 Irfan Ali 2

Overview

•  Power Conservation in UE Ø  High Power: “Connected Mode” when UE has both its

transmitter and receiver always on. Ø  Low Power: “Idle Mode” when UE turns off it transmitter. It

turns on its receiver periodically •  Transition between the states •  Mobility in Idle Mode

Ø  Cell Selection and Re-selection Ø  Tracking Area Update

•  Mobility in Connected Mode: Handovers


Power Management in LTE High Power Mode Connected Mode

Low Power Mode Idle Mode

•  UE’s radio is in ON state. •  UE is constantly communicating with the network.

•  Network controls UE’s movement through handover.

•  Location of the UE is known to the network at granularity of a cell.

Mobility Mobility

Tracking Area 1 Tracking Area 2

•  Network does not control UE’s movement. UE autonomously selects new cell as it moves.

•  Network only knows the location of the UE to the granularity of a tracking-area.

•  UE’s radio is in low-power state. UE’s transmitter is off.

•  UE only listens periodically to control channel. If UE enters a new location area, based on hearing information from base-station, the UE informs the network of the new tracking area it has entered.

UE is like a dog on a leash J UE is like a dog without a leash enclosed in an electronic fence


Activity State Management

•  A UE in LTE can be in two states: Ø  Connected Mode: The UE is transmitting and receiving data from the

network. Ø  Idle Mode: The UE is only monitoring the paging and broadcast channel.

•  After the UE stops transmitting/receiving data/signal for a period of time, called inactivity period, the network moves the UE’s state to idle-state

Data/Signal activity

Yes

No

Connected

Idle

UE’s State

Connected -> Idle Inacitivity Timer

Connected -> Idle Inacitivity Timer

Time

Time


UE’s Activity States for AS and NAS

AS

NAS EMM-IDLE EMM-

Connected

RRC connection Established

RRC connection Released

RRC-IDLE RRC-Connected

RRC connection Established

RRC connection Released

EMM-IDLE EMM-Connected

NAS connection Established

NAS connection Released

S1-MME

UE eNB MME

EMM Enhanced Mobility Management NAS Non Access Stratum AS Access Stratum RRC Radio Resource Control

RRC State

RRC-Connected

RRC-Idle

EMM-Connected

EMM-Idle

EMM State Time

Time

Time MME Request S1 connection to be torn down

eNB tears down RRC Connection

UE has a packet to send UE sets up RRC Connection

MME Request eNB to setup data radio bearers

eNB sets up data radio bearers

Events

UE’s State Machine


Key Points about UE’s AS (RRC) and NAS State Machines

•  The RRC state machine transitions are very clear Ø  When the RRC Connection is setup, the UE transitions from RRC-Idle

to RRC-Connected, and vice-versa •  The NAS state machine transitions are based on RRC events

Ø  The NAS specification (TS 24.301), does not have EMM-Connected and EMM-Idle shown in a state-transition diagram. TS 24.301 has more detailed NAS state machine diagrams, with states such as EMM-Registered, EMM-Deregistered, etc. The following two statements buried deep in 24.301 provide details of state-transitions:

•  In S1 mode, when the RRC connection has been released, the UE shall enter EMM-IDLE mode and consider the NAS signalling connection released

•  In S1 mode, when the RRC connection has been established successfully, the UE shall enter EMM-CONNECTED mode and consider the NAS signalling connection established.

Ø  Details of NAS specifications for MME are not explicitly provided in TS 24.301. One needs to infer these from TS 24.301, which is written from a UE implementation point of view.


Activity States of UE

EMM-IDLE RRC connection established

RRC connection released

EMM-IDLE EMM-CONNECTED NAS Connection established

NAS Connection released

UE

MME

Idle Mode •  UE monitors paging channels periodically (DRX cycle)

and some System Information channel •  No NAS signalling connection between UE and MME •  UE (independently) performs cell selection/re-selection

based on broadcast information •  No UE information in the eNB •  Location of UE is known to the MME at granularity of

Tracking Area. •  UE performs TAU when UE enters a new TAI or when the

periodic TAU timer expires. •  UE enters connected mode when RRC signaling

connection is established. •  For MME there is no clear indication when the

UE’s state transitions to EMM-Connected. Typically this happens when the S1-MME connection is established for the UE.

Connect Mode •  UE monitors System Information channel and control

channels associated with shared data channels. •  NAS signalling connection between UE and MME x •  Network (eNB) controls UE’s movement through

handover. •  UE context in the eNB •  Location of the UE is known to the MME at granularity

of eNB. •  UE performs TAU when UE enters a new TAI broadcast •  UE enters idle mode when RRC connection is

released. •  For MME there is no clear indication when the

UE’s state transitions to EMM-Idle. Typically this happens when the S1-MME connection is released for the UE.

