  In this paper, we investigate the performance of reconfigurable intelligent
surface (RIS)-aided spatial shift keying (SSK) wireless communication systems
with imperfect channel state information (CSI). Specifically, we study the
average bit error probability (ABEP) of two RIS-SSK systems based on
intelligent reflection and blind reflection modes. For the intelligent RIS-SSK
scheme, we first derive the conditional pairwise error probability of the
composite channel through maximum likelihood (ML) detection. Subsequently, we
derive the probability density function of the combined channel. Due to the
intricacies of the composite channel formulation, an exact closed-form ABEP
expression is unattainable through direct derivation. To this end, we resort to
employing the Gaussian-Chebyshev quadrature method to estimate the results.
Additionally, we employ Q-function approximation to derive the non-exact
closed-form expression in the presence of channel estimation errors. For the
blind RIS-SSK scheme, we derive both closed-form ABEP expression and asymptotic
ABEP expression with imperfect CSI by adopting the ML detector. To offer deeper
insights, we explore the impact of discrete reflection phase shifts on the
performance of the RIS-SSK system. Lastly, we extensively validate all the
analytical derivations via Monte Carlo simulations.
