Transmitter CSI-Free RIS-Randomized CUMA for Extreme Massive Connectivity

This paper proposes a transmitter channel state information (CSI)-free, massive connectivity scheme in the downlink that requires no optimization from the base station (BS) side. Inter-user interference is dealt with solely by the mobile users themselves, each equipped with a position-flexible fluid antenna system (FAS). Specifically, we adopt the compact ultra massive antenna (CUMA) receiver architecture at the FAS of each user, which protects the desired user’s signal from interference without the need of the interferers’ CSI. Different from previous work, we incorporate the return loss and isolation parameters into the channel model to properly reflect realistic mutual coupling countermeasures and deploy randomized reconfigurable intelligent surfaces (RISs) in the environment to restore rich scattering channel conditions. Our simulation results demonstrate that under these conditions at $26~{\textrm {GHz}}$ , CUMA can accommodate 50 users per channel use to achieve a rate of $45~{\textrm {bps/Hz}}$ , without power control and precoding at the BS and also no successive interference cancellation (SIC) at each user. The rate will be increased to $54~{\textrm {bps/Hz}}$ if 100 users are served, revealing the possibility of massive connectivity without the CSI at the BS.