Fluid Antenna System—Part III: A New Paradigm of Distributed Artificial Scattering Surfaces for Massive Connectivity

Reconfigurable intelligent surface (RIS) has recently emerged as a promising technology to extend the coverage of a base station (BS) in wireless communication networks. However, the adoption of RIS comes with the challenges of highly complex joint optimization of the multiple-input multiple-output (MIMO) precoding matrix at the BS and the phase shifters of the RIS as well as estimation of the cascaded channels. To circumvent this, this letter presents a new paradigm that uses RISs as distributed artificial scattering surfaces (DASSs) to produce a rich scattering environment that enables fluid antenna system (FAS) to prevent multiuser interference at each user equipment (UE). The use of fluid antenna multiple access (FAMA) liberates MIMO and RIS and greatly simplifies their optimization. Our simulation results show that with DASS, slow FAMA can obtain a high multiplexing gain without precoding and phase shifter design when the direct link does not exist. In the presence of the direct link, nonetheless, BS precoding becomes essential. Our results further reveal that fast FAMA with 20 DASSs can accommodate 64 co-channel UEs to achieve a multiplexing gain of 59.3 without precoding at the BS nor RIS phase shifter optimization and the direct link.