Uplink Time Synchronization for Non-Terrestrial Networks

Uplink time synchronization techniques for Non-terrestrial Network (NTN) in 3GPP Release 17 are introduced. Uplink time synchronization is important for NTN because the uplink waveform is based on OFDM (thus requiring the transmission from all users to arrive within the cyclic prefix), and it is challenging in the case of LEO and MEO, because the propagation delay changes orders-of-magnitude faster than that in a typical terrestrial network. The uplink time synchronization follows a dual-loop approach: an open loop that calculates the service link delay based on the UE location and the satellite location and the feeder link delay based on parameters received from the network, and a closed loop that accumulates timing advance (TA) commands, intended to correct any residual timing error from the open loop. We examine the double correction issue resulting from the interaction between the two control loops and explain the 3GPP specification that addresses the issue. We also explain how to calculate the timing advance with examples.