Reconfigurable Intelligent Surface-Based Hybrid Phase and Code Modulation for Symbiotic Radio

Reconfigurable intelligent surface (RIS)-assisted symbiotic radio is a spectrum-and energy- efficient communication paradigm, in which an RIS per-forms passive beamforming to enhance active transmission, while using the electromagnetic waves from the active transmission for additional information transfer (i.e., passive transmission). In this paper, a hybrid RIS-based modulation, termed hybrid phase and code modulation (HPCM), is proposed to improve the reliability of RIS-assisted symbiotic radio. In RIS-HPCM, the RIS simultaneously performs direct sequence spread spectrum and passive beamforming on incident signals. Moreover, both the spreading code and phase offset are exploited to carry the RIS's own information. A low-complexity detector is designed, in which the receiver first detects the spreading codes and then demodulates the constellation symbols. We analyze the bit error rate (BER) performance of RIS-HPCM over Rician fading channels. BER upper bounds and approximate BER expressions are derived in closed-form for maximum-likelihood and low-complexity detectors, respectively. Simulation results in terms of BER verify the analysis and show the superiority of RIS-HPCM over the existing RIS-based modulation.