In this paper, we are interested in reconfigurable intelligent surface (RIS)-assisted symbiotic radio (SR) systems, where an RIS assists a primary transmission by passive beamforming and simultaneously acts as an information transmitter by periodically adjusting its reflecting coefficients. The above modulation scheme innately enables a new multiplicative multiple access channel (M-MAC), where the primary and secondary signals are superposed in a multiplicative and additive manner. To pursue the fundamental performance limits of the M-MAC, we focus on the characterization of the capacity region of such systems. Due to the passive nature of RISs, the transmitted signal of the RIS should satisfy the peak power constraint. Under this constraint at the RIS as well as the average power constraint at the primary transmitter (PTx), we analyze the capacity-achieving distributions of the transmitted signals and characterize the capacity region of the M-MAC. Then, theoretical analysis is performed to reveal insights into the RIS-assisted SR. It is observed that: 1) the capacity region of the M-MAC is strictly convex and larger than that of the conventional TDMA scheme; 2) the secondary transmission can achieve the maximum rate when the PTx transmits the constant envelope signals; 3) and the sum rate can achieve the maximum when the PTx transmits Gaussian signals and the RIS transmits the constant envelope signals. Finally, extensive numerical results are provided to evaluate the performance of the RIS-assisted SR and verify the accuracy of our theoretical analysis.

翻译：本文研究可重构智能表面（RIS）辅助的共生无线电（SR）系统。在该系统中，RIS通过被动波束赋形辅助主传输，同时通过周期性调整其反射系数作为信息发射器。上述调制方案天然形成一种新的乘性多址接入信道（M-MAC），其中主信号与次信号以乘性和加性方式叠加。为探究M-MAC的极限性能，我们专注于刻画此类系统的容量区域。由于RIS的被动特性，其发射信号需满足峰值功率约束。在RIS受此约束且主发射机（PTx）受平均功率约束的条件下，我们分析了发射信号的容量可达分布，并刻画了M-MAC的容量区域。随后通过理论分析揭示RIS辅助SR的内在机理，发现：1）M-MAC的容量区域严格凸且大于传统TDMA方案的容量区域；2）当PTx发射恒包络信号时，次传输可达最大速率；3）当PTx发射高斯信号且RIS发射恒包络信号时，和速率可达最大值。最后，通过大量数值结果评估RIS辅助SR性能，并验证理论分析的准确性。