RIS Design for Symbiotic Radio: A Mutualistic Spectrum Sharing Perspective

In the reconfigurable intelligent surface (RIS)-based symbiotic radio (SR) system, the RIS acts as a secondary transmitter by modulating its information over the primary signal and simultaneously assists the primary transmission, which leads to a mutualistic spectrum sharing relationship. To ensure that the primary transmission can gain more benefits from the secondary transmission, we propose a novel RIS design scheme for SR, which divides the phase-shift matrix into two sub-phase-shift matrices, one to assist the primary transmission and the other to transmit the secondary signal. To optimize the two sub-phase-shift matrices, we first introduce a mutualistic factor which describes the performance requirement of the primary transmission and then formulate the problem to minimize the bit error rate (BER) of the secondary transmission, under the constraint that the BER performance of the primary transmission is higher than the performance requirement controlled by the mutualistic factor. To solve this non-convex problem, we resort to the successive convex approximation technique to obtain a suboptimal solution. Finally, simulation results reveal an interesting tradeoff between the BER performance of the primary and secondary transmissions by adjusting the mutualistic factor.