Capacity Maximization for FAS-assisted Multiple Access Channels

TL;DR

This paper explores capacity maximization in multiuser mmWave uplink systems with fluid antenna systems (FAS), proposing optimization algorithms that significantly enhance system capacity over benchmarks.

Contribution

It introduces joint optimization of transmit covariance and antenna positions for FAS-assisted channels, providing closed-form solutions and efficient algorithms for multiuser scenarios.

Findings

FAS significantly improves MAC capacity.

Proposed algorithms outperform benchmarks.

Closed-form solutions for single-user, two-path cases.

Abstract

This paper investigates a multiuser millimeter-wave (mmWave) uplink system in which each user is equipped with a multi-antenna fluid antenna system (FAS) while the base station (BS) has multiple fixed-position antennas. Our primary objective is to maximize the system capacity by optimizing the transmit covariance matrices and the antenna position vectors of the users jointly. To gain insights, we start by deriving upper bounds and approximations for the capacity. Then we delve into the capacity maximization problem. Beginning with the simple scenario of a single user equipped with a single-antenna FAS, we demonstrate that a closed-form optimal solution exists when there are only two propagation paths between the user and the BS. In the case where multiple propagation paths are present, a near-optimal solution can also be obtained through a one-dimensional search method. Expanding our focus to multiuser cases, in which users are equipped with either single- or multi-antenna FAS, we show that the original capacity maximization problems can be reformulated into distinct rank-one programmings. Then, we propose alternating optimization algorithms to deal with the transformed problems. Simulation results indicate that FAS can improve the capacity of the multiple access channel (MAC) greatly, and the proposed algorithms outperform all the benchmarks.