Tomlinson Harashima Precoding for Multi-User MIMO-OTFS with Reduced Overhead

This paper investigates spatial domain hybrid digital-analog beamforming (BF) and Delay-Doppler (DD) domain Tomlinson-Harashima precoding (THP) for downlink mul-tiuser multiple-input and multiple-output (MU-MIMO) Orthogonal Time Frequency Space (OTFS) transmission over a millimeter wave (mmWave) channel. The main challenge of this problem is that the matrix transform of the equivalent channel matrix has high complexity, which scales cubically with the product of the numbers of subcarriers, time slots, and antennas. To avoid high-complexity computation, we propose a two-step precoding scheme to cancel interference involved in the MU-MIMO-OTFS downlink transmission, with low computational complexity and signaling overhead. Our numerical results demonstrate that our proposed scheme achieves a good bit error rate (BER), which decreases linearly with transmit SNR for all users without a floor. Numerical results on the achievable sum rate are also presented, revealing that the scheme can utilize the full degree of freedom (DoF) and will not be damaged severely by the overhead.