Timing Advance

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Some information related to Timing Advance in LTE (RL10)

Basics

4 © Nokia Siemens Networks LTE RL1/CH_4 (SFS BLSv8.3) / Peter Stöckl / January 2010 Confidential

L1 Procedures & eNB measurements

PHY Layer Procedures

Timing Advance

• Used for alignment of UL/DL sub-frame start at eNB

– eNB shall measure the initial timing advance and perform period update of UL timing correction

– eNB shall send timing advance command to UE

• TA measurements on

– RACH preamble

– Periodic update of UL timing for UE using ▪ Available UL transmission on PUSCH

▪ SRS

▪ Periodic CQI reports on PUCCH

• Two types of TA commands

– Initial TA command  11-bit timing advance command

– Continuous TA measurement  6-bit timing advance command

• Timing adjustment shall apply from sub-frame n+6 in case TA command is received on sub-frame n

5 © Nokia Siemens Networks LTE RL1/CH_4 (SFS BLSv8.3) / Peter Stöckl / January 2010 Confidential

L1 Procedures & eNB measurements

PHY Layer Procedures

Timing Advance

Uplink-Downlink frame timing

Transmission of the UL radio frame from UE shall start at (NTA+ NTA offset) x TS

before the start of the corresponding DL radio frame at the UE TS= 1 / 30.72MHz = 32.552ns

NTA offset= 0 for FDD / NTA offset= 624 for TDD / 0 ≤ NTA≤ 20512 Initial Timing Advance

11-bit TA command with TA= 0, 1, 2, …, 1282

NTA= TAx 16 Therefore TA granularity =

16 x 32.552ns ≈ 0.52µs

Continuous Timing Advance

6-bit TA command with TA= 0, 1, 2, …, 63 NTA,NEW= NTA,OLD+ (TA - 31) x 16 Therefore pos/neg. adjustment in

steps of 0.52µs

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Maintenance of Uplink Time Alignment (according to 3GPP)

The UE has a configurable timer timeAlignmentTimer which is used to control how long

the UE is considered uplink time aligned.

The UE shall:

• when a Timing Advance Command MAC control element is received:

o apply the Timing Advance Command;

o start or restart timeAlignmentTimer.

• when a Timing Advance Command is received in a Random Access Response

message:

o if the Random Access Preamble was not selected by UE MAC:

 apply the Timing Advance Command;

 start or restart timeAlignmentTimer.

o else, if the timeAlignmentTimer is not running:

 apply the Timing Advance Command;

 start timeAlignmentTimer;

 when the contention resolution is considered not successful, stop

timeAlignmentTimer.

o else:

 ignore the received Timing Advance Command.

• when timeAlignmentTimer expires:

o flush all HARQ buffers;

o notify RRC to release PUCCH/SRS;

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Involved Parameters

Abbreviated Name

taTimer

Parameter Id 53557

MO Class LNCEL

Parameter relationships taTimerMargin must be less or equal than (taTimer-10ms)

Short description Determines the number of subframes after which a UE assumes it is out-of-sync in UL if no Time Alignment command was received.

Description Determines the number of subframes after which a UE assumes it is out-of-sync in UL if no Time Alignment command was received.

Full name Time Alignment timer

3GPP Name timeAlignmentTimer

Range and Step 500 (0), 750 (1), 1280 (2), 1920 (3), 2560 (4), 5120 (5), 10240 (6)

Default value 1280 (2)

Default value notes Hidden

Internal value

Modification BTS restart needed

Abbreviated Name

taMaxOffset

Parameter Id 53556

MO Class LNCEL

Parameter relationships

Short description Determines the maximum allowed Time Alignment offset.

Description Determines the maximum allowed Time Alignment offset. If the value is exceeded, TA command is sent to the UE to adjust UL timing.

Full name Time Alignment maximum offset 3GPP Name

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Default value 0.52 us

Default value notes Corresponds to 16*Ts Hidden

Internal value internal_value = gui_value * 100 Modification Requires object locking

Abbreviated Name

taTimerMargin

Parameter Id 65249

MO Class LNCEL

Parameter relationships taTimerMargin must be less or equal than (taTimer - 10ms)

Short description The parameter defines lead with respect to the taTimer expiration time for starting to send the periodic timing advance command.

Description The parameter defines lead with respect to the taTimer expiration time for starting to send the periodic timing advance command.

Full name Time alignment timer margin 3GPP Name

Range and Step 0...2560 subframes, step 1 subframes Default value 89 subframes

Default value notes Hidden

Internal value internal_value = gui_value Modification Not modifiable

Abbreviated Name

taCmdMaxRetry

Parameter Id 53117

MO Class LNBTS

Parameter relationships

Short description The number of times the timing advance command is retried before the LTE MAC assumes the UE has gone out-of-synch.

Description The number of times the timing advance command is retried before the LTE MAC assumes that the UE has gone out-of-synch. This parameter is vendor-specific.

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Full name Maximum number of time alignment command retries 3GPP Name

Range and Step 1...10, step 1

Default value 1

Default value notes

Hidden Yes

Internal value internal_value = gui_value Modification Not modifiable

Abbreviated Name

taOffScheMarg

Parameter Id 53118

MO Class LNBTS

Parameter relationships

Short description Determines the time alignment offset limit for the uplink sceduler to stop considering the UE for scheduling.

Description Determines the time alignment offset limit for the uplink scheduler to stop considering the UE for scheduling. This parameter is vendor-specific.

Full name Time alignment offset margin for scheduling 3GPP Name

Range and Step 1.5...4 us, step 0.1 us

Default value 2 us

Default value notes

Hidden Yes

Internal value internal_value = gui_value * 10 Modification Not modifiable

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Maintenance of Uplink Time Alignment (according SFS)

(in figures)

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Maintenance of Uplink Time Alignment (according SFS)

(in bullets)

• UL TA update shall be done

o Periodically

o On per-need basis

• Timing Alignment Timer shall be started/restarted whenever an initial TA or TA

update command is received. (Exception: see 3GPP block above)

• If timer expires UE detects out-of-sync status

• Interval between periodic TA update commands is calculated as

taTimer - taTimerMargin

• Per-need-TA update is required if

reported time alignment offset > taMaxOffset

• If reported time alignment offset > taSchedulingThreshold then UL scheduler is

informed that UE is drifting out of alignment

taSchedulingThreshold = max(taOffScheMarg; taMaxOffset + 0.5

μs)

• HARQ ACK feedback is received for TA command

 For a per-need TA command and when taSchedulingThreshold was

exceeded inform UL scheduler that the TA is OK again

 The periodic timing alignment timer is (re)started

• NO HARQ ACK feedback is received for a TA command within max number of DL

HARQ transmissions

 TA update command shall be repeated up to taCmdMaxRetry or until the

timing alignment timer expires

 If maximum number of retries is exceeded or the timing alignment timer has

expired, then status UE UL out-of-sync id detected.

Figure

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References

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