Partial Fluid Antenna System: Port Selection Via Statistical Analysis

The fluid antenna system (FAS) enables position reconfigurability, granting the transceiver access to a high-resolution spatial signal. A potential drawback of real-time FAS, however, is that it requires complete channel state information (CSI) for each FAS port at every communication time slot, an approach referred to as ideal-FAS. Recognizing the difficulties of achieving ideal-FAS, we propose a FAS scheme based on incomplete CSI, referred to as partial-FAS. This paper first introduces the spatial-temporal framework of FAS, upon which the proposed partial-FAS is developed. The proposed partial-FAS is lightweight and computationally efficient, scalable to an arbitrary number of ports and time slots, and operates without pre-training or deep learning structures. The scheme effectively exploits incomplete historical CSI to estimate the conditional distribution across all FAS ports at the desired time slot, thereby identifying the statistical optimal port for signal reception. Generally, the key idea of partial-FAS is to select the optimal port through conditional distribution analysis, from a statistical perspective, with optimality defined according to the scenario of interest. Moreover, we derive a closed-form expression for the placement of optimal port, where optimality is defined as the port that minimizes the outage probability, in the special case where only a single port CSI is available. Inspired by information-theoretic entropy, we further develop the residual entropy power ratio to characterize how physical parameters influence the performance gap between partial-FAS and ideal-FAS. Our analysis reveals that estimation performance depends not only on the number of sampled ports and time slots, but also on the specific indices of ports with given CSI at each time slot, i.e., the port sampling strategy. This critical factor has been largely overlooked in existing port estimation studies. In addition, we establish the Markov condition for the proposed partial-FAS, which provides insights into the physical design of time slot duration and the number of historical CSI samples required for partial-FAS to closely approximate ideal-FAS. Numerical results demonstrate that the proposed partial-FAS achieves performance comparable to, and in some cases indistinguishable from, that of ideal-FAS, while requiring significantly fewer port CSI measurements and lower port switching speeds.