Pre-Equalized Multi-User OTFS

OTFS faces inter-symbol interference (ISI) due to delay and/or Doppler spreads, which requires compensation with an equalizer. In addition, in multiple access scenarios where users traverse distinct channels, OTFS also encounters multi-user interference (MUI). Hence, MUI is more challenging to mitigate than intra-user ISI and becomes more severe in high-mobility scenarios, where OTFS is typically deployed. This paper proposes a novel method called pre-equalized multi-user OTFS (PreEQ-MU-OTFS), which applies pre-equalization at the transmitter, using linear precoders for each user to fundamentally mitigate MUI. Specifically, the precoder is derived by initializing with the modified Wiener filter and employing a single step of iterative minimization. Additionally, the receiver in this method requires only per-user scaling, instead of full equalization. By leveraging the property of OTFS whereby fast-varying channels exhibit nearly time-invariant characteristics in the delay-Doppler (DD) domain, PreEQ-MU-OTFS performs effectively in high-mobility channels. The proposed method demonstrates superior bit error rate (BER) performance compared to conventional post-equalization receivers such as minimum mean square error (MMSE), MMSE-successive interference cancellation (MMSE-SIC), message passing (MP), and maximum-ratio combining (MRC) receivers. This method is validated in realistic channel scenarios with moving reflectors and practical channel aging levels.