Owing to the rapid growth in wireless data traffic, millimeter-wave (mm-wave) communications have shown tremendous promise and are considered an attractive technique in fifth-generation (5G) wireless communication systems. However, to design robust communication systems, it is important to understand the channel dynamics with respect to space and time at these frequencies. Millimeter-wave signals are highly susceptible to blocking, and they have communication limitations owing to their poor signal attenuation compared with microwave signals. Therefore, by employing highly directional antennas, co-channel interference to or from other systems can be alleviated using line-of-sight (LOS) propagation. Because of the ability to shape, switch, or scan the propagating beam, phased arrays play an important role in advanced wireless communication systems. Beam-switching, beam-scanning, and multibeam arrays can be realized at mm-wave frequencies using analog or digital system architectures. This review article presents state-of-the-art phased arrays for mm-wave mobile terminals (MSs) and base stations (BSs), with an emphasis on beamforming arrays. We also discuss challenges and strategies used to address unfavorable path loss and blockage issues related to mm-wave applications, which sets future directions.

Review of Recent Phased Arrays for Millimeter-Wave Wireless Communication.

This paper presents the design and realization of a metasurface-based low-profile wideband Circularly Polarized (CP) patch antenna with high performance for Fifth-generation (5G) communication systems. The antenna consists of a modified patch, sandwiched between an array of $4\times4$ symmetrical square ring Metasurface (MTS) and a ground plane. Initially, the intrinsic narrow bandwidth of the conventional patch antenna is increased using a diagonal rectangular slot. For further performance enhancement, the additional resonances and CP radiations are achieved for wideband operation in terms of impedance and Axial Ratio (AR) by effective excitation of surface waves propagating along the MTS. The stacking of MTS on the modified patch without any air gap resulted in an overall compact size of $1.1\lambda _{0} \times 1.1\lambda _{0} \times 0.093\lambda _{0}$ . Simulated and measured results show that the MTS-based antenna offers a wide impedance bandwidth ranging from 24 – 34.1 GHz (34.7%) for $\vert \text{S}_{11}\vert with a maximum gain of 11 dBic and a 3-dB AR bandwidth of 24.1 – 29.5 GHz (20.1 %). Moreover, the proposed antenna has a smooth gain response with a small variation in its gain (9.5 – 11 dBic) and a stable left-hand CP radiation in the desired frequency range. The operating bandwidth of this antenna is covering the proposed entire global millimeter-wave spectrum (24.2 – 29.5 GHz) for 5G communication systems.

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A Metasurface-Based Low-Profile Wideband Circularly Polarized Patch Antenna for 5G Millimeter-Wave Systems

In this letter, a specific absorption rate (SAR) reduced 28 GHz beam-steering array antenna with dual-mode operation for a fully metal-covered fifth-generation (5G) cellular handset is proposed. The proposed antenna consists of two subarrays, each of which has eight rotated slot antenna elements, which are arranged on the upper frame and on the part of the back cover of the handset. The subarrays are selected according to the modes by using a switch. The proposed array demonstrates good reflection coefficients in the frequency band ranging from 27.2 to 28.2 GHz and the mutual couplings between antenna elements are less than –11.8 dB. The proposed antenna has good beam-steering properties and a hemispherical beam coverage for a millimeter-wave (mm-wave) 5G cellular handset. The calculated peak SAR values on the head phantom by the proposed structure, for the beam scan angles of 0° and 40°, are 0.53 and 0.88 W/kg, respectively, when the input power of each subarray is 24 dBm.

A SAR Reduced mm-Wave Beam-Steerable Array Antenna With Dual-Mode Operation for Fully Metal-Covered 5G Cellular Handsets