EMM Enhanced Mobility Management NAS Non Access Stratum RRC Radio Resource Control

Both the UE and MME keep track of the state of the UE

EMM-CONNECTED


State in Network for Connected and Idle mode

UE

MME

eNB

SGW

PGW

NAS (logical)

S1-MME

S1-u S11

S5

DRB SRB

UE

MME

eNB

SGW

PGW

S11

S5

No UE Context

UE Context

No S1-U tunnel

Connected Mode Idle Mode DRB Data Radio Bearer SRB Signaling Radio Bearer


Transition from Connected to Idle State – S1 Release Procedure

SGW HSS eNB UE

PGW

MME

Internet

Data Radio Bearer-10 GTP-U-10 Tunnel GTPU-10 Tunnel

GTPC Tunnel GTPC Tunnel S1-MME SRB-2

SRB-1

SRB-0

UE reamains inactive for sometime

S1 UE Context Release Request

EMM-Connected

GTPC

Release Access Bearer Resp. (IMSI, TEIDs)

Release Access Bearer Req. (IMSI, TEIDs, )

S1 UE Context Release Command DL-SCH:DCH SRB1

RRC Connection Release

S1 UE Context Release Complete

EMM-Idle

RRC-Idle

EMM-Idle No UE Context in eNB

GTP-U-10 Tunnel

GTPC Tunnel GTPC-1 Tunnel

SGW does not have DL S1-U TEIDs for UE

RRC-Idle

Irfan Ali 10

Packet arrives at Serving GW for idle UE: Where to page the UE?


Concept of Tracking Area-1 •  Tracking Area consists of a set of eNBs. •  The concept of tracking area is introduced to reduce the amount of location

reporting (Tracking Area Update TAU) signaling that a UE does when in idle-state Ø  The UE only signals to the network (MME) when the UE enters a TA to which it is not

admitted. Ø  The MME knows the location of the UE to the granularity of TAs.

•  Tracking areas are non-overlapping in LTE. •  The identity of each tracking area is called Tracking Area Identity (TAI). •  Each cell in a eNB can belong to only one TAI. •  Each cell advertises in broadcast message the TAI to which it belongs. •  The MME tells the UE which Tracking areas the UE is registered in.

Ø  This is done in EMM-Connected mode.

TA-1

TA-2

TA-3

TA-4

TA-5

TA-6

TA-7


Concept of Tracking Area-2 •  A UE in LTE can be admitted to multiple tracking areas. The list of tracking areas

to which the UE is admitted is called the tracking area list (TAI List) is provided to the UE.

•  When a UE is idle and the MME needs to locate the UE, the MME pages the UEs in the set of eNB which belong to the TAI that the UE is registered in. Ø  Larger the tracking area, less frequent will be the UE’s need to signal to the network;

however larger the number of eNBs that the UE will need to be paged in.

TA-1

TA-2

TA-3

TA-4

TA-5

TA-6

TA-7

UE-1 is admitted to TAI-1 UE-2 is admitted to {TAI-2, TAI-4}

Perimeter-crossing where UE-2 performsTAU Area to page UE-2

Area to page UE-1 Perimeter-crossings where UE-1 performs TAU


Tracking Area Identity (TAI)

World

US Turkey India

Turkcell Vodafone Avea

Izmir Istanbul Antalya

MCC

MCC MNC

MCC MNC TAC

MCC: Mobile Country Code 3 digits

2-3 digits

MNC: Mobile Network Code

2 Octets

TAC: Tracking Area Code

310 286 404

01 02 03 Uniquely identifies an operator

TAI: Tracking Area Identifier

Source for MCC and MNC codes: www.wikipedia.org

1-400 401-2000 3000-3500

Irfan Ali 14

Idle-mode: When to page the UE?

In the next few set of slides we figure out when the UE turns on its receiver to figure out if the network is paging the UE.


DL Frame Structure – Type 1 (FDD)

•  1 subframe = 1ms •  10 subframes make up Radio Frame •  Each subframe consists of 14 symbols •  DL control signalling is in the first 1-3 symbols

Ø  The rest of the symbols (11-13) are used for data and dedicated control channels.

#0 #1 #9 … … …

Sub-frame (1 ms)

One Radio Frame (10 ms)

CCH 1

CCH 2

CCH 3

CCH 4

RB 0 RB 1 RB 2

RB n-1

.

.

.

CCH 1

CCH 2

CCH 3

CCH 4

RB 0 RB 1 RB 2

RB n-1

.

.

.

CCH 1

CCH 2

CCH 3

CCH 4

RB 0 RB 1 RB 2

RB n-1

.

.

.

