RIS-Assisted Receive Quadrature Space-Shift Keying: A New Paradigm and Performance Analysis

TL;DR

This paper introduces RIS-RQSSK, a novel RIS-assisted modulation scheme that improves spectral efficiency and error performance in 6G wireless systems through innovative optimization and simple detection methods.

Contribution

The paper proposes RIS-RQSSK, a new RIS-assisted modulation technique with an analytical optimization solution and demonstrates its superior performance over existing schemes.

Findings

RIS-RQSSK significantly outperforms benchmark schemes.

The optimization reduces to a single-variable equation, simplifying design.

Performance improves with more receive antennas.

Abstract

Reconfigurable intelligent surfaces (RISs) represent a promising candidate for sixth-generation (6G) wireless networks, as the RIS technology provides a new solution to control the propagation channel in order to improve the efficiency of a wireless link through enhancing the received signal power. In this paper, we propose RIS-assisted receive quadrature space-shift keying (RIS-RQSSK), which enhances the spectral efficiency of an RIS-based index modulation (IM) system by using the real and imaginary dimensions independently for the purpose of IM. Therefore, the error rate performance of the system is improved as all RIS elements reflect the incident transmit signal toward both selected receive antennas. At the receiver, a low-complexity but effective greedy detector (GD) can be employed which determines the maximum energy per dimension at the receive antennas. A max-min optimization problem is defined to maximize the received signal-to-noise ratio (SNR) components at both selected receive antennas; an analytical solution is provided based on Lagrange duality. In particular, the multi-variable optimization problem is shown to reduce to the solution of a single-variable equation, which results in a very simple design procedure. In addition, we investigate the average bit error probability (ABEP) of the proposed RIS-RQSSK system and derive a closed-form approximate upper bound on the ABEP. We also provide extensive numerical simulations to validate our derivations. Numerical results show that the proposed RIS-RQSSK scheme substantially outperforms recent prominent benchmark schemes. This enhancement considerably increases with an increasing number of receive antennas.