DOES UPLINK TIME ALIGNMENT REMAIN VALID FOR THE UE ALL THE TIME?

Once uplink time alignment has been achieved for the UE, as long as a UE keeps on carrying out uplink data transmission, time alignment will continue to take place for the UE. The continuous transmission can be used by the receiving cell site to estimate the uplink receive timing and can be used as a source for the timing-control commands. When there is no data available for uplink, the UE may still carry out transmission of sounding reference signals (SRS) with a certain period, to continue to enable uplink receive-timing estimation and retain uplink time alignment. In this way, the UE can immediately restart uplink-orthogonal data transmission without the need for a timing re-alignment phase.

If the UE does not have uplink data to transmit for a longer period, no uplink transmission should be carried out. In that case, uplink time alignment may be lost and restart of data transmission must then be preceded by an explicit timing-realignment phase (random access) to restore the uplink time alignment.