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This paper studies a downlink system that combinesorthogonal-time-frequency-space (OTFS) modulation and sparse code multipleaccess (SCMA) to support massive connectivity in high-mobility environments. Wepropose a cross-domain receiver for the considered OTFS-SCMA system whichefficiently carries out OTFS symbol estimation and SCMA decoding in a jointmanner. This is done by iteratively passing the extrinsic information betweenthe time domain and the delay-Doppler (DD) domain via the corresponding unitarytransformation to ensure the principal orthogonality of errors from eachdomain. We show that the proposed OTFS-SCMA detection algorithm exists at afixed point in the state evolution when it converges. To further enhance theerror performance of the proposed OTFS-SCMA system, we investigate thecooperation between downlink users to exploit the diversity gains and develop adistributed cooperative detection (DCD) algorithm with the aid of beliefconsensus. Our numerical results demonstrate the effectiveness and convergenceof the proposed algorithm and show an increased spectral efficiency compared tothe conventional OTFS transmission.

OTFS-SCMA: A Downlink NOMA Scheme for Massive Connectivity in High Mobility Channels