Time

Frequency

Indication of page message for UE will be contained in the Common Control Channel (CCH) Pages may only be present in the subframe {0, 4, 5, 9}

CCH Common Control Channel RB Resource Block


UEs DRX cycle in idle mode: Paging DRX •  The UE’s paging DRX cycle period is one of the following:

Ø  {32, 64, 128, 256} frames (each frame is 10 msec), i.e Ø  {0.32, 0.64,1.28, 2.56} seconds

•  The UE determines its idle-mode DRX paging cycle either Ø  From the information in System Information Block (SIB) Ø  Or is provided to the UE via dedicated signal before UE goes idle.

•  Not all radio frames contain page messages. Ø  Paging Occasion (PO) is a subframe that contains paging message Ø  Paging Frame (PF) is a radio frame that contains one or more paging occasions.

•  The UE needs to monitor only one paging occasion per DRX cycle. •  Changes in the system information are indicated by the network using a

Paging message. Ø  Hence UE only monitors PDCCH. Ø  If there is a page message, the ID in the PDCCH is P-RNTI. All UEs share the same P-

RNTI (FFFE). Ø  Once the UE finds PRNTI, it looks at the appropriate Resource Block in the PDSCH

pointed to by the PDCCH message. If it finds its P-TMSI in the PDSCH, then page is destined for the UE.

Ø  When the Paging message indicates system information changes then UE shall re-acquire all system information.

PDCCH Physical Downlink Common Control Channel DRX Discontinuous Reception P-RNTI Paging Radio Network Temporary Identity S-TMSI S Temporary Mobile Service Identity


Low Power (Idle Mode)

•  UE’s radio is in low-power state. UE’s transmitter is off. •  UE listens periodically to control channel.

•  To receive pages from the network.

DRX Cycle

ON Duration UE Montiors

PDCCH

DRX Sleep

UE’s Receiver


Formula to determine which radio frame number (SFN) and which subframe within the SFN for UE to monitor for page message

SFN mod T = (T/N) X (UE_ID mod N) i_s = floor(UE_ID/N) mod Ns

T = min (Tc, Tue) N = min (T, number of paging subframes per frame X T)

Ns = max (1, number of paging subframes per frame(Nf) )

where, Tc cell specific paging cycle {32,64,128,256} radio frames Tue UE specific paging cycle {32,64,128,256} radio frames N number of paging frames within the paging cycle of the UE UE_ID IMSI mod 1024 i_s index to a table containing the subframes with a radio frame used for paging N_f number of paging subframes in a radio frame that is used for paging. {4, 2, 1, 1/2, 1/4, 1/8, 1/16,1/32}

SFN System Frame Number

Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=3 1 9 N/A N/A N/A 2 4 9 N/A N/A 4 0 4 5 9

Table to determine the subframe within a radio frame that is used for paging

Source: 36.304 Section 7.1


Transition from Idle to Active: Network Triggered – Part 1 of 1

SGW HSS eNB UE

PGW

Internet MME

RRC-Idle

GTP-U-10 Tunnel

GTPC Tunnel GTPC-1 Tunnel

SGW does not have DL S1-U TEIDs for UE.

EMM-Idle

GTPC

Downlink Data Nofic. Ack

Downlink Data Notification

eNB eNB

S1AP Page (S-TMSI)

DL-SCH: Common CC: SRB0

RRC Paging (S-TIMSI)

UE Trigerred Service Request Procedure

IP Packet


Transition from Idle to Active: UE Triggered (1 of 2)

DL-SCH: Common CC

Random Access Preamble RACH

Random Access Preamble

UL-SCH: SRB0 RRC Connection Request

DL-SCH: Common CC: SRB0 RRC Connection Setup

UL-SCH: SRB1 RRC Connection Complete

NAS MSG

SGW HSS eNB UE

PGW

Internet MME

Random Access Procedure

RRC Setup Procedure

RRC-Idle

RRC-Connected

UE needs to send data GTPC Tunnel GTPC-1 Tunnel

SGW does not have DL S1-U TEIDs for UE.

EMM-Idle

GTP-U-10 Tunnel

EMM-Connected


Transition from Idle to Active – UE Triggered (2 of 2)

SGW eNB UE

PGW

Internet MME

Initial UE Message NAS MSG: Service Request, GUTI, UE Network Capability

MME looks up EMM Context based on GUTI

S1-MME

Initial Context Setup Request (UE Context Info: UE Security Capability, KeNB

DL-SCH:CCH SRB1 RRC Security Mode Command, AS Algorithm

UL-SCH: SRB1 RRC Security Mode Complete

Initial Context Setup Complete

AS Security

Data Radio Bearer-10 GTPU-10 Tunnel

GTPC Tunnel S1-MME

DL-SCH:CCH SRB1 RRC Connection Reconfig

UL-SCH: SRB1 RRC Reconfig Complete

SRB-2

SRB-1

SRB-0

GTPC

Modify Bearer Resp (IMSI, TEID)

Modify Bearer Req. (IMSI, eNB TEIDs…)

EMM-Connected


Tracking Area Update, Inter-MME – Part 1 of 3

DL-SCH: Common CC

Random Access Preamble RACH

Random Access Preamble

UL-SCH: SRB0 RRC Connection Request

DL-SCH: Common CC: SRB0 RRC Connection Setup

UL-SCH: SRB1 RRC Connection Complete

NAS MSG

SGW HSS eNB UE

PGW

Internet MME-1

Random Access Procedure

RRC Setup Procedure

RRC_Idle

RRC-Connected

TA-3 TA-5

MME-1 MME-2

SGW

MME-2

UE reads the TAI advertised by eNB and realizes that it is in a new TA.

