LTE Timing Advance Command

What is Timing advance in LTE and why is it required?

As UE is not always stationary with respect to eNB, the variation in distance between UE and eNB will reflect in UL transmission time (msg received at eNB). Due to the change, eNB encounters interference as UL transmission of different UE's can be overlapping.

How is it achieved?

To avoid this kind of interference, eNB will either periodically or need basis correct the UE to either advance or delay the UL transmission with a granularity of 16 Ts(basic time unit).
UE receives the Timing Advance (TA) in either the MAC CE Timing Advance command or in RAR during RACH procedure.

1. if UE receives the TA in MAC CE or
   if UE receives the TA in RAR and if preamble index is not selected by UE or
   if UE receives the TA in RAR and if time alignment timer is not running
• apply the Timing Advance command
• start or restart time alignment timer
2. else ignore the Timing Advance command

The new correction is valid till the expiry of time alignment timer. As cyclix prefix (CP) is added to start of each OFDM symbol, the overlapping interference at eNB can be ignored till the time it's not exceeding the Tcp (Duration of Cyclix Prefix).

What happens if Time alignment timer is expired?

When Time alignment timer is expired, UE is assumes to have lost UL sync with eNB and

1. releases PUCCH/SRS configuration
2. flush all HARQ buffers
3. clear any configured UL & DL grants

How does UE obtain the time alignment timer configuration?

Time alignment timer value is received in SIB-2 (common to all UE's) and mac-MainConfig (UE specific value). 

UE applies the value received in SIB-2 when moving from idle to connected mode or during rrc connection reestablishment radio link recovery phase.

When UE receives dedicated time alignment timer in either the rrc connection setup or rrc connection reconfiguration, UE will apply the new value during the next timer restart.

TimeAlignmentTimer ::= ENUMERATED {
sf500, sf750, sf1280, sf1920, sf2560, sf5120,
sf10240, infinity}

References

1. 3GPP 36.213
2. 3GPP 36.321