Timing Advance Calculation and Timing alignment timer

For calculating timing advance, parameters to consider are below.

1. Propagation delay – The time taken for UE to reach node B and vice versa. This is variable and need to be calculate every time when there is a change in distance between UE and node B. keep in mind that this is to be calculated in RTT(round trip time).
2. DL/UL switch time – This is the time taken by the node B to switch between uplink to downlink and vice versa. this is constant depending on the operating frequency range i.e for FR1 it should be 10usec and for FR2 it should be 3us.
3. nodeB to nodeB sync time – This is the time for Node B transmitter to become active and the delay associated with it.

Timing advance  = 2*propagation delay + NTA offset*Tc

NTA offset * Tc To guarantee that the uplink frame is finished before the start of the next downlink frame, this is mentioned in terms of  Tc whose value is constant  1/(480000*4096) sec.

n-timing advance offset is the information element to instruct the UE which value to be applied for NTA offset. this indication will happen through SIB1 or by dedicated signalling. possible values are 0, 25600 and 39936. default value is 25600 which UE applies when no specific indication to UE.

Timing advance needs to be maintained at the MAC control element which can be included as part of the Mac header. below is the description of the MAC header for timing advance.

TAG – timing advance group 2bits is a group of cells which share same uplink transmission timing i.e they share same downlink timing reference and same timing advance commands. cells belonging to different TAG are likely to be geographically separated and subsequently have different propagation delays.

Time Alignment Timer:

This timer is configured for each TAG defines the maximum time a UE can remain uplink synchronised without having received TA command from base station. ie UE assumes it lost the sync once it receives timing advocate command. This timer will be restarted each time UE receives a timing advance command for TAG.

Once this timer expires all HARQ buffers are flushed, all PUSCH and SRS configurations are released and all uplink and downlink resource allocations are cleared up and until the start random access procedure to resync.