RIS-Aided Symbiotic Radio: A Number and Phase Modulation Approach

In this paper, a novel backscatter modulation scheme, termed number and phase modulation (NPM), is proposed for reconfigurable intelligent surface (RIS)-aided symbiotic radio (SR). In RIS-NPM, the number and phase shifts of RIS elements are jointly utilized to carry backscatter information. A bit-mapping method is designed by using the two information-bearing units. We develop two types of maximum-likelihood (ML) detectors that are based on instantaneous and statistic channel state information, respectively. A low-complexity detector that achieves a trade-off between performance and complexity is then designed. Closed-form upper bounds on the bit error rate (BER) are derived for both ML detectors over Rician fading channels. We further extend RIS-NPM to the scenario with multiple receive antennas, enhancing the spectral efficiency by embedding partial information into the antenna indices. For this extension, BER performance is analyzed by assuming energy-based ML detection. Computer simulations verify the analysis and show the superiority of RIS-NPM over the state-of-the-art RIS-based modulation schemes in SR.