In reconfigurable intelligent surface (RIS)-assisted symbiotic radio (SR), the RIS acts as a secondary transmitter by modulating its information bits over the incident primary signal and simultaneously assists the primary transmission, then a cooperative receiver is used to jointly decode the primary and secondary signals. Most existing works of SR focus on using RIS to enhance the reflecting link while ignoring the ambiguity problem for the joint detection caused by the multiplication relationship of the primary and secondary signals. Particularly, in case of a blocked direct link, joint detection will suffer from severe performance loss due to the ambiguity, when using the conventional on-off keying and binary phase shift keying modulation schemes for RIS. To address this issue, we propose a novel modulation scheme for RIS-assisted SR that divides the phase-shift matrix into two components: the symbol-invariant and symbol-varying components, which are used to assist the primary transmission and carry the secondary signal, respectively. To design these two components, we focus on the detection of the composite signal formed by the primary and secondary signals, through which a problem of minimizing the bit error rate (BER) of the composite signal is formulated to improve both the BER performance of the primary and secondary ones. By solving the problem, we derive the closed-form solution of the optimal symbol-invariant and symbol-varying components, which is related to the channel strength ratio of the direct link to the reflecting link. Moreover, theoretical BER performance is analyzed. Finally, simulation results show the superiority of the proposed modulation scheme over its conventional counterpart.

翻译：在可重构智能表面（RIS）辅助共生无线电（SR）中，RIS通过将自身信息比特调制到入射主信号上，作为次级发射机工作，同时辅助主传输，随后使用协作接收机联合解码主信号和次级信号。现有SR研究大多聚焦于利用RIS增强反射链路，却忽略了由主信号与次级信号的乘积关系导致的联合检测模糊性问题。特别是在直射链路受阻的情况下，若RIS采用传统开关键控和二进制相移键控调制方案，联合检测将因模糊性而遭受严重性能损失。针对这一问题，我们提出一种新型RIS辅助SR调制方案，将相移矩阵分为两部分：符号不变分量和符号变化分量，分别用于辅助主传输和承载次级信号。为设计这两个分量，我们关注由主信号和次级信号构成的复合信号的检测问题，通过构建最小化复合信号误比特率（BER）的优化问题，提升主信号与次级信号的BER性能。通过求解该问题，我们推导出最优符号不变分量和符号变化分量的闭式解，该解与直射链路与反射链路的信道强度比相关。此外，还分析了理论BER性能。最后，仿真结果表明所提调制方案相较于传统方案具有优越性。