Abstract
In this paper, we investigate the two-way communication between two users assisted by a reconfigurable intelligent surface (RIS). The scheme that two users communicate simultaneously 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 (log(ρ)/ρ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sup> rate where L is the number of elements, whereas the spectral efficiency increases at log(ρ) rate at large average SINR p. For non-reciprocal channels, the minimum user SINR is targeted to be maximized. For single-element RIS, closed-form solution is derived whereas for multiple-element RIS the problem turns out to be non-convex. The latter one is solved through semidefinite programming relaxation and a proposed greedy-iterative method, which can achieve higher performance and lower computational complexity, respectively.
Highlights
Multiple antenna systems exploit spatial diversity to increase throughput and to enhance the reliability of the wireless channel
Since the energy efficiency in turn is a function of data rate, power consumption and frequency/time resource usage, this significant efficiency improvement with Reconfigurable Intelligent Surface (RIS) can address several major issues arising from future wireless applications such as increasing demand for data rates, spectrum crunch, high energy consumption and environment impact
Our analysis reveals that the outage decreases at log(ρ)/ρ rate, whereas the spectral efficiency increases at log(ρ) rate for asymptotically large signal-to-interference-plus-noise ratio (SINR), ρ
Summary
Multiple antenna systems exploit spatial diversity to increase throughput and to enhance the reliability of the wireless channel. Since the energy efficiency in turn is a function of data rate, power consumption and frequency/time resource usage, this significant efficiency improvement with RIS can address several major issues arising from future wireless applications such as increasing demand for data rates, spectrum crunch, high energy consumption and environment impact. This brand-new concept has already been proposed to incorporated into various wireless techniques – multi-cell multipleinput multiple-output (MIMO) systems [3], massive MIMO [4], non-orthogonal multiple access (NOMA) [5], energy harvesting [6], optical communications [7] to name a few. The remainder of this section has an overview of related work, followed by a summary on contributions of this work
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