Reconfigurable intelligent surface (RIS) is regarded as an important enabling technology for the sixth-generation (6G) network. Recently, modulating information in reflection patterns of RIS, referred to as reflection modulation (RM), has been proven in theory to have the potential of achieving higher transmission rate than existing passive beamforming (PBF) schemes of RIS. To fully unlock this potential of RM, we propose a novel superimposed RIS-phase modulation (SRPM) scheme for multiple-input multiple-output (MIMO) systems, where tunable phase offsets are superimposed onto predetermined RIS phases to bear extra information messages. The proposed SRPM establishes a universal framework for RM, which retrieves various existing RM-based schemes as special cases. Moreover, the advantages and applicability of the SRPM in practice is also validated in theory by analytical characterization of its performance in terms of average bit error rate (ABER) and ergodic capacity. To maximize the performance gain, we formulate a general precoding optimization at the base station (BS) for a single-stream case with uncorrelated channels and obtain the optimal SRPM design via the semidefinite relaxation (SDR) technique. Furthermore, to avoid extremely high complexity in maximum likelihood (ML) detection for the SRPM, we propose a sphere decoding (SD)-based layered detection method with near-ML performance and much lower complexity. Numerical results demonstrate the effectiveness of SRPM, precoding optimization, and detection design. It is verified that the proposed SRPM achieves a higher diversity order than that of existing RM-based schemes and outperforms PBF significantly especially when the transmitter is equipped with limited radio-frequency (RF) chains.

翻译：可重构智能超表面（RIS）被视为第六代（6G）网络的重要使能技术。近年来，在RIS反射模式中调制信息的技术称为反射调制（RM），理论上已被证明具有比现有RIS无源波束赋形（PBF）方案更高的传输速率潜力。为充分释放RM的这一潜力，我们针对多输入多输出（MIMO）系统提出一种新颖的叠加RIS相位调制（SRPM）方案，其中可调相位偏移被叠加到预定的RIS相位上以承载额外的信息消息。所提出的SRPM建立了RM的通用框架，将多种现有基于RM的方案作为特例纳入其中。此外，通过从平均误码率（ABER）和遍历容量角度对SRPM性能进行解析表征，从理论上验证了其在实际中的优势与适用性。为最大化性能增益，我们针对非相关信道下单流场景，在基站（BS）处构建通用预编码优化问题，并通过半定松弛（SDR）技术获得最优SRPM设计。进一步地，为避免SRPM的最大似然（ML）检测导致极高复杂度，我们提出一种基于球形译码（SD）的层叠检测方法，其性能接近ML且复杂度显著降低。数值结果验证了SRPM、预编码优化及检测设计的有效性。结果表明，所提出的SRPM具有比现有基于RM的方案更高的分集阶数，并且显著优于PBF，尤其当发射机配备有限射频（RF）链时。