Joint Precoding and RIS Phase Shift Design for Sum-Rate Maximization in RIS-Aided MISO Symbiotic Radio

Symbiotic radio (SR) has emerged as a spectral-efficient paradigm for Internet-of-Things networks. However, simultaneously optimizing coexisting primary and secondary transmissions remains a challenge. To address this, a joint precoding and RIS phase shift design is proposed to maximize the sum-rate in the reconfigurable intelligent surface (RIS)-aided multiple-input single-output SR system. Specifically, based on an optimization problem that maximizes the sum-rate of primary and secondary data, an iterative algorithm that sequentially updates the precoder and the RIS phase shift matrix is developed. To address practical aspects, channel training overhead and minimum signal-to-interference-plus-noise ratio (SINR) constraints are also considered in the precoder design. Simulation results verify that the precoder design based on perfect successive interference cancellation achieves the highest sum-rate. The results also highlight the necessity of the proposed SINR-constrained precoding schemes in guaranteeing data rates of the primary and secondary links under various channel environments, proving the practical validity of our joint precoder and RIS phase shift design.