PGW

EMM-Connected



SGW HSS eNB UE

PGW

Internet MME-1 MME-2

Initial UE Message NAS MSG: TAU Request, GUTI, UE Network Capability

MME-2 does DNS lookup based on GUTI

Context Req (GUTI)

Context Resp (IMSI, MM Cntxt, SM Cntx)

NAS MSG

GTPC Tunnel

GTPC

Modify Bearer Resp (IMSI, S1U TEID)

Modify Bearer Req. (IMSI, TEIDs…)

Location Update Request IMSI, …

Location Update Response Subscription Data

Cancel Location Request (IMSI,..)

Cancel Location Resp (IMSI,..)

MME-1 checks msg integrity

Downlink NAS transport NAS: TAU Accept( new GUTI, TAI,..)

DL-SCH: Dedicated CC: SRB1 DL Information Transfer

MME-2 allocates new GUTI to UE

NAS: TAU Accept

S1-MME



SGW HSS eNB UE

PGW

Internet MME-1 MME-2

UL-SCH: SRB1 UL Information Transfer

NAS: TAU Accept Complete

UL NAS Transport

S1-MME

S1 UE Context Release Command DL-SCH:DCH SRB1

RRC Connection Release

S1 UE Context Release Complete

RRC-Idle

EMM-Idle EMM-Idle No UE Context in eNB

NAS Msg

Irfan Ali 25

Idle mode procedures in network: Selecting an MME and finding context of UE in MME


Tracking Area and MME Service Area

•  MME Service Area is defined as the set of TAIs served by the MME. Ø  MME Service Area consists of complete TAI(s).

•  The Service Area of two MMEs can be overlapping.

TA-1

TA-2

TA-3

TA-4

TA-5

TA-6

TA-7

MME-1 MME-2

Service Area of MME-1 {TA-1, TA2, TA-3, TA-4}

Service Area of MME-2 {TA-3, TA-4, TA-5, TA-6, TA-7}


?

UE performing Tracking Area Update

•  UE in idle-mode informs the MME about its current location by performing Tracking Area Update either Ø  When the UE enters a new Tracking area (not in the UE’s TAI List), or

•  The new tracking area may be under the same MME serving the UE current TAI, or served by a new MME.

Ø  When the periodic Tracking Area Update timer expires (to let the network know that it is alive)

•  Routing to get to the old MME. Ø  For periodic TAU, the UE should provide sufficient information to the eNB to route the UE’s

message to the MME that currently holds the UE’s context.

Ø  For normal tracking area to a new MME, the new MME should be able to identify the old MME inorder to get the UE’s context from the old MME.

•  The identity used to perform routing is the UE’s temporary identity, called Globally Unique Temporary identity (GUTI) Ø  The next few slides provides and overview of how GUTI is used to route to the MME that contains

the UE’s context.

MME-1 MME-2 MME-3

(Periodic) TAU message

MME-1

MME-2 MME-3

MME-4

?

(normal) TAU message

MME-5

TAI-1 TAI-2


MME Pooling Concept

S-GW

eNB eNB eNB Cell Cell Cell Cell Cell

eNB Cell Cell

eNB Cell Cell

PDN GW

TA1 TA2

S-GW

eNB Cell Cell

MME MME

S-GW

MME

MME Pool-1 MME Pool-2

•  Pool areas can be overlapping.

•  A cell in an eNB belongs to only one TA.

•  A eNB (single cell) can be connected to multiple MMEs (belonging to more than one MME pools).

TA3 TA4 Pool Area-1

Pool Area-2


MME Identification in a pool

S-GW

eNB eNB eNB Cell Cell Cell Cell Cell

eNB Cell Cell

eNB Cell Cell

PDN GW

TA1 TA2

S-GW

eNB Cell Cell

MME MME

S-GW

MME

MME Pool-1 MME Pool-2 MME Group ID = 1 MME Group ID = 2

MMEC=1 MMEC=2 MMEC=3

MMEGI (MME Group ID)

MMEC (MME Color) Code

16 bits 8 bits

MME Pool # MME # within Pool

TA3 TA4

Pool Area-1 Pool Area-2

MMEC cannot be 1 or 2 due to overlapping pool area


UE’s NAS Temporary ID in LTE

3 BCD digits

2 or 3 BCD digits MMEGI

(MME Group ID) MMEC (MME Code)

16 bits 8 bits 32 bits

Globally Unique Temporary ID

S-TMSI M-TMSI MMEC

8 bits

40 bits

•  An M-TMSI is the unique part of a GUTI within the domain of one MME.

