UAV-Enabled Short-Packet Communication via Fluid Antenna Systems

This paper develops a framework for analyzing UAV-enabled short-packet communication, leveraging fluid antenna system (FAS)-assisted relaying networks. Operating in the short-packet regime and focusing on challenging urban environments, we derive novel, closed-form expressions for the block error rate (BLER). This is achieved by modeling the spatially correlated Nakagami-$m$ fading link via a tractable eigenvalue-based approach. A high-signal-to-noise ratio (SNR) asymptotic analysis is also presented, revealing the system’s fundamental diversity order. Building on this analysis, we formulate a novel energy efficiency (EE) maximization problem that, unlike idealized models, uniquely incorporates the non-trivial time and energy overheads of FAS port selection. An efficient hierarchical algorithm is proposed to jointly optimize key system parameters. Numerical results validate our analysis, demonstrating that while FAS provides substantial power gains, the operational overhead creates a critical trade-off. This trade-off dictates an optimal number of FAS ports and a non-trivial optimal UAV deployment altitude, governed by the balance between blockage and path loss. This work provides key insights for FAS-aided UAV communications.

💡 Research Summary

This paper presents a comprehensive analytical framework for UAV‑enabled short‑packet (finite‑blocklength) communication that incorporates fluid‑antenna‑system (FAS) assisted relaying in dense urban environments. Recognizing that ultra‑reliable low‑latency communication (URLLC) requires performance metrics beyond classical Shannon capacity, the authors model the BS‑UAV and UAV‑UE links with probabilistic line‑of‑sight (LoS) / non‑LoS conditions and spatially correlated Nakagami‑m fading. The UAV follows a circular trajectory at a variable altitude, while the UE is equipped with an N‑port FAS that can reconfigure its antenna position electronically, thereby achieving spatial diversity with a single RF chain.

A key technical contribution is the eigenvalue‑based representation of the correlated Nakagami‑m channel for the second hop. By decomposing the Jakes correlation matrix, the authors define an effective number of independent diversity branches, N_eff, and express the selected channel gain as the maximum of weighted Gamma‑distributed variables. This tractable model enables the derivation of closed‑form average block error rate (BLER) expressions for both hops under finite blocklength theory. Lemma 1 and Lemma 2 give the cumulative distribution functions (CDFs) of the instantaneous SNRs, which are then integrated using a piecewise linear approximation of the Q‑function. The resulting BLER formulas (13) and (15) are validated against Monte‑Carlo simulations and shown to be highly accurate.

High‑SNR asymptotic analysis (Theorem 1) reveals that the diversity order of the FAS‑enabled link equals m_k · N_eff, where m_k is the Nakagami‑m parameter for LoS or NLoS conditions. Theorem 2 demonstrates a “first‑hop bottleneck”: even if the UAV transmit power grows without bound, the overall BLER converges to a floor determined solely by the BS‑UAV link. This insight emphasizes that improving the UAV‑side link alone yields diminishing returns, and that system design must prioritize the BS‑UAV link quality.

Beyond performance analysis, the paper introduces a realistic energy‑efficiency (EE) model that explicitly accounts for the time (T_sw = N·τ_p) and power (P_sw) overhead associated with FAS port selection. EE is defined as successfully delivered bits per joule, incorporating transmit power, static circuit power, and switching power. The authors formulate a non‑convex mixed‑integer nonlinear program (MINLP) to maximize EE subject to a BLER constraint, power limits, altitude bounds, blocklength limits, and the requirement that switching time does not exceed the total block duration.

To solve the MINLP, a hierarchical algorithm is proposed: (i) for given blocklength, altitude, and port number, the minimum UAV transmit power satisfying the BLER constraint is found via a one‑dimensional bisection search; (ii) the optimal number of FAS ports is obtained by a discrete search over N, using the power obtained in step (i); (iii) the optimal UAV altitude is then searched over the feasible interval; finally, an exhaustive search over admissible blocklength values yields the global optimum.

Numerical results validate the analytical expressions and illustrate several design trade‑offs. The FAS provides a notable power gain over a conventional fixed‑position antenna (approximately 3 dB for a target BLER of 10⁻⁴). Increasing the antenna aperture W improves BLER by reducing spatial correlation, but gains saturate beyond about 2 λ. The UAV altitude that minimizes required transmit power exhibits a convex shape, with an optimal altitude around 450 m that balances blockage avoidance (favoring higher altitudes) against increased path loss (favoring lower altitudes). Energy‑efficiency analysis shows that EE first increases with the number of ports, reaches a peak (typically for N ≈ 4–8), and then declines due to the growing switching overhead. Similarly, moderate blocklengths (e.g., 500–1000 bits) achieve the highest EE, reflecting the trade‑off between coding gain and overhead.

In summary, the paper makes four major contributions: (1) a tractable eigenvalue‑based model for spatially correlated Nakagami‑m fading in urban UAV‑FAS links; (2) closed‑form finite‑blocklength BLER expressions and high‑SNR asymptotics that expose the diversity order and first‑hop bottleneck; (3) a realistic EE formulation that incorporates FAS selection overhead; and (4) a hierarchical optimization algorithm that jointly selects blocklength, UAV altitude, transmit power, and number of FAS ports. The results provide concrete guidelines for designing energy‑efficient, ultra‑reliable UAV communication systems that exploit fluid‑antenna technology in the challenging urban landscape.