4.3 Timing advance
25.2243GPPPhysical layer procedures (TDD)TS
UTRAN may adjust the UE transmission timing with timing advance. The initial value for timing advance (TAphys) will be determined in the UTRAN by measurement of the timing of the PRACH or E-RUCCH. The required timing advance will be represented as an 8 bit number (0-255) ‘UL Timing Advance’ TAul, being the multiplier of 4 chips which is nearest to the required timing advance (i.e. TAphys = TAul 4 chips).
When Timing Advance is used the UTRAN will continuously measure the timing of a transmission from the UE and send the necessary timing advance value. On receipt of this value the UE shall adjust the timing of its transmissions accordingly in steps of ±4chips. The transmission of TA values is done by means of higher layer messages. Upon receiving the TA command the UE shall adjust its transmission timing according to the timing advance command at the frame number specified by higher layer signaling. The UE is signaled the TA value in advance of the specified frame activation time to allow for local processing of the command and application of the TA adjustment on the specified frame. Node-B is also signaled the TA value and radio frame number that the TA adjustment is expected.to take place.
If TA is enabled by higher layers, after handover the UE shall transmit in the new cell with timing advance TA adjusted by the relative timing difference t between the new and the old cell:
TAnew = TAold + 2t.