Symbiotic radio (SR) is a promising technology of spectrum- and energy-efficient wireless systems, for which the key idea is to use cognitive backscattering communication to achieve mutualistic spectrum and energy sharing with passive backscatter devices (BDs). In this paper, a reconfigurable intelligent surface (RIS) based SR system is considered, where the RIS is used not only to assist the primary active communication, but also for passive communication to transmit its own information. For the considered system, we investigate the EE trade-off between active and passive communications, by characterizing the EE region. To gain some insights, we first derive the maximum achievable individual EEs of the primary transmitter (PT) and RIS, respectively, and then analyze the asymptotic performance by exploiting the channel hardening effect. To characterize the non-trivial EE trade-off, we formulate an optimization problem to find the Pareto boundary of the EE region by jointly optimizing the transmit beamforming, power allocation and the passive beamforming of RIS. The formulated problem is non-convex, and an efficient algorithm is proposed by decomposing it into a series of subproblems by using alternating optimization (AO) and successive convex approximation (SCA) techniques. Finally, simulation results are presented to validate the effectiveness of the proposed algorithm.

翻译：共生无线电（SR）是一种具有频谱和能量效率的无线系统前景技术，其核心思想是利用认知反向散射通信，实现与无源反向散射设备（BD）的互惠频谱与能量共享。本文研究了一种基于可重构智能表面（RIS）的SR系统，其中RIS不仅用于辅助主有源通信，还用于无源通信以传输自身信息。针对所考虑的系统，我们通过刻画能效区域，探究了有源与无源通信之间的能效权衡。为获得深入理解，我们首先分别推导了主发射机（PT）和RIS的最大可达个体能效，随后利用信道硬化效应分析渐进性能。为表征非平凡的能效权衡，我们建立了一个优化问题，通过联合优化发射波束成形、功率分配和RIS无源波束成形，寻求能效区域的帕累托边界。该问题非凸，我们提出了一种高效算法，利用交替优化（AO）和逐次凸近似（SCA）技术将其分解为一系列子问题。最后，仿真结果验证了所提算法的有效性。