Reconfigurable intelligent surface (RIS)-assisted transmission has become a novel concept for future wireless communications. The multiple-input multiple-output (MIMO)-based RIS-assisted Alamouti (RIS-Alamouti) scheme has been recently proposed in the literature. The RIS-Alamouti scheme models the conventional Alamouti scheme with two transmit antennas and \(N_r\) receive antennas. In this paper, we propose a novel MIMO-based scheme, hereinafter referred to as the N-ary RIS-assisted Alamouti (N-RIS-Alamouti) scheme. In the proposed N-RIS-Alamouti scheme, \(\log _2N\) additional information bits can be conveyed by transmitting a pair of N-ary phase shift keying (NPSK) symbols in one of two transmit antennas over four time slots. The pair of NPSK symbols can be regarded as one channel’s additional phase component over four time slots. The proposed N-RIS-Alamouti scheme not only preserves the error performance of the RIS-Alamouti scheme but also enhances its spectral efficiency; hence, it allows the transmission of a larger number of bits per channel use without trading off on reliability. The achievable order N of the N-RIS-Alamouti scheme increases as the number of receive antennas increase meaning that a larger spectral efficiency is achievable for a larger number of receive antennas. Simulation results show that at a bit error rate (BER) of \(2\times {10^{-5}}\) , the proposed 128-RIS-Alamouti and 256-RIS-Alamouti with 64-ary phase shift keying and 64 reflector elements achieve the BER performance of the RIS-Alamouti for two and three receive antennas, respectively. Equivalently, an additional 7 bits and 8 bits are transmitted in four time slots, respectively.
