Reconfigurable Intelligent Surface assisted Two–Way Communications:
Performance Analysis and Optimization
Abstract
In this paper, we investigate the two-way communication between two
users assisted by a re-configurable intelligent surface (RIS). The
scheme that two users communicate simultaneously in the same time slot
over Rayleigh fading channels is considered. The channels between the
two users and RIS can either be reciprocal or non-reciprocal. For
reciprocal channels, we determine the optimal phases at the RIS to
maximize the signal-to-interference-plus-noise ratio (SINR). We then
derive exact closed-form expressions for the outage probability and
spectral efficiency for single-element RIS. By capitalizing the insights
obtained from the single-element analysis, we introduce a gamma
approximation to model the product of Rayleigh random variables which is
useful for the evaluation of the performance metrics in multiple-element
RIS. Asymptotic analysis shows that the outage decreases at
$\left(\log(\rho)/\rho\right)^L$
rate where $L$ is the number of elements, whereas the spectral
efficiency increases at $\log(\rho)$
rate at large average SINR $\rho$. For non-reciprocal
channels, the minimum user SINR is targeted to be maximized. For
single-element RIS, closed-form solutions are derived whereas for
multiple-element RIS the problem turns out to be non-convex. The latter
is relaxed to be a semidefinite programming problem, whose optimal
solution is achievable and serves as a sub-optimal solution.