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.

翻译：环境反向散射通信（AmBC）通过无源反向散射设备（BD）反射环境信号来传输信息，实现了超低功耗连接。然而，级联的AmBC信道存在严重的双路径损耗和乘性衰落，同时由于反向散射信号微弱且BD资源受限，准确获取信道状态信息（CSI）极具挑战性。为应对这些挑战，本文研究了一种配备基于像素的流体天线系统（FAS）阅读器的AmBC系统。通过在紧凑孔径内从密集像素集中动态选择天线位置，配备FAS的阅读器通过测量驱动的端口选择来利用空间分集，无需显式获取CSI或多条射频链。本文还通过联合优化能量收集（EH）中性约束下的反向散射调制系数，考虑了BD固有的速率-能量权衡问题。为高效求解该问题，开发了基于粒子群优化（PSO）的框架，以“先优化后平均”（OTA）的方式联合确定FAS端口选择和调制系数。仿真结果表明，与传统单天线阅读器相比，所提方案显著提高了可达速率，且在非完美观测、严格EH约束和不同像素间距下仍能保持性能增益。