Enabling Decentralized Access Policy Selection by Exploiting RIS-Induced Channel Diversity

Novel random access (RA) protocols have been developed to leverage the radio resources and ensure fair connectivity in the challenging task of handling high numbers of machine-type devices (MTDs). In combination with promising technologies such as reconfigurable intelligent surfaces (RISs), such protocols can expand the network coverage and spectral efficiency. Although most works addressing RISs neglect the direct radio link between the base station (BS) and MTDs, in practical scenarios, devices with a non-negligible direct link with the BS might coexist with those without. Hence, ignoring that might produce unrealistic performance evaluations. This paper proposes a grant-free RA protocol where the MTDs choose the access policy based on channel observations to determine whether they need RIS assistance for data transmission. This selection is decentralized at the MTDs and is oriented by the spatiotemporal diversity induced by the RIS beam configurations. MTDs with an impaired direct link might sense higher variations among the estimated channel coefficients, being programmed to select an access policy that exploits the RIS-induced diversity. Conversely, MTDs with a robust direct link experience low channel variability, selecting another policy. Simulation results reveal that, when the MTDs adapt the access policy according to the cell region they are located, the overall system throughput can be substantially increased, especially under medium and high channel load scenarios.