Low-Complexity Precoder Design for MU-MIMO OTFS Networks

Abstract

Delay-Doppler (DD) domain transmission schemes, such as Orthogonal Time Frequency Space (OTFS), have attracted significant attention due to their unique properties, particularly their robustness in doubly-dispersive channels. One challenging problem in OTFS framework is the precoding for Multi-User MIMO (MU-MIMO) transmissions due to the increased computational complexity. In this letter, we propose two methods to perform low-complexity precoding for OTFS in MU-MIMO systems in Delay-Doppler (DD) domain. The first method leverages the diagonalization properties of the Doubly Block Circulant (DBC) channel matrix under the assumption of ideal pulse shaping. The second method benefits from the equivalent Time-Frequency (TF) domain representation of the signal to perform precoding in DD domain without any ideal pulse shaping assumptions. We describe how both methods can be employed to perform Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE) precoding in OTFS signals in a low-complexity manner. Numerical results show that our proposed methods minimize multi-user interference and achieve higher received SNR compared to benchmark schemes.