Reconfigurable intelligent surfaces (RISs) is a new technology that can
be used to create a virtual line-of-sight (LOS) link when the direct link is
blocked. The signal-to-noise ratio (SNR) can be significantly improved by
optimizing the reflecting elements’ phases to make the reflected signals add
coherently at the receiver. Nevertheless, the RIS cannot control the phase of
the direct link if it exists. In such scenarios, the RIS phase can still be controlled
such that the direct and reflected signals add coherently, however, the gain
obtained by using the RIS might be hindered. Therefore, this paper considers
the performance analysis of such scenarios where a novel analytical framework
is developed to evaluate the SNR, outage probability and bit error rate (BER).
To capture a broad range of fading conditions, the channels are modeled as
independent but not identically distributed generalized <i>κ</i>-<i>µ </i>shadowed fading
channels. The Laguerre expansion is used to derive the probability density
function (PDF) and cumulative distribution function (CDF) of the instantaneous
channel fading, which are used to derive the PDF and CDF of the instantaneous
SNR. The paper also considers deriving the asymptotic PDF, CDF, moment
generating function (MGF) of the SNR, as well as the outage probability and
BER. The derived expressions are used to evaluate the system performance in
various fading environments such as Rayleigh, One-Sided Gaussian, Nakagami-<i>m</i> , Rician, and Rician-shadowed distributions since they are special cases
of the <i>κ</i>-<i>µ </i>shadowed
distribution. The obtained analytical results corroborated by Monte Carlo
simulation show that a strong direct link can generally eliminate the gain
obtained using the RIS. Therefore, the overhead required for the RIS operation
becomes a burden on the network and may cause severe throughput degradation. In
such scenarios, the RIS involvement should dynamically controlled to avoid
unnecessary complexity and throughput reduction.