A Framework of FAS-RIS Systems: Performance Analysis and Throughput Optimization

TL;DR

This paper develops a comprehensive framework for FAS-RIS systems, optimizing throughput in static and challenging environments by designing transmission schemes with and without channel state information.

Contribution

It introduces a novel framework for FAS-RIS systems that includes both CSI-based and CSI-free transmission design approaches, with new algorithms for throughput optimization.

Findings

The proposed algorithms effectively maximize throughput in FAS-RIS systems.

The framework performs well in static and CSI-challenged environments.

Simulation results confirm the effectiveness of the proposed methods.

Abstract

In this paper, we investigate reconfigurable intelligent surface (RIS)-assisted communication systems which involve a fixed-antenna base station (BS) and a mobile user (MU) that is equipped with fluid antenna system (FAS). Specifically, the RIS is utilized to enable communication for the user whose direct link from the base station is blocked by obstacles. We propose a comprehensive framework that provides transmission design for both static scenarios with the knowledge of channel state information (CSI) and harsh environments where CSI is hard to acquire. It leads to two approaches: a CSI-based scheme where CSI is available, and a CSI-free scheme when CSI is inaccessible. Given the complex spatial correlations in FAS, we employ block-diagonal matrix approximation and independent antenna equivalent models to simplify the derivation of outage probabilities in both cases. Based on the derived outage probabilities, we then optimize the throughput of the FAS-RIS system. For the CSI-based scheme, we first propose a gradient ascent-based algorithm to obtain a near-optimal solution. Then, to address the possible high computational complexity in the gradient algorithm, we approximate the objective function and confirm a unique optimal solution accessible through a bisection search method. For the CSI-free scheme, we apply the partial gradient ascent algorithm, reducing complexity further than full gradient algorithms. We also approximate the objective function and derive a locally optimal closed-form solution to maximize throughput. Simulation results validate the effectiveness of the proposed framework for the transmission design in FAS-RIS systems.