Interference Cancellation and Iterative Detection for Orthogonal Time Frequency Space Modulation
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The recently proposed orthogonal time frequency space (OTFS) modulation technique was shown to provide significant error performance advantages over orthogonal frequency division multiplexing (OFDM) in Doppler channels. In this paper, we first derive the explicit input-output relation describing OTFS modulation and demodulation (mod/demod) for delay-Doppler channels. We then analyze the cases of (i) ideal pulse-shaping waveforms that satisfy the bi-orthogonality conditions, and (ii) rectangular waveforms which do not. We show that while only inter-Doppler interference (IDI) is present in the first case, additional inter-carrier interference (ICI) and inter-symbol interference (ISI) occur in the second case. We next analyze 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 proposed 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 pulseshaping waveforms. Finally, simulations results demonstrate the superior error performance gains of the proposed uncoded OTFS schemes over OFDM under various channel conditions.
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