A Study of Interference Effect on the Performance of RIS-Equipped Source and Relays DF Relay Network

This paper studies the interference impact on the performance of reconfigurable intelligent surface (RIS)-equipped decode-and-forward (DF) relay networks, where RIS is used as internal part of both the source and relay nodes. For that purpose, approximate closed-form expression is derived for the system outage probability assuming Rayleigh fading channels and opportunistic relaying scheme. Furthermore, to get more insights at the system performance, an approximate but accurate expression is achieved for the outage probability at the high signal-to-noise ratio (SNR) regime, where the system diversity order and coding gain are obtained and analyzed. The findings show that the system can achieve a diversity order of G_d=min(N₁,N₂)K, where N₁ and N₂ are the numbers of reflecting elements at the source and relays, respectively, and K is the number of relays. Additionally, results illustrate that for the same diversity order, utilizing one relay with multiple reflecting elements gives better performance than utilizing multiple relays each with a single reflecting element. Moreover, findings show that the number of relays not only affects the system diversity order, but also the coding gain of the first hop when it is dominating the system performance. Furthermore, results illustrate that when N₁=N₂, the system performance is dominated by the largest interference at either the relay node or the destination. With unequal interference powers, changing number of interferers at the node where interference is larger affects the performance, whereas changing number of interferers at the other node has no effect on the system behavior. On the other hand, when both nodes have the same interference power, changing number of interferers at either node by the same amount results in the same effect on the system performance.