  Index modulation schemes for reconfigurable intelligent surfaces
(RIS)-assisted systems are envisioned as promising technologies for
fifth-generation-advanced and sixth-generation (6G) wireless communication
systems to enhance various system capabilities such as coverage area and
network capacity. In this paper, we consider a receive diversity RIS-assisted
wireless communication system employing IM schemes, namely, space-shift keying
(SSK) for binary modulation and spatial modulation (SM) for M-ary modulation
for data transmission. The RIS lies in close proximity to the transmitter, and
the transmitted data is subjected to a fading environment with a prominent
line-of-sight component modeled by a Rician distribution. A receiver structure
based on a greedy detection rule is employed to select the receive diversity
branch with the highest received signal energy for demodulation. The
performance of the considered system is evaluated by obtaining a series-form
expression for the probability of erroneous index detection (PED) of the
considered target antenna using a characteristic function approach. In
addition, closed-form and asymptotic expressions at high and low
signal-to-noise ratios (SNRs) for the bit error rate (BER) for the SSK-based
system, and the SM-based system employing M-ary phase-shift keying and M-ary
quadrature amplitude modulation schemes, are derived. The dependencies of the
system performance on the various parameters are corroborated via numerical
results. The asymptotic expressions and results of PED and BER at high and low
SNR values lead to the observation of a performance saturation and the presence
of an SNR value as a point of inflection, which is attributed to the greedy
detector's structure.
