RIS-Assisted Space-Shift Keying with Non-Ideal Transceivers and Greedy Detection

Reconfigurable intelligent surfaces (RIS) and index modulation (IM) represent key technologies for enabling reliable wireless communications with high energy efficiency. However, to fully take advantage of these technologies in practical deployments, comprehending the impact of the non-ideal nature of the underlying transceivers is paramount. In this vein, this paper introduces two RIS-assisted IM communication models, in which the RIS is part of the transmitter and space-shift keying (SSK) is employed for IM, and assesses their performance in the presence of hardware impairments. In the first model, the RIS acts as a passive reflector only, reflecting the incoming SSK modulated signal intelligently towards the desired receive diversity branch. The second model employs RIS as a modulator for sensing data, employing M-ary phase-shift keying to implement reflection phase modulation (RPM) for such, and as a reflector for the incoming SSK modulated signal. Considering transmissions subjected to Nakagami-m fading, and greedy detection at the reception, the performance of the RIS-assisted SSK and RIS-assisted SSK-RPM systems are evaluated in terms of the pairwise probability of erroneous index detection and the probability of erroneous index detection, for which closed-form expressions are obtained in both system configurations. Monte-Carlo simulations are carried out to validate the analytical framework, and numerical results are presented to study the dependency of the error performance on the main system parameters. The findings highlight the effect of hardware impairments on the system performance. In particular, minimal to no impact is observed in the low ranges of average signal-to-noise ratio (SNR), while the effect is prominent for higher average SNRs. The study emphasizes using such greedy detectors, which are robust to the said impairments.