Downlink OTFS Non-Orthogonal Multiple Access Receiver Design based on Cross-Domain Detection

Orthogonal time frequency space (OTFS) modulation has recently been recognized as a promising modulation scheme for high-mobility wireless communications systems. This paper considers a new code-domain non-orthogonal multiple access (CD-NOMA) scheme for downlink OTFS systems, and the sparse code multiple access (SCMA), a kind of CD-NOMA, is utilized to support ultra-massive communication devices in high-mobility environments. Inspired by cross-domain detection, we propose a cross-domain-based receiver to jointly carry out the OTFS symbol detection and SCMA decoding, in the presence of fractional Doppler shifts. The proposed OTFS-SCMA detector performs equalization (linear estimation) in the time domain and performs denoising and SCMA decoding in the DD domain. Extrinsic information is passed between two domains via the corresponding unitary transformation to ensure the orthogonality of errors from each domain and is updated iteratively. Our numerical results demonstrate the effectiveness of the proposed algorithm, which enables an increased spectral efficiency compared to the conventional OTFS transmission and a good error performance.