Interference Mitigation Using Delay-Doppler Channel Projection in OTFS Receivers

In orthogonal time frequency space (OTFS) modulation, the delay-Doppler (DD) spread in the channel causes interference across DD domain information symbols, degrading detection performance. In this paper, we propose and investigate a novel idea of DD channel projection in OTFS receivers towards the mitigation of interference due to DD channel spread. In conventional OTFS receivers, the received time domain (TD) signal is first transformed to frequency-time (FT) domain through a projection operation matched to the transmit waveform. The proposed idea in this paper is to achieve the TD to FT domain conversion without much distortion through the use of better orthonormal functions for projection. Towards this goal, the proposed receivers use projection functions that are based on DD channel knowledge. Two such receivers are proposed. The first receiver, termed as the effective path (EP) receiver, uses a projection function that is obtained based on an approximate characterization of the multipath channel as a channel with a single effective path, and the received signal is projected onto this effective path. A cost function is defined for this purpose, which is maximized to obtain the effective path. In the second receiver, termed as the concatenated paths (CP) receiver, the received signal is projected onto multiple paths and the resulting projected signals are concatenated to obtain the overall DD channel matrix. Simulation results show that the proposed EP and CP receivers offer good interference mitigation, resulting in significantly better performance compared to the conventional receiver.