Fluid antenna systems (FAS) have recently emerged as a promising solution for sixth-generation (6G) ultra-dense connectivity. These systems utilize dynamic radiating and/or shaping techniques to mitigate interference and improve spectral efficiency without relying on channel state information (CSI). The reported improvements achieved by employing a single dynamically activated radiating position in fluid antenna multiple access (FAMA) are significant. To fully realize the potential of FAMA in multi-user multiplexing, we propose leveraging the unique fast-switching capabilities of a single radio-frequency (RF)-chain meta-fluid antenna structure to achieve multi-activation. This allows for a significantly larger set of independent radiating states without requiring additional signal processing. Simulations demonstrate that multi-activation FAMA enables robust multi-user multiplexing with a higher signal-to-interference ratio (SIR) under various Rayleigh-fading environments compared to other single RF-chain technologies. We further show that the SIR can be optimized within a 15~$μs$ timeframe under a multi-user Rayleigh-fading channel, making the proposed scheme highly suitable for fast-changing wireless environments. Verified through the theoretical Jakes' model, full three-dimensional (3D) electromagnetic (EM) simulations and experimental validation, multi-activation FAMA enables effective CSI-free, multi-user communication, offering a scalable solution for high-capacity wireless networks.

Fluid antenna systems (FAS) are emerging as a transformative enabler for sixth-generation (6G) wireless communications, providing unprecedented spatial diversity through dynamic reconfiguration of antenna ports. However, the inherent spatial correlation among ports poses significant challenges for accurate analysis. Conventional models such as Jakes are analytically intractable, while oversimplified constant-correlation models fail to capture the true behavior. In this work, we address these challenges by applying the variable block-correlation model (VBCM) -- originally proposed by Ramírez-Espinosa \textit{et al.} in 2024 -- to FAS security analysis, and by developing comprehensive optimization methods to enhance analytical accuracy. We derive new closed-form expressions for average secrecy capacity (ASC) and secrecy outage probability (SOP), demonstrating that the VBCM framework achieves simulation-aligned accuracy, with relative errors consistently below $5\%$ (compared to $10$--$15\%$ for constant-correlation models). To maximize ASC, we further design two algorithms: a grid search (GS) method and a gradient descent (GD) method. Numerical results reveal that the VBCM-based approach not only provides reliable insights into FAS security performance, but also yields substantial gains -- ASC improvements exceeding $120\%$ in high-threat scenarios and $18$--$19\%$ performance enhancements for compact antenna configurations. These findings underscore the practical value of integrating VBCM into FAS security analysis and optimization, establishing it as a powerful tool for advancing 6G communication systems.

The rapid development and proliferation of millimeter-wave (mmWave) networks have significantly boosted interest in wireless energy harvesting within this band. This article delineates a novel broadband, high-gain, multibeam ambient mmWave energy harvesting system. The system incorporates a modified Luneburg lens, a dual-polarized ME-dipole rectenna array, and a simple dc combiner to adeptly address the challenges associated with battery charging and replacement within the scope of industrial Internet of Things (IoT) applications. The system leverages frequency, spatial, and polarization diversity to optimize mmWave energy harvesting. The modified Luneburg lens realizes RF combination and multibeam characteristics, while a simplified rectifier network allows for efficient energy conversion. The antenna array has 29 antenna elements, each achieving 50% relative bandwidth covering 24–40 GHz. Beams have a gain of up to 27 dBi, providing 130° of 3-D spatial coverage. The rectifier design has been validated for high RF-to-DC efficiency across the operating frequency. Our prototype shows the system can provide an output dc voltage of 3.5 V, an output dc power of 6 dBm, and an RF-to-DC efficiency of about 40% when two transmitters produce a power density of 0.15 mW/cm2.

