A cost-effective, compact and high-performance antenna element for beamforming applications in all 5G New Radio bands in the [24.25-29.5] GHz spectrum is proposed. The novel antenna topology adopts a square patch, an edge-plated air-filled cavity, and an hourglass-shaped aperture-coupled feed to achieve a very high efficiency over a wide frequency band in a compact footprint ${(0.48\lambda_{0}\times0.48\lambda_{0})}$ . Its compliance with standard PCB fabrication technology, without complex multi-layer PCB stack, ensures low-cost fabrication. The antenna feedplane offers a platform for compact integration of active electronic circuitry. Two different modular 1×4 antenna arrays were realized to demonstrate its suitability for broadband multi-antenna systems. Measurements of the fabricated antenna element and the antenna array prototypes revealed a -10-dB impedance bandwidth of 7.15 GHz (26.8%) and 8.2 GHz (30.83%), resp. The stand-alone antenna features a stable peak gain of 7.4±0.6 dBi in the [24.25- 29.5] GHz band and a measured total efficiency of at least 86%. The 1±4 array features a peak gain of 10.1±0.7 dBi and enables grating-lobe-free beamsteering from -50° to 50°.

This article presents a novel 4 $\times$ 4 Butler matrix implemented in the grounded co-planar waveguide (GCPW) technology, compactly integrated with a highly efficient and broadband 1 $\times$ 4 air-filled substrate integrated waveguide (AFSIW) cavity-backed patch antenna array (AA), giving rise to a broad operational frequency range 23.75, 31 GHz (26.5%) covering the n257, n258, and n261 fifth-generation (5G) bands. Three novel quadrature hybrid couplers and two crossovers are designed and compared to obtain the optimal building blocks for the Butler matrix. Within each of the supported 5G bands, the measured excess insertion loss of the optimized Butler matrix remains smaller than 3.5 dB with a maximal amplitude imbalance of $\pm$ 0.9 dB. Isolation between input ports is higher than 16.4 dB. A maximal measured realized gain of 12.3 dBi is obtained for the Butler matrix with integrated 1 $\times$ 4 AA while ensuring a $-$ 3-dB beamwidth coverage of 110 $^\circ$ . The main beamsteering directions of $-$ 40 $^\circ$ , $-$ 14 $^\circ$ , 14 $^\circ$ , 40 $^\circ$ exhibit measured deviations that stay within $\pm$ 3 $^\circ$ . The fabricated Butler matrix with AA features a very compact footprint of 21.4 mm $\times$ 46.0 mm $\times$ 2 mm 2 $\lambda_0 \times$ 4.3 $\lambda_0 \times$ 0.2 $\lambda_0$ .

A textile patch antenna is an attractive package for wearable applications as it offers flexibility, less weight, easy integration into the garment and better comfort to the wearer. When it comes to wearability, above all, comfort comes ahead of the rest of the properties. The air permeability and the water vapor permeability of textiles are linked to the thermophysiological comfort of the wearer as they help to improve the breathability of textiles. This paper includes the construction of a breathable textile rectangular ring microstrip patch antenna with improved water vapor permeability. A selection of high air permeable conductive fabrics and 3-dimensional knitted spacer dielectric substrates was made to ensure better water vapor permeability of the breathable textile rectangular ring microstrip patch antenna. To further improve the water vapor permeability of the breathable textile rectangular ring microstrip patch antenna, a novel approach of inserting a large number of small-sized holes of 1 mm diameter in the conductive layers (the patch and the ground plane) of the antenna was adopted. Besides this, the insertion of a large number of small-sized holes improved the flexibility of the rectangular ring microstrip patch antenna. The result was a breathable perforated (with small-sized holes) textile rectangular ring microstrip patch antenna with the water vapor permeability as high as 5296.70 g/m² per day, an air permeability as high as 510 mm/s, and with radiation gains being 4.2 dBi and 5.4 dBi in the E-plane and H-plane, respectively. The antenna was designed to resonate for the Industrial, Scientific and Medical band at a specific 2.45 GHz frequency.

The fifth generation (5G) cellular network is expected to include the millimeter wave spectrum, to increase base station density, and to employ higher-order multiple-antenna technologies. The centralized radio access network architectures combined with radio-over-fiber (RoF) links can be the key enabler to improve fronthaul networks. The sigma-delta modulated signal over fiber (SDoF) architecture has been proposed as a solution leveraging the benefits of both digitized and analog RoF. This work proposes a novel distributed antenna system using sigma-delta modulated intermediate-frequency signal over fiber (SDIFoF) links. The system has an adequately good optical bit-rate efficiency and high flexibility to switch between different carrier frequencies. The SDIFoF link transmits a signal centered at a 2.5 GHz intermediate frequency over a 100 m multi-mode fiber and the signal is up-converted to the radio frequency (24-29 GHz) at the remote radio unit. An average error vector magnitude (EVM) of 6.40% (-23.88 dB) is achieved over different carrier frequencies when transmitting a 300 MHz-bandwidth 64-QAM OFDM signal. The system performance is demonstrated by a 2×1 multiple-input single-output system transmitting 160 MHz-bandwidth 64-QAM OFDM signals centered at 25 GHz. Owing to transmit diversity, an average gain of 1.12 dB in EVM is observed. This work also evaluates the performance degradation caused by asynchronous phase noise between remote radio units. The performance shows that the proposed approach is a competitive solution for the 5G downlink fronthaul network for frequency bands above 24 GHz.

Exploring mmWave frequencies and adopting small-cell architectures are two key enablers for increased wireless data rates. To make these evolutions economically viable, centralized architectures based on radio-over-fiber (RoF) are devised. To reduce the complexity of the cellular network even further, RF-over-Fiber transmission schemes are adopted in combination with reflective uplink operation. This paper relies on a very low complexity narrowband GaAs electronics / Si photonics transceiver for scalable RFoF architectures with which we demonstrate a fiber-wireless link capable of transmitting over 7 Gb/s in down- and uplink for a 2km fiber and 5m wireless link in the 28 GHz band. Furthermore, it is shown that Rayleigh degradation caused by reflective uplink operation can be avoided by using a coherent detection scheme.

A novel hybrid integration strategy for compact, broadband, and highly efficient millimeter-wave (mmWave) on-chip antennas is demonstrated by realizing a hybrid on-chip antenna, operating in the [27.5–29.5] GHz band. A cavity-backed stacked patch antenna is implemented on a 600 $\mu$ m thick silicon substrate by using air-filled substrate-integrated-waveguide technology. A hybrid on-chip approach is adopted in which the antenna feed and an air-filled cavity are integrated on-chip, and the stacked patch configuration is implemented on a high-frequency printed circuit board (PCB) laminate that supports the chip. A prototype of the hybrid on-chip antenna is validated, demonstrating an impedance bandwidth of 3.7 GHz. In free-space conditions, a boresight gain of 7.3 dBi and a front-to-back ratio of 20.3 dB at 28.5 GHz are achieved. Moreover, the antenna is fabricated using standard silicon fabrication techniques and features a total antenna efficiency above 90% in the targeted frequency band of operation. The high performance, in combination with the compact antenna footprint of $\text{0.49}\,\lambda _{\rm min} \times \text{0.49}\,\lambda _{\rm min}$ , makes it an ideal building block to construct broadband antenna arrays with a broad steering range.