Extra‐large MIMO enabling slow fluid antenna massive access for millimeter‐wave bands

Abstract

Fluid antenna multiple access (FAMA) is a new way of accommodating a large number of users on a single channel for massive connectivity, with slow FAMA (s-FAMA) being the practical version for achieving this. The impressive performance is understood to be achievable if the users have independent Rayleigh fading envelopes. With mobile networks vamping up the operating frequencies in 5G and beyond, nevertheless, the channel will have less multipath and become more directional. It is unclear if s-FAMA still performs well and how its performance is affected by different channel parameters. To address this, this letter first develops a multipath fading channel model capable of modelling a mixture of directional line-of-sight (LoS) and non-LoS paths over the ports of a fluid antenna system. The results indicate that a large number of paths in the channel is essential to the performance and a large Rice factor will degrade the performance rapidly. Also, contrast to the initial belief, the size of the fluid antenna plays a more important role than the number of ports or resolution of the fluid antenna. To restore the gain of s-FAMA, it is proposed to employ extra-large multiple-input multiple-output (XL-MIMO) at the base station (BS) to scramble the channel and create artificial multipath so that the users' envelopes can become independent Rayleigh again. The results confirm that XL-MIMO enabling s-FAMA is an effective technique for massive connectivity in the directional millimeter-wave (mmWave) bands.