In this paper, a novel amplitude phase shift keying (APSK) modulation scheme for cooperative backscatter communications aided by a reconfigurable intelligent surface (RIS-CBC) is presented, according to which the RIS is configured to modulate backscatter information onto unmodulated or PSK-modulated signals impinging on its surface via APSK. We consider both passive and active RISs, with the latter including an amplification unit at each reflecting element. In the passive (resp. active) RIS-CBC-APSK, backscatter information is conveyed through the number of RIS reflecting elements being on the ON state (resp. active mode) and their phase shift values. By using the optimal APSK constellation to ensure that reflected signals from the RIS undergo APSK modulation, a bit-mapping mechanism is presented. Assuming maximum-likelihood detection, we also present closed-form upper bounds for the symbol error rate (SER) performance for both passive and active RIS-CBC-APSK schemes over Rician fading channels. In addition, we devise a low-complexity detector that can achieve flexible trade-offs between performance and complexity. Finally, we extend RIS-CBC-APSK to multiple-input single-output scenarios and present an alternating optimization approach for the joint design of transmit beamforming and RIS reflection. Our extensive simulation results on the SER performance corroborate our conducted performance analysis and showcase the superiority of the proposed RIS-CBC-APSK schemes over the state-of-the-art RIS-CBC benchmarks.

翻译：本文提出了一种新颖的振幅相位键控（APSK）调制方案，用于可重构智能表面（RIS）辅助的协作反向散射通信（RIS-CBC）。在该方案中，RIS被配置为通过APSK将反向散射信息调制到入射其表面的未调制或PSK调制信号上。我们同时考虑了无源和有源RIS，其中有源RIS在每个反射单元中包含一个放大单元。在无源（或有源）RIS-CBC-APSK中，反向散射信息通过处于“开”状态（或有源模式）的RIS反射单元数量及其相位偏移值来传递。通过使用最优APSK星座图确保来自RIS的反射信号经历APSK调制，本文提出了一种比特映射机制。假设采用最大似然检测，我们还给出了瑞利衰落信道下无源和有源RIS-CBC-APSK方案的符号错误率（SER）性能的闭式上界。此外，我们设计了一种低复杂度检测器，能够在性能和复杂度之间实现灵活权衡。最后，我们将RIS-CBC-APSK扩展到多输入单输出场景，并提出了一种交替优化方法，用于联合设计发射波束成形和RIS反射。我们关于SER性能的大量仿真结果验证了所进行的性能分析，并展示了所提出的RIS-CBC-APSK方案相较于现有最先进的RIS-CBC基准方法的优越性。