**Contents**



## **About the Editors**

**Issa Tamer Elfergani** (Member, IEEE, IET) obtained his M.Sc. and Ph.D. degrees in electrical and electronic engineering from the University of Bradford, UK, in 2008 and 2013, respectively. After his Ph.D., and since 2013, he has worked as a "Postdoctoral Researcher", and then as an "Investigador junior" at Instituto de Telecomunicac¸oes Aveiro, