Sparsity-Exploiting Channel Estimation For Unsourced Random Access With Fluid Antenna

TL;DR

This work addresses channel estimation for uplink URA with a fluid antenna system (FAS), leveraging the extra spatial diversity to improve activity detection and decoding under limited RF chains. It introduces two port-activation CE strategies—alternate-ports CE (AP-CE) and partial-ports CE (PP-CE)—and uses a channel dictionary to estimate AoAs and refine channel reconstruction under planar propagation with LOS and scatterers. A regularized estimator for PP-CE and an optimization over port index gaps are proposed to balance AoA precision and channel refinement, with Simultaneous Orthogonal Matching Pursuit (SOMP) enabling joint activity and channel recovery. Numerical results indicate that AP-CE delivers superior AoA accuracy while PP-CE, particularly with dictionary-based refinement, yields the best overall CE performance versus a conventional ULA baseline, highlighting the potential of integrating URA with fluid antennas for enhanced massive connectivity.

Abstract

This work explores the channel estimation (CE) problem in uplink transmission for unsourced random access (URA) with a fluid antenna receiver. The additional spatial diversity in a fluid antenna system (FAS) addresses the needs of URA design in multiple-input and multiple-output (MIMO) systems. We present two CE strategies based on the activation of different FAS ports, namely alternate ports and partial ports CE. Both strategies facilitate the estimation of channel coefficients and angles of arrival (AoAs). Additionally, we discuss how to refine channel estimation by leveraging the sparsity of finite scatterers. Specifically, the proposed partial ports CE strategy is implemented using a regularized estimator, and we optimize the estimator's parameter to achieve the desired AoA precision and refinement. Extensive numerical results demonstrate the feasibility of the proposed strategies, and a comparison with a conventional receiver using half-wavelength antennas highlights the promising future of integrating URA and FAS.

Figures (9)

• Figure 1: Illustration of planar propagation with LOS path and multiple scattered paths under possible FAS and ULA structures. Only finite RF-chains are assumed, i.e., $M\ll N$.
• Figure 2: Illustration of the proposed AP-CE strategy and possible ports activation patterns (vicinity/interval activation).
• Figure 3: Illustration of the proposed PP-CE strategy and ports activation patterns with index gap $\Delta$.
• Figure 4: Performance with different power allocation under geometric model.
• Figure 5: The value of subject function \ref{['eq:17']} under different index gaps and number of ports.