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 performs 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.