Impacts of Time-Varying Channel Spreading Function on the OTFS Modulation System

The channel spreading function (CSF) could be time-varying due to the interference and “birth and death” of inseparable propagation paths, whose impacts need to be considered when designing delay Doppler (DD) domain multiplexing systems. In this correspondence, the digital input/output (I/O) relationship under rectangle and bi-orthogonal pulses for one of the DD domain multiplexing schemes, orthogonal time frequency space (OTFS) modulation, is derived considering the time-varying CSF with off-grid delay and off-grid Doppler shift. Therein, the time-varying CSF is modeled by the first-order autoregressive random process (AR(1)). The impacts of time-varying CSF on OTFS are studied regarding the sparsity of effective DD domain channel, the achievable bit rate, and the bit error rate (BER) under different equalizers. Through simulation, it is revealed that time-varying CSF reduces the bit rate under least square (LS) and zero-forcing decision feedback equalization (ZF-DFE) equalizers, but decreases BER with higher multipath diversity.