On Design of Tomlinson-Harashima Precoded OTFS System

An orthogonal time frequency space (OTFS) scheme that operates in the delay–Doppler (DD) domain has competitive performance in high-mobility wireless environments. However, its complexity for tackling the interferences among the DD grids remains too high for many practical applications. In this paper, we study a DD-domain Tomlinson-Harashima precoding (THP) approach for OTFS system. First, a new DD-domain selective zero-padding (SZP) scheme is developed, which addresses the non-causality of the time–frequency doubly-selective channel (DSC) matrix is introduced. Based on that, THP that pre-cancels the DD domain interference on a symbol-by-symbol basis is carried out, where the requirement of large matrix inversion is avoided. Further, we derive the exact a posteriori probability (APP) for optimal soft detection in the THP-OTFS system with state-of-the art channel codes. Numerical results demonstrate that our proposed THP-OTFS scheme brings up to 5 dB improvement in complex multipath scenario over the benchmark system operated with linear precoding. For the channel-coded THP-OTFS system, our developed optimal soft detection brings about 2 dB improvement over existing THP method. Also, the complexity is significantly reduced relative to the linear precoding scheme.