Interference Cancellation and Iterative Detection for Orthogonal Time Frequency Space Modulation

TLDR

The recently proposed orthogonal time‑frequency‑space (OTFS) modulation offers significant error‑performance advantages over OFDM in delay‑Doppler channels. The paper derives the explicit input‑output relation for OTFS modulation/demodulation and develops a low‑complexity message‑passing algorithm for joint interference cancellation and symbol detection. The authors analyze ideal bi‑orthogonal and practical rectangular pulse shapes, characterize resulting inter‑Doppler, inter‑carrier, and inter‑symbol interferences, and propose a message‑passing algorithm that removes ICI/ISI through phase shifting and mitigates IDI by targeting the strongest interference terms. The proposed algorithm eliminates ICI/ISI, mitigates IDI, and enables OTFS with practical rectangular waveforms to match the performance of ideal pulse shaping and to outperform OFDM in error rates across diverse channel conditions.

Abstract

The recently proposed orthogonal time-frequency-space (OTFS) modulation technique was shown to provide significant error performance advantages over orthogonal frequency division multiplexing (OFDM) over delay-Doppler channels. In this paper, we first derive the explicit input-output relation describing OTFS modulation and demodulation (mod/demod). We then analyze the cases of: 1) ideal pulse-shaping waveforms that satisfy the bi-orthogonality conditions and 2) rectangular waveforms which do not. We show that while only inter-Doppler interference (IDI) is present in the former case, additional inter-carrier interference (ICI) and inter-symbol interference (ISI) occur in the latter case. We next characterize the interferences and develop a novel low-complexity yet efficient message passing (MP) algorithm for joint interference cancellation (IC) and symbol detection. While ICI and ISI are eliminated through appropriate phase shifting, IDI can be mitigated by adapting the MP algorithm to account for only the largest interference terms. The MP algorithm can effectively compensate for a wide range of channel Doppler spreads. Our results indicate that OTFS using practical rectangular waveforms can achieve the performance of OTFS using ideal but non-realizable pulse-shaping waveforms. Finally, simulation results demonstrate the superior error performance gains of the proposed uncoded OTFS schemes over OFDM under various channel conditions.