A miniature feeding network for aperture-coupled wearable antennas is proposed. In wearable systems, it allows to minimize the dimensions of the rigid printed circuit board (PCB) carrying the electronics and feeding the textile antenna. This optimizes the comfort of the user. Simultaneously, it avoids traditional probe feeding that has the disadvantage of requiring a single soldering point, which has a high risk of being broken with time due to movements of the user or washing. On top, since the aperture is implemented on the PCB, it can be fabricated with excellent dimensional tolerances that do not change during use. The proposed feeding topology is prototyped for applications in the industrial, scientific, and medical band (2.4-2.4835 GHz), resulting in dimensions of 0.073×0.061 λ ² 4 at 2.45 GHz, realized on a PCB substrate of 10 × 10 mm ² (0.0817 × 0.0817 λ ² ₀ at 2.45 GHz). The demonstration antenna incorporating the new feeding network has a realized gain of 5.6 dBi, a total efficiency of 47%, a low cross-polarization, and a high front-to-back ratio. Simulations and experiments agree well, proving the validity of the new concept.

An omni-directional circularly polarized wearable antenna for 5 GHz applications is proposed. The antenna is constructed on a button-like setup, using the concept of a magnetic dipole combined with a flexible patch located on a textile substrate to optimize the antenna performance. With the proposed topology, an omni-directional pattern involving a robust circular polarization (CP) for on-body communication is reached. The -10 dB impedance bandwidth ranges from 5.68 to 5.91 GHz. The 3 dB axial ratio bandwidth ranges from 5.72 to 5.9 GHz. The omni-directional pattern is ideal for indoors and the CP is able to cope with any possible polarization mismatch caused by human body motions. Comparing with literature, this antenna takes the special case of wearable applications into account, which makes it the first practical wearable button antenna that features an omni-directional radiation pattern featuring CP.

A reconfigurable dual-band antenna with pattern diversity is proposed. By switching the dispersion curve of a composite right-/left-handed transmission-line-inspired structure, the antenna is able to realize both a broadside pattern and an omnidirectional pattern in both the 2.4835-2.5 GHz industrial scientific medical band and the 3.4-3.5 GHz WiMAX band. Despite its complex reconfigurability, only one feeding port is used. A small dual-frequency separation ratio is reached, which makes the antenna suitable for low-frequency range wireless communication systems. A prototype is fabricated and the measured results agree well with the simulated ones.

A dual-band dual-polarized wearable array is proposed, based on a miniaturized innovating button radiator topology. The diameter of the rigid button is only 19.5 mm (0.29 λ at 4.5 GHz), which optimizes the users' comfort, and makes it the smallest up to date in literature. The operational bands are 4.50-4.61 GHz and 5.04-5.50 GHz. The antenna thus covers the 4.5-4.6 unlicensed future 5th generation (5G) communication band for the internet of things (IoT), and the 5.1-5.5 GHz wireless local area network (WLAN) band, respectively. Two orthogonal linear polarizations are obtained in each band. A low mutual coupling between the button antenna elements (below -18 dB) and between the two ports within each element (below -20 dB) is achieved, guaranteeing a good diversity performance. The envelope correlation coefficient (ECC) and the specific absorption rate (SAR) performance are also analyzed. In order to demonstrate the robustness of the button antenna and to mimic realistic situations, a more complicated asymmetrical ground plane model of the button antenna is studied for the first time. A prototype of a two-element button array has been fabricated. The measurement results match well with the simulations. A 10-element button array is studied within the context of a 3-D channel model, taking into account the button element's radiation pattern. A high achievable spectral efficiency (SE) is obtained.

An automatic AI-driven design procedure for mutual coupling reduction and a novel isolator are proposed for a frequency reconfigurable antenna array. The design process is driven and expedited by the parallel surrogate model-assisted differential evolution for antenna synthesis (PSADEA) method. The reconfigurable array element can switch its operation between the 2.5 GHz ISM band and the 3.4 GHz WiMAX band. By introducing the proposed isolator, the mutual coupling in the higher and lower band is reduced by 8 dB and 7 dB, respectively. The reconfigurable array was prototyped, and measurements agree well with simulations, verifying the validity of the proposed concept. Although used for a specific antenna in this communication, the proposed AI-driven design strategy is generic and can easily be employed for other array topologies.

Increased demand for higher spectrum efficiency, especially in the space-limited chip, base station, and vehicle environments, has spawned the development of full-duplex communications, which enable the transmitting and receiving to occur simultaneously at the same frequency. The key challenge in this full-duplex communication paradigm is to reduce the self-interference as much as possible, ideally, down to the noise floor. This paper provides a comprehensive review of the self-interference cancellation (SIC) techniques for co-located communication systems from a circuits and fields perspective. The self-interference occurs when the transmitting antenna and the receiving antenna are co-located, which significantly degrade the system performance of the receiver, in terms of the receiver desensitization, signal masking, or even damage of hardwares. By introducing the SIC techniques, the self-interference can be suppressed and the weak desired signal from the remote transmitter can be recovered. This, therefore, enables the full-duplex communications to come into the picture. The SIC techniques are classified into two main categories: the traditional circuit-domain SICs and the novel field-domain SICs, according to the method of how to rebuild and subtract the self-interference signal. In this review paper, the field-domain SIC method is systematically summarized for the first time, including the theoretical analysis and the application remarks. Some typical SIC approaches are presented and the future works are outlooked.