This study introduces a novel Wireless Power Transfer (WPT) system designed explicitly for distance-tolerant and angle-insensitive. The system incorporates a Fabry-Perot Cavity (FPC) and a Rectifying Metasurface (ReMS), effectively addressing the challenge of maintaining stable Direct Current (DC) output power across varying distances. The FPC antenna serves as the transmitter, while the ReMS, as one layer of the multi-layer FPC, combines the functionalities of a receiver and rectifier. The ReMS captures Radio Frequency (RF) energy from diverse incident angles through its top surface’s cross dipoles. Its bottom rectification network is designed based on the performance of the FPC antenna, aiming to improve the efficiency of RF-to-DC conversion. Comprising 64 cells, the ReMS achieves an energy harvesting efficiency of up to 94.5% at various incident angles. Additionally, the rectifier’s design has been validated with a peak rectifying efficiency of 62.6%. Prototype results indicate that the system, at an average unit input power of 10 dBm, attains considerably stable DC output power over a range of distances, with a total efficiency of approximately 40%-50%. The outcomes of this work lay the groundwork for developing self-powered units with wireless transmission capabilities, ideal for intelligent devices subject to frequent locational changes.

Millimeter-wave (mmWave) communication technologies offer abundant spectrum resources and higher data rates that present new opportunities to facilitate fifth-generation (5G) Internet of Things (IoT) systems. As IoT devices are randomly distributed in the environment, mmWave antennas with multiple switched directional beams are a better choice to reduce data collisions. In this context, a wideband multibeam antenna with a compact structure is presented for various mmWave IoT applications. First, a wideband dual-polarized magnetoelectric dipole (ME-dipole) antenna is developed as the excitation for the quasi-spherical lens. Next, by incorporating the lens with a group of the source antennas positioned at each edge of a hexagonal ground plane, six tilted beams with dual polarization are generated for pattern and polarization diversities. A prototype is constructed and measured, demonstrating a measured bandwidth from 27 to 37 GHz. Moreover, the maximum measured gain for each beam is around 18 dBi. Benefiting from the advantages of wideband operation and excellent radiation performance, the proposed mmWave-powered multibeam antenna can be mounted on the ceiling to communicate with multiple users and smart devices beneath it, offering a reliable and cost-effective solution for 5G IoT.

In this paper, stringent requirements imposed on the design of base station antennas for mobile communications are summarized. Conventional techniques for implementing base station antennas are reviewed. The complementary antenna concept of combining an electric dipole with a magnetic dipole is reconsidered. Recently, this kind of antenna has been commonly called a “Huygen's source.” The purpose is to develop wideband unidirectional antennas with stable frequency characteristics and low back radiation. Based on this concept, the magnetoelectric dipole was invented by integrating an electric dipole with an -probe fed shorted quarter-wave patch antenna. A number of magnetoelectric dipoles with different radiation patterns and different polarizations have been developed in recent years. An overview of the characteristics of this new class of complementary antennas is presented. Major design challenges are explained. Finally, a new magnetoelectric dipole that is low in profile and robust in structure is presented. The magnetic dipole part of this antenna is realized by a triangular-shaped loop antenna. The antenna is inherently direct current (dc) grounded, which satisfies the requirement for outdoor applications.

The objective of this paper is to provide the context, physical insight, and perspective on antennas in wireless communications. Although it does not mean to be comprehensive, the four key technologies related to small antenna designs to be reviewed and discussed, including multiband planar inverted-F antennas, broadband folded patch antennas, compact differentially fed antennas, and miniature circularly polarized patch antennas, would cover a wide range of topical interests and practical applications. A brief overview of computer software for analyzing these antennas is also provided. Hopefully, this paper will be beneficial for the diverse engineering readership of the IEEE.

With the blooming of modern wireless communication technologies, antenna research is entering another phase of significant development. In general, a wireless communication antenna is required to be available to cover a wide frequency bandwidth or several frequency bands. It is expected to be small in size or low in profile. It is no longer an isolated element and should be integrated with other circuit components. To achieve high performance, it must be codesigned with its mounting body and if possible with its environment. We are very pleased to note that a special issue that will discuss the technical design details of antennas in wireless communications is scheduled to be published by the Journal in 2012.