• A GUTI is globally unique. •  A GUTI is allocated to each UE by the serving MME. •  An M-TMSI is the uniqueness part of a GUTI within the domain of one MME.

• An S-TMSI is unique within the domain of an MME Pool. •  A UE is paged with its S-TMSI •  The UE identifies itself in a service request with the S-TMSI

S-TMSI

MME Pool # MME # within Pool

GUTI

MNC MCC MMEI (MME ID) M-TMSI GUMMEI

UEs ID used for Paging

UEs ID used In Signaling

1 1

24 bits 8 bits

1


Routing parameters provided by UE and used by eNB for Selecting MME

UE eNB MME

RRC Connection Setup Complete ( selectedPLMN-Identity, registeredMME: plmn-Identity, mmegi, mmec dedicatedInfoNAS )

S1-MME for UE

Select MME: Service request/periodic TAU: based on S-TMSI Attach w GUTI or TAU in new TA: MME ID+PLMN Attach w/o GUTI: selected PLMN-ID

RRC Connection Request ( UE Identity: S-TMSI or rand,..)

Signaling channel- SRB0

RRC Connection Setup

Signaling channel- SRB1

S-TMSI is only provided by upper layer if the cell belongs to UE’s registered TA. If S-TMSI is not provided UE generates random number

The “registered MME” ID is not provided by upper layer if the cell is in a TA the UE is already registered to, i.e in service request or periodic TA. [Ref: Section 5.3.1.1 TS24.301]

Attach request

MME Code: uniquely identifies an MME in case of over-lapping pools. Selected PLMN is used for MOCN to get to the right MME.

Irfan Ali 32

Cell-Selection and Cell-Reselection in Idle-mode: Which cell should UE “camp” on?


Low Power (Idle Mode)

Mobility

Location Area 1 Location Area 2

•  Network does not control UE’s movement. UE autonomously selects new cell as it moves.

•  Network only knows the location of the UE to the granularity of a location-area.

•  UE’s radio is in low-power state. UE’s transmitter is off. •  UE only listens periodically to control channel. If UE enters a new location area, based on

hearing information (SIB) from base-station, the UE informs the network of the new location area it has entered.

DRX Cycle

ON Duration UE Montiors

PDCCH

DRX Sleep

UE’s Receiver

Cell Reselection Instances

TAU TAU


Cell Selection vs Cell Reselection

•  Cell selection or cell-reselection is the process of UE choosing a cell.

•  Camped on a cell: UE has completed the cell selection/reselection process and has chosen a cell. The UE monitors system information and (in most cases) paging information.

Power-on

Return from Out-of-Coverage

RRC-Connected to RRC-Idle

Camped on a Cell

Camped on a different

Cell

Cell Selection Cell Re-Selection


What does the UE measure to determine if it can camp on a cell? (1 of 3) •  Reference Symbols

Ø  In order for receiver to estimate the channel, known reference symbols also referred to as pilot symbols are inserted at regular intervals within the OFDM time-frequency grid.

Ø  Using knowledge of the reference symbols the receiver can estimate the frequency- domain channel around the location of the reference symbol

Ø  The reference symbols should have sufficient high density in time and frequency to provide estimates of the entire time/frequency grid.

Ø  There are four resource elements per resource block that are dedicated to Reference Symbols.

Ø  The location of Reference Symbols depends on the Physical layer cell identity of the cell.

Ø  Once the UE has decoded the Primary and Secondary Synchronization Signals and consequently identified the Physical Layer Cell Identity, the UE is able to deduce the resource elements allocated to the Reference Signal.

7 symbols = 0.5 ms (Slot)

12 s

ubca

rrie

rs =

180

kH

z

Resource Block

Resource Elements used for Reference Symbols


What does the UE measure to determine if it can camp on a cell? (2 of 3) •  Reference Signal Received Power (RSRP)

Ø  The RSRP is the average power (in watts) received from a single Reference Signal resource element •  The power measurement is based upon the energy received during the useful part of the OFDMA symbol and

excludes the energy of the cyclic prefix.

Ø  Knowledge of absolute RSRP provides the UE with essential information about the strength of cells from which path loss can be calculated for power-control calculations.

•  Reference Signal Received Quality (RSRQ) Ø  RSRP on its own it gives no indication of signal quality. Ø  The Received Signal Strength Indicator RSSI parameter represents the entire received power

including the wanted power from the serving cell as well as all co-channel power and other sources of noise.

