MULTI-HOP COMMUNICATION VIA RIS AND RELAY

Abstract

Multi-hop technique is used in wireless networks for effective communication. In this technique, the transmitter and the receiver exchange information through an intermediate node instead of direct communication. The Multi-hop communication is an energy efficient scheme which achieves better throughput and increases the network coverage area when compared to a conventional cellular network. In our work, we have studied two such multi-hop communication techniques namely, relay and reconfigurable intelligent surface (RIS). We first investigate a multi-pair full-duplex (FD) two-way communication system assisted by an RIS, where each user-pair exchanges information via RIS. Unlike most existing works related to FD systems with RIS, we take into account an inevitable electromagnetic interference (EMI) and also consider a practical Rician channel between RIS and users, and spatial correlation owing to size constraint at the RIS. In addition, we consider low-cost hardware at RIS and the users resulting in impairments. We first derive an approximate expression for the achievable sum-rate of the system. We then optimize RIS phases to achieve the optimum sum-rate using particle swarm optimization (PSO) algorithm. We, through the simulation results, show the optimality of PSO algorithm and the impact of EMI and hardware impairments on the sum-rate. We also study a FD multi-pair communication system assisted by relay in which the user pairs communicate through the relay. We formulate the weighted sum energy efficiency (WSEE) optimization problem for this system and solve the same using quadratic transform (QT) technique. We also analyze the impact of exposure index (EI), a measure of electromagnetic radiation exposure, on the performance of the system. This EI effect is ignored by many of the existing works and we prove in our work that this effect is significant and cannot be ignored in practical systems.