LTE: Timing Advance and Time Alignment Timer

Timing advance is a negative offset, at the UE, between the start of a received downlink subframe and a transmitted uplink subframe. This offset at the UE is necessary to ensure that the downlink and uplink subframes are synchronised at the eNodeB.

A UE far from the eNodeB encounter a larger propagation delay so its uplink transmission is somewhat in advance as compared to a UE closer to the eNodeB. As an example (shown in the figure below), let us consider two UEs; UE1 is located far from the eNodeB and UE2 is located close to the eNodeB. Let δ1 be the propagation delay experienced on the downlink for UE1 and δ2 is the propagation delay experienced on the downlink for UE2. Since UE1 is located far from the eNodeB as compared to UE2, we can safely assume that δ1 > δ2. Let us say that the eNodeB has transmitted subframe #n at time t1 which is seen by UE1 at time t1+δ1 and UE2 at time t1+δ2. Both UE1 and UE2 take the downlink subframe arrival (together with Timing Advance) as a reference to calculate uplink subframe timing.

The Timing Advance is equal to 2 x propagation delay assuming that the same propagation delay value applies to both downlink and uplink directions. So, UE1 needs to start it’s uplink at t1+2δ1 whereas UE2 should start it’s uplink at t1+2δ2. This will ensure that both of the uplink transmissions (from UE1 and UE2) reach the eNodeB at the same time which means that at the eNodeB, both uplink and downlink subframes are time aligned.

If the Timing Advance is not applied, then the start of uplink transmission from UE2 for subframe #n+1 will overlap with the end of uplink transmission from UE1 for subframe #n. Assuming that same resource blocks are assigned to UE1 in subframe #n and UE2 in subframe #n+1, this overlap creates interference which causes reception failures at the eNodeB. If a proper value of Timing Advance is applied, then these subframes won’t collide.

The eNodeB continuously measures timing of uplink signal from each UE and adjusts the uplink transmission timing by sending the value of Timing Advance to the respective UE. As long as a UE sends some uplink data (PUSCH/PUCCH/SRS), the eNodeB can estimate the uplink signal arrival time which can then be used to calculate the required Timing Advance value.

Timing Advance Command in LTE

The eNodeB estimates the initial Timing Advance from PRACH sent by the UE. PRACH is used as timing reference for uplink during UE’s initial access, radio link failure, during Handover etc…The eNodeB sends Timing advance command in Random Access Response (RAR). Once the UE is in connected mode, the eNodeB keep estimating Timing Advance and sends Timing Advance Command MAC Control Element to the UE, if correction is required.

The UE shall adjust the timing of its uplink transmission at subframe #n+6 for a Timing Advance Command received in subframe #n

The UE shall adjust the timing of its transmissions with a relative accuracy better than or equal to ±4*T_S seconds to the signalled timing advance value compared to the timing of preceding uplink transmission

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

N_TA 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

    Timing Advance Command in MAC RAR

The Timing Advance Command in RAR takes 11 bits and it can indicate an index value T_A (0, 1, 2… 1282) which used to control the amount of timing adjustment that UE has to apply. The amount of the time alignment is given by N_TA = T_A × 16. The Timing Advance obtained via RAR is always positive

Example1 (T_A = 0): When the received T_A = 0 ⇨ N_TA = 0 so no timing adjustment required.

Example2 (T_A = 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

Example3 (T_A = 1282): When the received 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

The maximum distance value (of slightly above 100Km) would facilitate cell radius of up to 100Km.

    Timing Advance Command MAC CE

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). 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

Timing Advance command in MAC RAR and MAC CE are illustrated below

Uplink Time Alignment

It was discussed above how the eNodeB controls Timing Advance that each UE has to apply. Once the UE gets a Timing Advance Command, UE applies it and now the question is for how long the UE uses the received Timing Advance value?

The eNodeB provides the UE with a configurable timer called timeAlignmentTimer. TimeAlignmentTimer is used to control how long the UE is considered uplink time aligned

The value of the timer is either UE specific (timeAlignmentTimerDedicated) and managed through dedicated signalling between the UE and the eNodeB, or cell specific (timeAlignmentTimerCommon) which is indicated in SIB2. In both cases, the timer is normally restarted whenever a new Timing Advance is given by the eNodeB. At the time of restart, the timer is restarted to a UE specific value if configured; otherwise it is restarted to a cell specific value

The UE starts/restarts the TimeAlignmentTimer based on the condition when it received the Timing Advance Command.

• The Timing Advance Command is received in MAC RAR but timeAlignmentTimer is not already running: This case may arise in situations like, timeAlignmentTimer has expired (connected mode), Initial access from RRC_IDLE, during RRC Connection Re-establishment procedure etc…After the reception of RAR, the UE shall apply the Timing Advance Command value received in RAR and start timeAlignmentTimer. If the contention is not resolved/successful, then the UE stops timeAlignmentTimer, else, the UE continues running the timer
• The Timing Advance Command is received in MAC RAR as part of non-contention based Random Access procedure (ex: PDCCH Order):  After the reception of RAR, the UE shall apply the Timing Advance Command value received in RAR and starts/restart the timeAlignmentTimer
• The Timing Advance Command is received in MAC RAR as part of contention based Random Access procedure in connected mode and timeAlignmentTimer is already running: This could be in situations like UE is requesting for uplink resources but UE doesn’t have valid PUCCH resources for SchedulingRequest etc…After the reception of RAR, the UE shall ignore the received Timing Advance Command value and shouldn’t restart the timeAlignmentTimer
• When a Timing Advance Command MAC CE is received, the UE shall apply the received value of Timing Advance Command value and start/restart timeAlignmentTimer

As discussed before, the eNodeB continuously measures timing of uplink signal from the UE and adjusts the uplink transmission timing by sending the Timing Advance Command to the UE. If the UE has not received a Timing Advance Command until the expiry of timeAlignmentTimer, the UE assumes that it has lost the uplink synchronization. Hence, prior to any PUSCH/PUCCH/SRS transmission in the uplink, an explicit timing-re-alignment phase using the random access procedure must be performed to restore the uplink time alignment

The UE shall perform the following actions upon the expiry of timeAlignmentTimer:

• Flush all HARQ buffers;
• If configured, release PUCCH resources of Periodic CQI and Scheduling Request, and also SRS configuration. By doing so, the UE doesn’t perform transmission of SRS/PUCCH while timeAlignmentTimer is not running. The eNodeB has to configure these parameters again in order for the UE to transmit SRS/Periodic CQI/Scheduling Request after UE starts timeAlignmentTimer
• Clear configured downlink assignments and uplink grants. i.e., release SPS grant (uplink) and assignment (downlink) if configured

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

For multiple Timing Advances in uplink Carrier Aggregation concepts in Release-11, refer to the post here.

Reference: 3GPP TS 36.321, 36.213, 36.331, 36.133, 36.300