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

TL;DR

This paper introduces a novel OTFS-SCMA downlink scheme that enhances massive connectivity and high-mobility performance through joint detection and cooperative algorithms, demonstrating improved spectral efficiency and convergence.

Contribution

It proposes a cross-domain joint detection algorithm for OTFS-SCMA and a distributed cooperative detection method to exploit diversity gains in high-mobility environments.

Findings

The joint detection algorithm converges at a fixed point.

The cooperative detection improves error performance.

The scheme achieves higher spectral efficiency.

Abstract

This paper studies a downlink system that combines orthogonal-time-frequency-space (OTFS) modulation and sparse code multiple access (SCMA) to support massive connectivity in high-mobility environments. We propose a cross-domain receiver for the considered OTFS-SCMA system which efficiently carries out OTFS symbol estimation and SCMA decoding in a joint manner. This is done by iteratively passing the extrinsic information between the time domain and the delay-Doppler (DD) domain via the corresponding unitary transformation to ensure the principal orthogonality of errors from each domain. We show that the proposed OTFS-SCMA detection algorithm exists at a fixed point in the state evolution when it converges. To further enhance the error performance of the proposed OTFS-SCMA system, we investigate the cooperation between downlink users to exploit the diversity gains and develop a distributed cooperative detection (DCD) algorithm with the aid of belief consensus. Our numerical results demonstrate the effectiveness and convergence of the proposed algorithm and show an increased spectral efficiency compared to the conventional OTFS transmission.