Low Complexity MRC Detection for OTFS Receiver With Oversampling

Orthogonal time-frequency space (OTFS) modulation shows superior performance in high-mobility wireless environments compared to orthogonal frequency division multiplexing (OFDM). In this paper, we consider maximal ratio combining (MRC) detection for an OTFS receiver with oversampling for channels with fractional delays and Doppler shifts. Specifically, we first reformulate input-output relations in delay-Doppler and delay-time domains for an oversampled OTFS receiver. Then we present a modified iterative MRC detection in both domains taking advantage of the oversampled received signal to improve error performance. The complexity of our detection method is equivalent to that of standard MRC detection scaled by the oversampling factor, while remaining much lower than message passing (MP) detection. We also develop a noise whitening approach to decorrelate the oversampled noise in time domain and derive the optimal combining weights of the MRC detection. Simulation results show that the proposed detection with receiver oversampling outperforms the MRC detection with Nyquist sampling, and the oversampling MP detection. Finally, we show that adding noise whitening can significantly improve error performance, compared to the MRC detection without noise whitening. This comes at a small additional computational cost, while still remaining much lower than MP detection.