Ambient backscatter communication (AmBC) enables ultra-low-power connectivity by allowing passive backscatter devices (BDs) to convey information through reflection of ambient signals. However, the cascaded AmBC channel suffers from severe double path loss and multiplicative fading, while accurate channel state information (CSI) acquisition is highly challenging due to the weak backscattered signal and the resource-limited nature of BDs. To address these challenges, this paper considers an AmBC system in which the reader is equipped with a pixel-based fluid antenna system (FAS). By dynamically selecting one antenna position from a dense set of pixels within a compact aperture, the FAS-enabled reader exploits spatial diversity through measurement-driven port selection, without requiring explicit CSI acquisition or multiple RF chains. The intrinsic rate-energy tradeoff at the BD is also incorporated by jointly optimizing the backscatter modulation coefficient under an energy harvesting (EH) neutrality constraint. To efficiently solve this problem, a particle swarm optimization (PSO)-based framework is developed to jointly determine the FAS port selection and modulation coefficient on an optimize-then-average (OTA) basis. Simulation results show that the proposed scheme significantly improves the achievable rate compared with conventional single-antenna readers, with gains preserved under imperfect observations, stringent EH constraints, and different pixel spacings.
A MBIENT backscatter communication (AmBC) has emerged as a key enabling technology for ultra-lowpower and battery-less connectivity in large-scale Internet-of-Things (IoT) networks [1], [2]. By modulating and reflecting incident radio-frequency (RF) signals instead of generating new waveforms, backscatter devices (BDs) can operate with minimal energy consumption and hardware complexity [3], [4]. Despite these advantages, AmBC performance is fundamentally limited by the cascaded channel formed by sourceto-BD and BD-to-reader links. The resulting double path loss and multiplicative fading cause severe signal attenuation, which significantly constrains AmBC coverage and reliability [5], [6]. In addition, acquiring accurate instantaneous channel state information (CSI) is particularly challenging due to the extremely weak backscattered signal and the inability of passive BDs to support explicit channel training [7], [8].
Recent studies have explored fluid antenna systems (FAS) [9], [10] as an effective means of mitigating channel impair-ments in backscatter and energy-constrained wireless systems [11]- [13]. Early analytical works show that antenna position flexibility at the reader can significantly improve outage performance and reliability in backscatter communications (BC) under spatially correlated fading, confirming the diversity gains of FAS over conventional fixed-antenna readers [11]. Related studies have extended FAS concepts to wireless powered systems, demonstrating notable improvements in outage probability, secrecy performance, and achievable capacity when a single FAS port is optimally selected [12], [13].
Despite the promising performance gains reported in [11]- [13], existing FAS-aided backscatter studies predominantly rely on idealized assumptions, such as analytically tractable fading models and perfect or statistical channel knowledge. In particular, the effect of channel gain observation uncertainty, an inherent characteristic of AmBC arising from the extremely weak backscattered signal and the absence of explicit channel training at passive BDs, has received little attention. Furthermore, prior works typically decouple communication design from the energy harvesting (EH) requirements of the BD by assuming fixed or unconstrained reflection coefficients. In practical AmBC deployments, however, the BD must sustain its operation through harvested energy while simultaneously enabling reliable information transfer, giving rise to an intrinsic rate-energy tradeoff that directly impacts system performance [14], [15]. These considerations expose a clear gap between existing analytical FAS performance characterizations and the development of robust, implementable antenna selection strategies suited to realistic AmBC conditions. Motivated by this gap, this paper investigates an AmBC system in which the reader is equipped with a pixel-based FAS to enhance the cascaded backscatter channel under practical operating constraints. Instead of assuming perfect CSI, antenna port selection is performed based solely on noisy observations of the cascaded channel gains. To explicitly account for energy sustainability at the BD, we formulate a reader-centric optimization problem that maximizes the achievable rate by jointly optimizing the FAS port selection and the backscatter modulation coefficient under an EH constraint. Owing to the nonconvex nature of the resulting problem, a particle swarm optimization (PSO)-based framework is developed that operates directly on observed channel gains. Simulation results
Fig. 1. The considered AmBC system with pixel-based FAS at the reader.
confirm that the proposed FAS-enabled reader consistently outperforms traditional fixed-antenna benchmarks across a wide range of operating conditions.
We consider an AmBC setup consisting of an RF source (illuminator), a single-antenna passive BD, and a reader equipped with a pixel-based FAS, as illustrated in Fig. 1. The RF source continuously transmits an unmodulated carrier (or a known waveform) with transmit power P s . The BD harvests energy from the incident RF signal and conveys information by switching its load impedance between two states, corresponding to binary backscatter modulation [16], [17]. The reader employs the FAS to activate one antenna port (pixel) at a time from a set of K preset ports distributed over a linear aperture of length W λ, where λ denotes the carrier wavelength and W is the normalized FAS size. The K FAS ports are uniformly spaced along the aperture with sub-wavelength inter-port distance d [18].
Let h s ∈ C denote the forward channel from the RF source to the BD, and h b,k ∈ C denote the backscatter channel from the BD to the k-th FAS port at the reader, where k ∈ {1, . . . , K}. The corresponding channel power gains are defined as
Due to the finite aperture of the FAS, the backscatter channels {h b,k } are spatially correlated. A commonly adopted correlation model is the Jakes’ correlation across
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