Fluid Antenna System: New Insights on Outage Probability and Diversity Gain

TL;DR

This paper introduces a novel fluid antenna system (FAS) for 6G communications, providing analytical insights into its outage probability and diversity gain, and proposing a suboptimal design that outperforms traditional systems.

Contribution

It derives closed-form expressions for FAS outage probability and diversity gain, and proposes a suboptimal FAS with significant performance improvements over existing systems.

Findings

Suboptimal FAS with N* ports outperforms systems with N*-1 ports.

Proposed FAS surpasses SISO and SC systems in outage probability.

Near-MRC performance achieved with a single RF-chain when space is λ/2.

Abstract

To enable innovative applications and services, both industry and academia are exploring new technologies for sixth generation (6G) communications. One of the promising candidates is fluid antenna system (FAS). Unlike existing systems, FAS is a novel communication technology where its antenna can freely change its position and shape within a given space. Compared to the traditional systems, this unique capability has the potential of providing higher diversity and interference-free communications. Nevertheless, the performance limits of FAS remain unclear as its system properties are difficult to analyze. To address this, we approximate the outage probability and diversity gain of FAS in closed-form expressions. We then propose a suboptimal FAS with $N^{*}$ ports, where a significant gain can be obtained over FAS with $N^{*}-1$ ports whilst FAS with $N^{*}+1$ ports only yields marginal improvement over the proposed suboptimal FAS. In this paper, we also provide analytical and simulation results to unfold the key factors that affect the performance of FAS. Limited to systems with one active radio frequency (RF)-chain, we show that the proposed suboptimal FAS outperforms single-antenna (SISO) system and selection combining (SC) system in terms of outage probability. Interestingly, when the given space is $\frac{\lambda}{2}$, the outage probability of the proposed suboptimal FAS with one active RF-chain achieves near to that of the maximal ratio combining (MRC) system with multiple active RF-chains.