On RF-Chain Limited Spatial Modulation Aided NOMA: Spectral Efficiency Analysis

Recently, the combination of spatial modulation (SM) and non- orthogonal multiple access (NOMA) is proposed to enhance the spectral efficiency (SE) of downlink multi-user (MU) systems. However, traditional SM aided NOMA (SM-NOMA) systems require many radio frequency (RF) chains, which increases the implementation cost and lowers the energy efficiency (EE). In this paper, we propose a novel single-RF SM-NOMA system, in which the spatial domain bits can be flexibly divided for different user equipments (UEs). In addition, we analyze the SE of our proposed single-RF SM-NOMA system, and a closed-form SE lower bound is also derived. The tightness of our proposed SE lower bound is validated via Monte Carlo simulations, and the SE comparisons of different SM-NOMA systems are also shown, which illustrates the effectiveness of the proposed single-RF SM-NOMA system.