Performance Analysis of Fluid Antenna System under Spatially-Correlated Rician Fading Channels

TL;DR

This work analyzes a Rx-SISO-FAS system operating over spatially correlated Rician fading, deriving exact and bounded expressions for outage probability (OP) and ergodic rate (ER) while comparing uniform linear (ULA) and planar (UPA) port configurations. It develops joint statistics of port gains, establishes a high-SNR diversity order approximately equal to the number of ports ($d_N\approx N$), and demonstrates that increasing port count enhances diversity but can degrade OP under strong LoS in certain regimes. The results show that FAS can outperform L-branch maximum ratio combining (MRC) in Rician channels, with UPA generally outperforming ULA for the same $N$, and provide practical guidance on when more space and ports yield the greatest gains. Overall, the paper substantiates the viability of FAS as a scalable DoF for 6G and offers closed-form tools for system designers to evaluate OP and ER under realistic fading and correlation structures.

Abstract

Fluid antenna systems (FAS) are among the most promising technologies for the sixth generation (6G) mobile communication networks. Unlike traditional fixed-position multiple-input multiple-output (MIMO) systems, a FAS possesses position reconfigurability to switch on-demand among $N$ predefined ports over a prescribed space. This paper explores the performance of a single-input single-output (SISO) model with a fixed-position antenna transmitter and a single-antenna FAS receiver, referred to as the Rx-SISO-FAS model, under spatially-correlated Rician fading channels. Our contributions include exact expressions and closed-form bounds for the outage probability of the Rx-SISO-FAS model, as well as exact and closed-form lower bounds for the ergodic rate. Importantly, we also analyze the performance considering both uniform linear array (ULA) and uniform planar array (UPA) configurations for the ports of the FAS. To gain insights, we evaluate the diversity order of the proposed model and our analytical results indicate that with a fixed overall system size, increasing the number of ports, $N$, significantly decreases the outage performance of FAS under different Rician fading factors. Our numerical results further demonstrate that: $i)$ the Rx-SISO-FAS model can enhance performance under spatially-correlated Rician fading channels over the fixed-position antenna counterpart; $ii)$ the Rician factor negatively impacts performance in the low signal-to-noise ratio (SNR) regime; $iii$) FAS can outperform an $L$ branches maximum ratio combining (MRC) system under Rician fading channels; and $iv)$ when the number of ports is identical, UPA outperforms ULA.

Figures (11)

• Figure 1: The ULA port configuration for FAS.
• Figure 2: The UPA port configuration for FAS.
• Figure 3: The OP against the number of ports $N$ under different Rician fading factor $\kappa$ and SNR threshold $\gamma_{th}$, where the analytical results are derived from \ref{['pout']}.
• Figure 4: The OP against the SNR threshold $\gamma_{th}$ under different Rician fading factor $\kappa$ and number of ports $N$.
• Figure 5: The OP against the Rician fading factor $\kappa$ under different number of ports $N$ when $\gamma_{th}$ = 3 dB and $\gamma_{th}$ = 6 dB.

Theorems & Definitions (38)

• Lemma 1
• proof
• Lemma 2
• proof
• Theorem 1
• proof
• Corollary 1
• proof
• Corollary 2
• proof