Different UEs within a cell will experience different propagation delay to/from the cell site, depending on their exact position within the cell coverage area. If UEs set their transmit timing based only on the timing of the received downlink timing, their corresponding uplink transmissions will arrive at the cell site with potentially very different timing. If these receive-timing differences are too large, the Orthogonality between the uplink transmissions of different UEs will not be retained. Thus, an active uplink transmit-timing control is needed to ensure that uplink transmissions from different UEs are received with approximately the same timing at the cell site.
The transmit-timing control operates such that the network measures the received uplink timing of the different UEs. If the received timing of a specific UE is “lagging” behind, the UE is commanded to advance its transmit timing a certain amount. Similarly, a UE can be ordered to retard it transmit timing.
The steps by which the UEs can advance/retard their timings are multiple of 16⋅Ts ≈ 0.52 μs. The timing-control commands are transmitted as higher-layer signaling (MAC) to the UEs.