  In this paper, we propose a novel reconfigurable intelligent surface
(RIS)-assisted wireless communication scheme which uses the concept of spatial
modulation, namely RIS-assisted receive quadrature spatial modulation
(RIS-RQSM). In the proposed RIS-RQSM system, the information bits are conveyed
via both the indices of the two selected receive antennas and the conventional
in-phase/quadrature (IQ) modulation. We propose a novel methodology to adjust
the phase shifts of the RIS elements in order to maximize the signal-to-noise
ratio (SNR) and at the same time to construct two separate PAM symbols at the
selected receive antennas, as the in-phase and quadrature components of the
desired IQ symbol. An energy-based greedy detector (GD) is implemented at the
receiver to efficiently detect the received signal with minimal channel state
information (CSI) via the use of an appropriately designed one-tap
pre-equalizer. We also derive a closed-form upper bound on the average bit
error probability (ABEP) of the proposed RIS-RQSM system. Then, we formulate an
optimization problem to minimize the ABEP in order to improve the performance
of the system, which allows the GD to act as a near-optimal receiver. Extensive
numerical results are provided to demonstrate the error rate performance of the
system and to compare with that of a prominent benchmark scheme. The results
verify the remarkable superiority of the proposed RIS-RQSM system over the
benchmark scheme.
