Mitigating Spatial Correlation in MIMO-OTFS

In this article, we present an in-depth analysis of whitening methods for mitigating the spatial correlation in a multiple-input multiple-output orthogonal time frequency space (MIMO-OTFS) system. After presenting the input-output relation of MIMO-OTFS with receive spatial correlation, we analyze the error performance in terms of the union bound and derive an upper bound on the pairwise error probability (PEP). This upper bound is dominated by diversity and coding gains, as well as given SNRs. However, evaluating the PEP upper bound involves high computational complexity. This motivates us to adopt average receive SNR as a parameter for performance evaluation. Next, we investigate the whitening transformation (WT) method and show that the effect of spatial correlation cannot be completely removed, leading to a limited performance improvement. To tackle this problem, we develop an alternative whitening technique, viz. the receive whitening filter (WF) method for MIMO-OTFS. Additionally, we investigate the effect of mismatched spatial correlation coefficient and noise variance on the performance of the WF method. Through a general proof based on the expectation of the average SNR ratios between the mismatched and the standard cases, we analytically demonstrate the performance degradation of the mismatched case. The proof also reveals the increasing performance degradation w.r.t. the magnitude of the mismatch error. Notably, for practical mismatch error of 5 - 10%, the performance degradation of the WF method is found to be insignificant. Finally, simulation results are presented to validate the analytical results in a practical LEO satcom channel model for various MIMO-OTFS systems.