Initial Time Synchronization for OTFS
Char‐Dir Chung, Min-Zhi Xu, Wei-Chang Chen · IEEE Transactions on Vehicular Technology · 2024
Initial time synchronization (ITS) is investigated for orthogonal time frequency space systems operating over the linear time-varying multipath channels exhibiting fractional delay and Doppler shifts on the delay-Doppler grid. Particularly, a comb-type preamble waveform carrying constant-amplitude preamble symbols is designed to meet the constraint of zero discrete ambiguity function and thereby facilitate accurate ITS. The receiver first measures the discrete cross-ambiguity values between the received signal and local discrete-frequency-shifted comb-type preamble signals for discrete time shifts sequentially. With the measured discrete cross-ambiguity values, the preamble-comb-based ITS approach using sequential threshold comparison and select-the-largest identification is then developed to estimate the start time of a received preamble frame on the leading channel path. Preamble-comb-based coarse and fine ITS systems are investigated to operate over a wide timing acquisition range (TAR) under the influence of data self-interference (DSI) and over an adjustable narrow DSI-free TAR, respectively. When the channel exhibits a strong leading path and wide ranges of fractional Doppler shifts, the preamble-comb-based ITS systems are shown to outperform several conventional ITS systems significantly in estimation robustness and accuracy for sufficiently high preamble signal-to-noise power ratios.