RIS-Aided SCMA-Based SWIPT Systems: Design and Optimization

The amalgamation of sparse code multiple access (SCMA) and simultaneous wireless information and power transfer (SWIPT) is envisioned to be a promising solution to massive connectivity while providing both information and power to myriads of wireless machines and sensors in Internet-of-Things (IoT) application. However, the benefit of the SCMA-based SWIPT system has not been fully exploited yet, especially when the communications link is hostile. Fortunately, the propagation-induced impairment can be mitigated by the emerging technique of reconfigurable intelligent surface (RIS). In this paper, we investigate the beneficial role of RIS in SCMA-based SWIPT systems, where $K$ single-antenna information receivers and one signal-antenna energy receiver are served by a multiple-antenna base station over $J$ orthogonal subcarriers with the aid of RIS. We propose transmit power minimization algorithms for both i) the perfect and ii) the imperfect channel state information (CSI) scenarios, by jointly optimizing the codebook assignment, the BS active beamforming, as well as RIS reflection coefficients, relying on advanced techniques both of alternating optimization and of successive convex approximations, and on the robust design framework. Furthermore, we compare and evaluate the detection performance of the system considered. The numerical results demonstrate that our proposed RIS-aided SCMA-based SWIPT system significantly outperforms the conventional SCMA-based SWIPT system without RIS and is capable of adapting to the imperfect full channel estimation results.