On the Impact of Control Signaling in RIS-Empowered Wireless Communications

The research on Reconfigurable Intelligent Surfaces (RISs) has dominantly been focused on physical-layer aspects and analyses of the achievable adaptation of the wireless propagation environment. Compared to that, questions related to system-level integration of RISs have received less attention. We address this research gap by analyzing the control signaling operations needed to integrate the RIS as a new wireless infrastructure element. As the main contribution of the paper, we build a systematic procedure for evaluating the impact of control operations on communication performance along two dimensions: i) the rate selection for the data channel (multiplexing or diversity), and ii) the allocated bandwidth of the control channels (in-band and out-of-band). Specifically, the first dimension results in two generic transmission paradigms: one focuses on optimizing RIS setting according to the propagation environment, labeled as optimization based on channel estimation (OPT-CE); the other is based on sweeping through predefined RIS phase configurations, labeled as codebook-based beam sweeping (CB-BSW). We analyze the communication performance in multiple setups built along these two dimensions. While necessarily simplified, our analysis reveals the basic trade-offs in RIS-assisted communications and the associated control operations: CB-BSW is better suited for high mobility scenarios since its operation is not conditional on performing channel estimation within the coherence time; OPT-CE performs significantly better when the channel coherence time is sufficiently long, but it requires exchanging more control data, necessitating higher control reliability and profiting more from out-of-band control channel design. Our systematic procedure can be easily adapted to include more complex systems and transmission modes.