  This paper investigates the reconfigurable intelligent surface (RIS) assisted
spatial scattering modulation (SSM) scheme for millimeter-wave (mmWave)
multiple-input multiple-output (MIMO) systems, in which line-of-sight (LoS) and
non-line-of-sight (NLoS) paths are respectively considered in the
transmitter-RIS and RIS-receiver channels. Based on the maximum likelihood
detector, the conditional pairwise error probability (CPEP) expression for the
RIS-SSM scheme is derived under the two cases of received beam correct and
demodulation error. Furthermore, we derive the closed-form expressions of the
unconditional pairwise error probability (UPEP) by employing two different
methods: the probability density function and the moment-generating function
expressions with a descending order of scatterer gains. To provide more useful
insights, we derive the asymptotic UPEP and the diversity gain of the RIS-SSM
scheme in the high SNR region. Depending on UPEP and the corresponding
Euclidean distance, we get the union upper bound of the average bit error
probability (ABEP). A new framework for ergodic capacity analysis is also
provided to acquire the proposed system's effective capacity. Finally, all
derivation results are validated via extensive Monte Carlo simulations,
revealing that the proposed RIS-SSM scheme outperforms the benchmarks in terms
of reliability.
