# Reconfigurable Intelligent Surface Assisted Two-Way Communications: Performance Analysis and Optimization

Saman Atapattu, Rongfei Fan, Prathapasinghe Dharmawansa, Gongpu Wang, Jamie Evans, Theodoros A. Tsiftsis

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)

## 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 $\left ({\log (\rho)/\rho }\right)^{L}$ 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 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.

Original language English 9138463 6552-6567 16 IEEE Transactions on Communications 68 10 https://doi.org/10.1109/TCOMM.2020.3008402 Published - Oct 2020

## Keywords

• Outage probability
• reconfigurable intelligent surface (RIS)
• spectral efficiency
• two-way communications

## ASJC Scopus subject areas

• Electrical and Electronic Engineering