Low-Complexity Coherent Iterative Receiver for SCMA-Based LEO Satellite Communications

In this paper, we consider the sparse code multiple access (SCMA) technique for low earth orbit (LEO) satellite communications to achieve high user capacity and provide technical support for satellite-based Internet of Things (IoT) applications. Note that the Doppler shift in the LEO satellite link cannot be ignored and the message passing algorithm (MPA) for SCMA detection is quite time-consuming due to high complexity. Thus, we propose a coherent iterative receiver with low complexity for SCMA-based LEO satellite communications. First, we design a data-aided carrier synchronization scheme using multiple disjoint pilot blocks to estimate the Doppler shift precisely. Then, a soft-out approximate message passing (SO-AMP) algorithm with linear complexity is derived from the first-order approximation of the MPA based on a novel framework of the SCMA mapper. Further, a damping method is introduced into the SO-AMP algorithm to improve the overall performance. Numerical results show that the designed receiver can achieve the performance close to ideal performance with no Doppler shift while maintaining low complexity.