  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.