In this paper, a miniaturized novel-shape dual-band implantable antenna operating in the industrial, scientific, and medical bands (902–928 MHz and 2.4–2.4835 GHz) is developed for battery-powered implants. The Rogers ULTRALAM ( $\varepsilon _{r} =2.9$ and tan $\delta =0.0025$ ) liquid crystalline polymer material with 0.1 mm thickness is used as both the substrate and superstrate. By employing the shorting strategy, a flower-shape radiating patch, and open-ended slots in the ground plane, the total volume of the proposed antenna is confined to $7 \times 7.2\times0.2$ mm3. In a homogeneous skin phantom, the presented antenna has a maximum gain of −28.44 and −25.65 dBi at 928 MHz and 2.45 GHz, respectively. The calculated maximum specific absorption rate values are in the safe limit and satisfy the IEEE C95.1-1999 and C95.1-2005 safety guidelines. The design, optimization, and analysis of the suggested antenna are performed using the finite difference time-domain- and finite-element method-based simulators. The results suggest that the flower-shape antenna exhibits fairly omnidirectional radiation patterns; therefore, it can be used for gastro applications and skin implantations. Through wireless communication link, we validated that at both frequencies (928 MHz and 2.45 GHz), 7 Kb/s and 78 Mb/s of data can be transmitted easily over more than 6 and 1.5 m, respectively. To verify the validity of the design and simulation results, measurements are performed by substituting the fabricated prototype in the American Society for Testing Materials and head phantoms containing saline solution. The proposed flower-shape antenna shows salient performance parameters compared with the recently proposed antennas.

A Miniaturized Novel-Shape Dual-Band Antenna for Implantable Applications

This paper presents a multifunctional reconfigurable frequency-selective surface (FSS) based on liquid-metal-filled microchannels engraved in an elastomeric substrate. The proposed FSS comprises two layers of periodic meandered patterns encased within the opposite sides of a dielectric, where the layers are arranged in orthogonal polarization with independent control of reconfigurability. The novelty of the design lies in its switching characteristic between four different working states (dual-polarized all pass, single-polarized low pass, single-polarized bandpass, and dual-polarized bandpass) with the injection of a liquid metal in the top and bottom microchannels. Moreover, the design exhibits miniaturization performance during one of the states (exhibiting dual-polarized bandpass response at 1.58 GHz) corresponding to the unit cell dimensions of $0.052\lambda _{0} \times 0.052\lambda _{0}$ . The equivalent circuit models and parametric variations are extensively analyzed for understanding the design principles. Because any liquid-metal-based reconfigurable FSS structure has not been reported earlier, a detailed description of the fabrication and measurement procedure are presented in this paper. The experimental validation of the fabricated prototype also confirms the potential use of microfluidic technology-based reconfigurable structures in future.

Fluidically Reconfigurable Multifunctional Frequency-Selective Surface With Miniaturization Characteristic

In this communication, we have proposed a nonuniform series-fed angled dipole array antenna with a high gain, a wide bandwidth, a good sidelobe level (SLL), and a high front-to-back ratio characteristics. The proposed array antenna structure consisted of eight dipole elements, and each dipole was connected to a parallel stripline printed on the front and back sides of the substrate. From the first dipole element near the ground plane to the eighth dipole element at the end of the parallel stripline, the length of each dipole was reduced by $\Delta L_{d}$ . As with dipole length, the spacing between the dipole elements decreased by $\Delta S_{d}$ from each previous dipole spacing. Compared to uniform array antennas, the impedance and gain bandwidths were greatly increased, and the SLL and front-to-back (F/B) ratio characteristics were improved. The proposed array antenna had an $\vert S_{11}\vert dB, an impedance bandwidth of 23.3–51.0 GHz, and a 3 dB gain bandwidth of 22.0–44.0 GHz. At 28 GHz, the E- and H-plane SLLs of the proposed array antenna were −24.1 and −18.8 dB, respectively. The F/B ratio of the proposed antenna was less than −30 dB.

A High-Gain and Wideband Series-Fed Angled Printed Dipole Array Antenna

A broadband, low-profile, compact, and cost-effective antenna consisting of a printed dipole loaded with split-ring resonators (SRRs) is presented. The antenna consists of two identical printed SRRs, each enclosing an arm of a printed dipole element. Each SRR and the center-fed half-wavelength dipole element are printed on the top and bottom sides of a thin dielectric substrate. The dipole is excited by a coaxial feed, while the SRRs are electromagnetically coupled to the dipole. The dipole, in conjunction with the SRRs, generates three resonant modes with stable omnidirectional radiation patterns. These resonances interact to provide broadband characteristics, that is, a measured impedance bandwidth covering 1.32–2.46 GHz for a reflection coefficient of less than −10 dB. The functionalities, mechanisms, and modes of operation of the composite antenna structure are discussed in relation to both the dipole and the SRRs. A prototype is fabricated and measured in order to validate the antenna design. It is computationally and experimentally confirmed that the antenna demonstrates broadband characteristics as well as a stable gain with non-deteriorating omnidirectional radiation patterns. The compact antenna, which has the overall dimensions of 9.6 mm $\times$ 74.4 mm $\times$ 0.508 mm ( $0.06\lambda \times 0.469\lambda \times 0.0032\lambda$ at 1.895 GHz), has a measured fractional bandwidth of 60.31%, an omnidirectional radiation pattern with a gain greater than 1.8 dBi, and a radiation efficiency greater than 88% within a broad impedance bandwidth. The compact size, simple structure, and stable radiation properties of the antenna render it suitable for many wireless communication applications.

