RIS-Assisted Generalized Receive Quadrature Spatial Modulation

In this paper, reconfigurable intelligent surface (RIS)-assisted generalized receive quadrature spatial modulation (RIS-GRQSM) is proposed to improve the spectral efficiency of RIS-aided quadrature spatial modulation (QSM) systems by utilizing the concept of generalized spatial modulation (GSM). That is, multiple antennas are activated at the receiver independently for both the real and imaginary parts. We propose a max-min optimization problem to adjust the phase shifts of all RIS elements to maximize the relevant signal-to-noise ratios (SNRs) at all activated receive antennas. Using Lagrange duality, the non-convex optimization problem involving the phase shifts of all RIS elements reduces to a convex optimization involving a number of variables equal to the number of activated receive antennas. A successive greedy detector (GD) can be used at the receiver to detect the active antennas, which simplifies the detection process. The numerical results show that the proposed scheme outperforms the benchmark schemes in terms of error rate performance, especially in systems with a larger number of receive antennas. In the special case where each receive antenna corresponds to a user and is activated, the RIS-GRQSM system becomes a multicast communication system. In this context, in contrast to existing phase shift optimization algorithms which exhibit an impractical level of complexity, our proposed solution offers the advantage of low complexity and practical feasibility of implementation.