Ø  where N is the number of Resource blocks over which the RSSI is measured

Ø  RSRQ is always less than 1 (< 0 dB, actually < -3dB)

7 symbols = 0.5 ms (Slot)

12 s

ubca

rrie

rs =

180

kH

z

Resource Block

Resource Elements used for Reference Signals

RSRP= Energy in one Reference Signal Resource Element

RSSI = Total energy in OFDMA symbol containing Reference Signal RE

OFDMA Symbol

RSRQ = RSRP RSSI / N


What does the UE measure to determine if it can camp on a cell? (3 of 3) •  Cell Selection Criteria

Cell is selected if: Srx > 0, and Sq > 0

Measured Rx Level (dBm)

Time (s)

Measured Cell Quality (dB)

Time (s)

Srx = Rx_measured – P_comp – Rx_min

Srx

P_compensation Rx_min

Sq = Q_measured – Q_min

Q_min

Sq

Cell Selected Cell Not Selected

Irfan Ali 38

Power savings in active state: DRX in connected mode in LTE


Overveiw •  DRX allows UE to not continuously monitor the PDCCH

Ø  Leads to power-savings for UE in active state.

Ø  Configured using RRC signaling by the eNB

Ø  Per UE mechanism

Ø  The eNB keep track of UE’s DRX cycle, so that it transmits DL data to the UE only during the subframe when the UE is listening to PDCCH.

•  DRX Cycle: Specifies the periodic repetition of the On Duration followed by a period of sleep Ø  Two types of DRX cycles: Long DRX cycle, and (optional) Short DRX cycle. The Long DRX cycle is a multiple of

short DRX cycle.

•  On Duration Timer: Specifies the number of consecutive PDCCH-subframe(s) at the beginning of a DRX Cycle

Long DRX Cycle

Short DRX Cycle

ON Duration

UE Montiors PDCCH

PDCCH Physical Downlink Common CHannel Source: 36.300 (Section 12), 36.321 (Section 5.7)

DRX Sleep

UE Montiors PDCCH

DRX Sleep


RRC State Transition in LTE with Connected Mode DRX

DRX

Continuous Reception

Short DRX Long DRX Inactivity Timer

Data Transfer

RRC-CONNECTED RRC-IDLE

DRX Inactivity Timer

DRX Short Cycle Timer

Timer Expiration Data Transfer

Source: A Close Examination of Performance and Power Characteristics of 4G LTE Networks, Junxian Huang, et al, 2012


Entering DRX operation

•  Inactivity Timer: Duration in downlink subframes that the UE waits from the last successful decoding of a PDCCH which contained data for UE, till entering DRX.

•  On-duration: Duration in downlink subframes that the UE waits for, after waking up from DRX, to receive PDCCHs. If the UE receives PDCCH with data for UE, the UE stays awake and starts the inactivity timer.

Inactivity timer DRX Start Offset On Duration

Long DRX Cycle DRX Short Cycle Timer

•  All DRX parameters are signalled by eNB during RRC Connection Setup message.

•  The frame-number, x, and the subframe number, y, to start the On-duration is computed as follows: Æ  [x * 10 + y] mod (Short_DRX_Cycle) = DRX_Start_Offset mod (Short_DRX_Cycle), for Short DRX cycle Æ  [x * 10 + y] mod (Long_DRX_Cycle) = DRX_Start_Offset, for Long DRX Cycle

•  DRX Start Offset: Number of subframes.

•  Short DRX Cycle: Value in number of subframes. 2,5, 8, 10,…, 320,512,640

•  DRX Short Cycle Timer: Number of short cycles before the UE enters Long DRX Cycle

•  Long DRX Cycle: Value in number of subrame.10, 20, .. 2560 (2.56s)

DRX Sleep

UE Montiors PDCCH

PDCCH contains DL data for UE


Exiting and re-entering DRX operation

•  Inactivity Timer: Duration in downlink subframes that the UE waits from the last successful decoding of a PDCCH which contained data for UE, till entering DRX.

•  On-duration: Duration in downlink subframes that the UE waits for, after waking up from DRX, to receive PDCCHs. If the UE receives PDCCH with data for UE, the UE stays awake and starts the inactivity timer.

Inactivity timer DRX Start Offset

On Duration

Long DRX Cycle DRX Short Cycle Timer

•  All DRX parameters are signalled by eNB during RRC Connection Setup message.

•  The frame-number, x, and the subframe number, y, to start the On-duration is computed as follows: Æ  [x * 10 + y] mod (Short_DRX_Cycle) = DRX_Start_Offset mod (Short_DRX_Cycle), for Short DRX cycle Æ  [x * 10 + y] mod (Long_DRX_Cycle) = DRX_Start_Offset, for Long DRX Cycle

•  DRX Short Cycle Timer: Number of short cycles before the UE enters Long DRX Cycle

•  Short DRX Cycle: Value in number of subframes. 2,5, 8, 10,…, 320,512,640

•  DRX Start Offset: Number of subframes.

