Fluid antenna systems (FAS) improve wireless reliability by selecting the strongest port within a compact aperture, but the same port-selection capability also changes the physical-layer security behavior of a wiretap channel. Exact secrecy outage analysis is difficult because the legitimate and eavesdropping links each contain many spatially correlated ports, leading to high-dimensional joint distributions. This paper develops a closed-form secrecy outage framework for FAS wiretap channels using the equivalent degrees of freedom (EDoF) principle. Under the Jakes spatial correlation model, the many correlated physical ports are represented by a much smaller set of aperture-limited dominant spatial modes. The resulting model converts the correlated wiretap channel into a tractable competition between the effective selection branches available to Bob and Eve. An eigenvalue-weighted refinement further captures unequal dominant-mode powers. Closed-form and asymptotic results reveal how Bob’s aperture determines secrecy diversity, how Eve’s aperture strengthens the eavesdropping link, and why proportional Bob-Eve signal-to-noise ratio scaling produces a nonzero secrecy outage floor. Numerical simulations verify the analytical trends and show that the decisive security resource is Bob’s aperture-induced EDoF advantage rather than the raw number of selectable ports.

Extremely large-scale multiple-input multiple-output (XL-MIMO) and terahertz (THz) communications are pivotal candidate technologies for supporting the development of 6G mobile networks. However, these techniques invalidate the common assumptions of far-field plane waves and introduce many new properties. To accurately understand the performance of these new techniques, spherical wave modeling of near-field communications needs to be applied for future research. Hence, the investigation of near-field communication holds significant importance for the advancement of 6G, which brings many new and open research challenges in contrast to conventional far-field communication. In this paper, we first formulate a general model of the near-field channel and discuss the influence of spatial nonstationary properties on the near-field channel modeling. Subsequently, we discuss the challenges encountered in the near field in terms of beam training, localization, and transmission scheme design, respectively. Finally, we point out some promising research directions for near-field communications.