Assistance-Transmission Tradeoff for RIS-assisted Symbiotic Radios

This paper considers the reconfigurable intelligent surface (RIS)-assisted symbiotic radio (SR) system, in which the RIS, acting as a secondary transmitter (STx), transmits its information by leveraging the primary signal as its RF carrier and simultaneously assists the primary transmission. In such a system, when the RIS transmits information, it could only provide limited assistance for the primary transmission. Thus, there exists a fundamental tradeoff between the assistance and information transmission capabilities of the RIS. To study the tradeoff, in this paper, we propose a novel RIS design scheme which partitions the RIS surface into two sub-surfaces, one to assist the primary transmission and the other to transmit the secondary signal. Considering the coupling effect between these two transmissions in SR, we focus on the composite signal formed by the primary and secondary signals. Then, to optimize the surface partitioning strategy, we formulate the assistance-transmission tradeoff problem to minimize the bit error rate (BER) of the composite signal, so as to improve the overall BER performance of the primary and secondary signals. By solving the problem, we show that optimal surface partitioning is related to the strength ratio of the direct link to the reflected link. Finally, simulation results show that the proposed design outperforms the conventional design significantly, which provides the best tradeoff.