  Reconfigurable intelligent surface (RIS)-assisted transmission and space
shift keying (SSK) appear as promising candidates for future energy-efficient
wireless systems. In this paper, two RIS-based SSK schemes are proposed to
efficiently improve the error and throughput performance of conventional SSK
systems, respectively. The first one, termed RIS-SSK with passive beamforming
(RIS-SSK-PB), employs an RIS for beamforming and targets the maximization of
the minimum squared Euclidean distance between any two decision points. The
second one, termed RIS-SSK with Alamouti space-time block coding
(RIS-SSK-ASTBC), employs an RIS for ASTBC and enables the RIS to transmit its
own Alamouti-coded information while reflecting the incident SSK signals to the
destination. A low-complexity beamformer and an efficient maximum-likelihood
(ML) detector are designed for RIS-SSK-PB and RIS-SSK-ASTBC, respectively.
Approximate expressions for the average bit error probabilities of the source
and/or the RIS are derived in closed-form assuming ML detection. Extensive
computer simulations are conducted to verify the performance analysis. Results
show that RIS-SSK-PB significantly outperforms the existing RIS-free and
RIS-based SSK schemes, and RIS-SSK-ASTBC enables highly reliable transmission
with throughput improvement.
