Joint Precoding Based on Interference Alignment and SVD for MU-MIMO OTFS With MRC Detector

The orthogonal time frequency space (OTFS) transmission scheme was shown to outperform orthogonal frequency division multiplexing (OFDM) under the doubly-dispersive channel. In this letter, a linear precoding scheme is studied for multiuser multiple-input and multiple-output (MU-MIMO) OTFS systems, in which a spatial-Doppler domain joint precoding scheme based on interference alignment (IA) and singular value decomposition (SVD) is proposed. At the transmitter, the IA is first utilized to align the inter-user interference into the dimensions which are othogonal to the user signals. Subsequently, the SVD precoder is exploited to convert the equivalent channel into a diagonal form, thereby improving reception reliability and reducing complexity. At the receiver, an iterative maximal ratio combining (MRC) detector is adopted in which multipath components of the transmitted symbols in the delay-Doppler grid are combined, so as to achieve the multipath diversity gain and increase the reception reliability. The feasible conditions for IA are derived and validated. The simulation results confirm that the joint precoding based on IA and SVD effectively mitigates the inter-user interference and enhances the error performance at high signal-to-noise ratio region for MU-MIMO OTFS systems.