DRX Sleep

UE Montiors PDCCH

PDCCH contains DL data for UE

Active Time


Difference between Connected mode DRX and Idle mode DRX

•  Typically the DRX period of connected mode DRX is shorter than that of idle mode DRX Ø  In connected mode, there is a higher probability of data activity from

UE. Longer connected mode DRX would mean higher delay in sending the first packet to the UE.

•  Power consumption for UE in connected-mode DRX is typically greater than that during idle-mode DRX. Ø  For more details, please refer to: A Close Examination of Performance

and Power Characteristics of 4G LTE Networks, Junxian Huang, et al, 2012

•  Since smart phones generate constant dribble of traffic, with several background processes doing keep-alives, and there is too much signaling overhead in transitioning the UE to idle and then back to connected state, operators keep smartphones in connected mode for long duration of time using connected mode DRX in LTE.

Irfan Ali 44

Mobility Management in LTE

Irfan Ali


Overview

Mobility Mangement in LTE

Mobility Management in Idle-Mode

Mobility Management in Connected Mode

Cell selection/reselection Covered in previous slides

Handovers Covered next

Irfan Ali 46

Handovers, or Mobility Management in Connected Mode


Overview of Handovers

•  All handovers in LTE are prepared handovers Ø  Resources are prepared in the target eNB, before the UE

connects to the target eNB •  All handovers in LTE are UE assisted network controlled

Ø  The UE is asked to make measurements of neighbouring cells by the source eNB and report back to the source eNB.

Ø  The source eNB decides as to which target eNB the UE should be handed over to and directs the UE to that particular target eNB.


Measurement (1 of 2)

•  There is no need to indicate neighbouring cell IDs to enable the UE to search and measure a cell i.e. E-UTRAN relies on the UE to detect the neighbouring cells

•  For the search and measurement of inter-frequency neighbouring cells, at least the carrier frequencies need to be indicated

•  eNB signals reporting criteria for event-triggered and periodical reporting Ø  Events can be defined eg to be low Rx threshold on current cell, etc.

•  An NCL (network cell list) can be provided by the serving cell by RRC dedicated signalling to handle specific cases for intra- and inter-frequency neighbouring cells. This NCL contains cell specific measurement parameters for specific neighbouring cells;

•  Black lists can be provided to prevent the UE from measuring specific neighbouring cells.


Measurement (2 of 2) •  Depending on whether the UE needs transmission/reception gaps to perform the

relevant measurements, measurements are classified as gap assisted or non-gap assisted. Ø  Gap patterns (as opposed to individual gaps) are configured and activated by RRC. Ø  Intra-frequency cell measurements are non-gap assisted. Ø  Inter-frequency cell measurements may be gap-assisted based on UE’s capabilities

and the current operating frequency. The UE determines whether a particular cell measurement needs to be performed in a transmission/reception gap and the scheduler needs to know whether gaps are needed

current cell UE target cell

fcfc

Scenario C


fcfc

Scenario A


fcfc

Scenario B


fcfc


fcfc

Scenario D Scenario E


fc

fc

Scenario F

Non-Gap Assisted Measurement

Gap Assisted Measurement


Types of handovers

IMS Internet

eNB-1 eNB-2 eNB-3

MME-A

S-GW-1

S-GW-2

P-GW HSS

S1-MME

S11

S1-U

S6a S5

X2 X2 eNB-4 eNB-5 eNB-6 X2

S-GW-3

MME-B

MME-C

1 2 3 4 5

1 X2 Handover with no SGW relocation

2 X2 Handover with SGW relocation

3 S1 Handover with MME and SGW relocation

S10 S10

4 X2 Handover with no SGW relocation

X2 Handovers cannot have an MME change, i.e for an X2 HO, both the source-eNB and target-eNB have to be under the control of the same MME.

5 S1 Handover with MME and no SGW relocation

Irfan Ali 51

X2 HO with S-GW relocation


X2 HO Basics •  X2 Handovers cannot have an MME change.

Ø  Both the source-eNB and target-eNB have to be under the control of the same MME. •  X2 Handovers with S-GW relocation assumes that there is connectivity

between the Source S-GW and the target eNB. Ø  The reason being that in X2 handover the MME is informed after the X2 HO is

complete, i.e the UE has already moved to the target eNB. If the target eNB has no connectivity to the source SGW, then packet in UL and DL will be dropped untill the MME moves the SGW.