Compact Broadband Omnidirectional Radiation Pattern Printed Dipole Antenna Incorporated With Split-Ring Resonators

This paper presents the performance of a planar, low-profile, and wide-gain-bandwidth leaky-wave slit antenna in different thickness values of high-permittivity gallium arsenide substrates at terahertz frequencies. The proposed antenna designs consisted of a periodic array of 5 × 5 metallic square patches and a planar feeding structure. The patch array was printed on the top side of the substrate, and the feeding structure, which is an open-ended leaky-wave slot line, was etched on the bottom side of the substrate. The antenna performed as a Fabry-Perot cavity antenna at high thickness levels (H = 160 μm and H = 80 μm), thus exhibiting high gain but a narrow gain bandwidth. At low thickness levels (H = 40 μm and H = 20 μm), it performed as a metasurface antenna and showed wide-gain-bandwidth characteristics with a low gain value. Aside from the advantage of achieving useful characteristics for different antennas by just changing the substrate thickness, the proposed antenna design exhibited a low profile, easy integration into circuit boards, and excellent low-cost mass production suitability.

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Performance of a Planar Leaky-Wave Slit Antenna for Different Values of Substrate Thickness

This paper presents low-profile broadband antennas, which are composed of four parasitic patches placed between planar radiators and a perfect electric conductor ground plane. Two types of planar radiators, a conventional dipole and a crossed dipole, are employed to produce linearly polarized (LP) and circularly polarized (CP) radiations, respectively. The radiator and parasitic patches are realized on thin substrates to lower the cost. Owing to the presence of parasitic patches, the antenna performance improves in terms of profile reduction, resonant frequency decrease, and bandwidth enhancement. These improvements are discussed and confirmed computationally and experimentally. The LP design with the overall dimensions of 120 mm × 120 mm × 16.3 mm (0.64λ0 × 0.64λ0 × 0.087λ0 at 1.6 GHz) has a |S11|   96%. The CP design, which has the same physical size as the LP case, has a |S11|   90%.

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Bandwidth-Enhanced Low-Profile Antenna with Parasitic Patches

This article proposes two reduced-size crossed-dipole antennas that comprise two printed crossed-dipole arms and a pair of vacant-quarter printed rings to generate circularly polarized (CP) radiation The T-shaped crossed-dipole antenna is similar to a conventional crossed-dipole antenna but differs that half of the crossed dipole is rotated 90° This rotation reduces the dipole’s area coverage by nearly half, resulting in a compact antenna that generates CP radiation with performance similar to that of a conventional crossed-dipole antenna A compact folded E-shaped crossed dipole with good CP radiation characteristics is formed by folding the arms of the T-shaped antenna The performance of these compact reduced-size crossed-dipole antennas is discussed The T-shaped antenna with an overall radiator size of 303 $\text {mm} \times 514\,\,\text {mm} \times 0508$ mm (029 $\lambda _{o} \times 05\lambda _{o} \times 0005\lambda _{o}$ at 297 GHz) has an $\vert \text {S}_{11} \vert dB bandwidth of 258–342 GHz (28%) and a 3 dB AR bandwidth of 278–316 GHz (128%) The folded E-shaped crossed-dipole antenna with overall dimensions of 196 $\text {mm} \times 31\,\,\text {mm} \times 0508$ mm ( $021\lambda _{o} \times 033\lambda _{o} \times 00055\lambda _{o}$ at 325 GHz) has an $\vert \text{S}_{11} \vert dB bandwidth of 292–344 GHz (164%) and a 3 dB AR bandwidth of 306–344 GHz (117%)

Kam Eucharist Kedze

Papers

A High-Gain and Wideband Series-Fed Angled Printed Dipole Array Antenna

Compact Broadband Omnidirectional Radiation Pattern Printed Dipole Antenna Incorporated With Split-Ring Resonators

Performance of a Planar Leaky-Wave Slit Antenna for Different Values of Substrate Thickness

Bandwidth-Enhanced Low-Profile Antenna with Parasitic Patches

Design of a Reduced-Size Crossed-Dipole Antenna