Timing Advance for LTE-A

LTE-A TIMING ADVANCE

·         Timing Advance is a MAC CE that is used to control Uplink signal transmission timing. Network (eNodeB in this case) keep measuring the time difference between PUSCH/PUCCH/SRS reception and the subframe time and can send a 'Timing Advance' command to UE to change the PUSCH/PUCCH transmission to make it better aligned with the subframe timing at the network side.

·         If PUSCH/PUCCH/SRS arrives at the network too early, network send a Timing Advance command to UE saying "Transmit your signal a little bit late", If PUSCH/PUCCH/SRS arrives at the network too late, network send a Timing Advance command to UE saying "Transmit your signal a little bit early".

Figure 1:  Figure showing Timing advance Mechanism

 Understanding the TA mechanism:

1.      In idle mode the Timing is in sync with Last received Timing Advance command received by the UE. And if the UE changes the Cell in Idle Mode, the New serving cell will again send the Timing Advance value according to which UE will restart the Time Alignment Timer.

2.      As soon as any service request is placed by the UE, In the RACH response eNodeB will send the TA command (TA_VALUE). It will be a 11 bit command ranging from 0 – 1282

3.      TA_VALUE = (TIMING COMMAND)*16

Timing command is 6 bit value ranging from 0-63.

Figure 2:  Figure showing Timing advance in Layer 3 message (RACH response)

4.      The timing advance command indicates the change of the uplink timing relative to the current uplink timing as multiples of 16Ts.

5.      NTA is the timing offset between uplink and downlink radio frames at the UE, expressed in units of Ts. where T_s = 1/ (2048x15000) = 1/30720000 sec.

6.      Example (T_A = 0): When the received TA = 0 ⇨ NTA = 0 so no timing adjustment required.

Example (TA = 1): If T_A = 1 ⇨ Timing Adjustment = N_TA = 16 Ts = 16/30720000 sec = 0.5208 μs ⇨ Distance = (3x10⁸x0.5208x10^-6)/2 = 78.12m which is the minimum

Example (TA = 1282): If T_A = 1282 ⇨ N_TA = 1282x16Ts = 1282x16/30720000 sec = 667.66 μs ⇨ Distance = (3x10⁸x667.66x10^-6)/2 = 100.15Km which is the maximum propagation distance.           

7.      In uplink the Timing advance information is sent with the name of “Time Alignment Timer”. It has two modes.

·         timeAlignmentTimerCommon (Common for All UEs in a Cell) included in SIB2.

·         TimeAlignmentTimerDedicated (UE specific value for Time Alignment Timer) is included in the RRC Connection Reconfiguration Message. It tells UE how long it should consider itself to be time aligned in uplink in unit of subframe. (500, 750, 1280, 1920, 2560, 5120, 10240 or infinity subframe).

Figure 3:  Figure showing Timing advance in Layer 3 message (SIB 2)

8.      In the case of Timing Advance Command MAC CE, it indicates relative Timing Advance which is 6-bit index value T_A (0, 1, 2… 63).

9.      In this case, N_{TA, new} = N_{TA, old} + (T_A − 31) ×16 where N_{TA, old} is the current timing adjustment and N_{TA, new} indicates new value. Here, adjustment of N_TA value by a positive or a negative amount indicates advancing or delaying the uplink transmission timing by a given amount respectively

10.  Example : TA = 30 then NTA,new = NTA,old + (30 - 31) x 16 Ts = NTA,old - 1 x 16 Ts. Distance variation equals -1 x 16 x 4.89 m = - 78.12 m
Distance variation maximum = - 31 x 16 x 4.89 m = - 2.42 km or + 32 x 16 x 4.89 m = + 2.5 km (1.55 mi)

11.  Timing Advance Command Received in the Nth Subframe Applied to (N+6)th subframe.

12.  The UE shall not perform any uplink transmission except the Random Access Preamble transmission when TA Timer is not running.

Multiple Timing Advance (Carrier Aggregation Scenario):

The use of multiple timing advances is required for the support of non-collocated cells with Carrier Aggregation. Assuming synchronization to the macro cell’s PCell is already obtained, the UE next has to synchronize to the SCell of the other site. Therefore the PCell eNB will request a RACH on SCell immediately after SCell activation. The RACH request is being sent by PDCCH signalling from the PCell

Figure 4: Multiple Timing Advance Transmission

In case several carriers require the same timing advance, these carriers will be grouped in so called timing advance groups (TAG) with the same timing advance.

Figure 5: Timing Advance Groups

·         If a TAG contains the PCell, it is referred to as the primary timing advance group (pTAG). If a TAG contains only SCeII(s), it is denoted as the secondary timing advance group (sTAG). There is one timing reference cell and one time alignment timer (TAT) per TAG, and each TAT may be configured with a different value.

·         For pTAG, the PCell is used as the timing reference cell, whereas for sTAG, the UE may use any activated SCell from the same sTAG as the timing reference cell.

·          From an RF requirement point of view. The number of component carriers is limited to two for LTE Ref-11: thus, if the sTAG is configured, there is only one SCell in the sTAG. The initial uplink timing alignment of the sTAG is obtained by an eNB initiated random access procedure similar to the pTAG.

·         The SCell in a sTAG can be configured with random access channel resources, and the eNB may instruct the UE to perform random access on the SCell. The Msg2 (or random access response) in response to the SCell preamble is transmitted on the PCell using an RA-RNTI that conforms to the LTE Rel-8 RACH procedure.

·         The grant in Msg2 is valid for the SCell in which the preamble was transmitted. The UE stops transmission of the random access preamble on the SCell when reaching the maximum number of transmissions. However, the UE will not indicate a random access problem to upper layers, if the maximum number of preamble transmissions is reached for the random access procedure on the SCell. The UE tracks the downlink frame timing change of the SCell and adjusts the uplink transmission timing following the timing advance commands from the eNB.

Figure 6: Timing Advance Groups (TAG) Format in L3 message

·         Previously reserved values are now modified to indicate a new 2-bit Timing Advance Group Identity (TAG Id). The 6-bit Timing Advance Command field is unchanged compared to Release-8. Since the TAG Id field is 2-bits, it can only indicate values from 0 to 3. The TAG containing the PCell has TAG Identity 0. So, at most three sTAGs can be configured.