Ø  In case the target eNB is not connected to the SGW to which the source eNB is connected, only S1-HO is allowed. In S1-HO, the MME in handover preparation tells the target SGW to be ready to accept packets from the target eNB. Thus there is no interruption in traffic from the target eNB.

eNB-2 eNB-3 X2 eNB-2 eNB-3 X2

PGW

SGW

SGW

MME

SGW SGW

PGW

MME

X2 handover not allowed; only S1 HO in this case X2 handover allowed


X2-HO with Serving GW change (1 of 2) S-SGW S-eNB UE PGW MME T-SGW T-eNB

1. MME provides area restrictions to eNB for UE 2. eNB Configures measurement reporting

3. Measurement Reports

4. HO Decision

5. Handover Request

6. Admission Control

7. Handover Request Ack

Transparent Container RRCConnReconfig (CRNTI, RACH preamble) 8. RRC Connection

Reconfig

DL-SCH:CCH SRB1

9. Detach from old Cell Synch to new Cell

DL-SCH: Common CC

10. Random Access Preamble RACH

11. Random Access Preamble

Random Access Procedure (Handover)

GTP-U UL Frwd

GTP-U DL Frwd

One per EPS Bearer

X2 AP


X2 AP

X2-HO with Serving GW change (2 of 2) S-SGW S-eNB UE PGW MME T-SGW T-eNB

UL-SCH: SRB0 12. RRC Connection Request

DL-SCH: Common CC 13. RRC Connection Setup

UL-SCH: SRB1 14. RRC Connection Complete

RRC Setup Procedure

15. Path Switch Req (UE S1AP ID, TAI)

16. Selects new SGW

GTPC

20. Create Session Response(IMSI, TEIDs)

17. Create Session Request (IMSI, TEIDs, PGW IP,…)

GTPC 18.Modify Bearer Req (IMSI, TEIDs)

19.Modify Bearer Rsp (IMSI, TEIDs)

S5 Bearer Setup

GTP-U-10 Tunnel

GTPC Tunnel GTPC Tunnel

GTP-U-10 Tunnel

21. Path Switch Req Ack (S1 TEID) 22. UE Cntxt

Release

23.Releases UE resources

GTPC

25. Delete Session Response(IMSI)

24. Delete Session Request (IMSI)

S1 MME

Target eNB forwards UL packets to the Source SGW

Target eNB forwards UL packets to the Target SGW

Irfan Ali 55

S1 HO with MME change and no SGW relocation


S1-HO without Serving GW change (1 of 3)

T-MME S-eNB UE PGW S-MME SGW T-eNB 1. MME provides area restrictions to eNB for UE 2. eNB Configures

measurement reporting

3. Measurement Reports

4. HO Decision

5. Handover Required (Target eNB, target TAI)

6. Target MME chosen

Transparent Src to Target Container

12. RRC Connection Reconfig

DL-SCH:CCH SRB2

13. Detach from old Cell Synch to new Cell

X2 AP

Transparent Source to Target Container

7. Frwd Reloc Req (IMSI, target eNB)

8. Handover Request

9. Handover Request Ack

Admission Control

Transparent Target to Src Container

10. Frwd Reloc Rsp (IMSI)

Transparent Target to Src Container

11. Handover Command Transparent Target to Src Container

S10

S10

X2 AP



T-MME S-eNB UE PGW S-MME SGW T-eNB

DL-SCH: Common CC

14. Random Access Preamble RACH

15. Random Access Preamble

Random Access Procedure (Handover)

UL-SCH: SRB0 16. RRC Connection Request

DL-SCH: Common CC 17. RRC Connection Setup

UL-SCH: SRB1 18. RRC Connection Complete

RRC Setup Procedure

19. Handover Notify

20. Forward Reloc Complete

21. Forward Reloc Complete Ack

GTPC 23. Modify Bearer Req (IMSI, TEIDs)

24. Modify Bearer Rsp (IMSI, TEIDs)

22. Start timer to release resources

S1 MME

S10

GTPC Tunnel

GTP-U-10 Tunnel



T-MME S-eNB UE PGW S-MME SGW T-eNB

UL-SCH: SRB2 25. UL Info Transport

26. Uplink NAS Transport S1-MME

HSS

27. Location Update Req. IMSI, …

30. Location Update Response Subscription Data

28. Cancel Location Request (IMSI,..)

29. Cancel Location Resp (IMSI,..)

32. Downlink NAS transport NAS: TAU Accept( new GUTI, TAI,..)

DL-SCH: Common CC: SRB1 33. DL Information Transfer

31. T-MME allocates new GUTI to UE

NAS: TAU Accept

NAS Msg NAS: TAU Request

35. UE Context Release Command

36. UE Context Release Complete

34. Timer from 22. Expires

S1-MME


Summary of LTE handover

•  All handovers are prepared and network controlled. Ø  The UE is provided the slot to attempt random access also during the

preparation phase from the target eNB. Ø  “Transparent Target to Source Container” is used by the target eNB to

provide preparation information to the UE.

•  The SGW in UL direction is expected to receive packets from target eNB for the UE and forward it to the PGW before receiving path switch message from MME Ø  So the UL GTP TEID allocated for the UE by SGW for S1-U should be

unique across all eNBs connected to the SGW. Ø  The same is true for PGW from SGW.

lte location management and mobility management

Technology