The eNB measures the required timing advance based on the received UE signal arrival time. It commands the UE to adjust the transmission time. This is performed on a per-need basis.
It is signaled by means of a special MAC control element; LCID = 11101. The signaled granularity is 16 Ts. The value range for adjustment is 8 bit and related to the current UL timing.
As Ts. = 1 / (2048 x 15000) sec = 1 / 30720000 sec the granularity is given by 0.52 μsec (corresponding to 78 m).
If UE is in-sync the timing advance is valid. Otherwise the RACH-procedure establishes a valid timing advance. An 11 bit value (range 0,1..1282) is signaled to establish the initial offset.
The BASIC Behind TA is as per Below.
For GSM A Timing Advance (TA) is used to compensate for the propagation delay as the signal travels between the Mobile Station (MS) and Base Transceiver Station (BTS). The Base Station System (BSS) assigns the TA to the MS based on how far away it perceives the MS to be. Determination of the TA is a normally a function of the Base Station Controller (BSC), bit this function can be handled anywhere in the BSS, depending on the manufacturer.
TA is basical calculation of Path (Propagation Delay) Distance covered by Signal (Movement from Mobile to BTS ), It may be Direct Path or Indirect Path it value is 0 to 63 ,Default value is 63 It is Maximum Distance B/w MS and BTS , 1 TA value equal to 550m, i.e 0.5×63 =31.5km we can restrict this Property by Dmax(BTS Parameter)