On Outage Probability for Two-User Fluid Antenna Multiple Access

Fluid antenna system (FAS) is an emerging flexible antenna technology that provides a new way for multiple access. In fluid antenna multiple access (FAMA), each user switches its fluid antenna to the location (i.e., port) in which the interfering users suffer from a deep fade for interference-free communication. Previous work has attempted to understand the interference immunity of FAMA but the results are limited to simplified spatial correlation models. In this paper, we revisit the FAMA system with only two users by characterizing the joint spatial correlation amongst all the ports. Using this model, however, the number of variables determining each channel coefficient scales with that of ports, hence making the analysis intractable. To tackle this, we first show that the channel model could be considerably simplified by taking into account only a few variables, and then derive the outage probability for the considered FAMA system by using the approximated model. Simulation results show that the simplified channel model can quickly approach the exact one and that the outage probability decreases with the number of ports but has an error floor unless the size of fluid antenna is increased.