Timing Advance Commands:-
After initializing a mobile’s timing advance using the random access procedure, the base
station updates it using MAC control elements known as Timing advance commands. Each
command adjusts the timing advance by an amount ranging from −496T s to +512T s , with a
resolution of 16T s [3]. This corresponds to a change of −2.4 to +2.5 km in the distance between
the mobile and the base station, with a resolution of 80 m.
The mobile expects to receive timing advance commands from the base station at regu-
lar intervals. The maximum permitted interval is a quantity known as timeAlignmentTimer,
which can take a value from 500 to 10 240 subframes (0.5 to 10.24 s) or can be infinite if
the cell size is small [4]. If the time elapsed since the previous timing advance command
exceeds this value, then the mobile concludes that it has lost timing synchronization with the
base station. In response, it releases all its PUSCH and PUCCH resources. Any subsequent attempt to transmit will trigger the random access procedure, through which the mobile can recover its timing
synchronization.