Low Complexity Receiver via Expectation Propagation for OTFS Modulation

Orthogonal time frequency space (OTFS) modulation has recently received considerable attention due to its capabilities to exploit full time-frequency diversity and deal with channel dynamics in high-mobility scenarios. However, the implementation of high-performance receivers in the delay-Doppler (DD) domain poses serious challenges in practice, especially in rich-scattering environments and when the fractional Doppler is considered. In this letter, we first propose an iterative receiver under the framework of an expectation propagation (EP) algorithm for the OTFS system. Then, a channel-coefficients-aware scheme is incorporated into the message passing by bundling multiple graph edges into one edge on the factor graph based on the sizes of associated channel coefficients to further reduce the computational complexity. Simulation results illustrate that the proposed approximate EP receiver achieves a good tradeoff between performance and complexity.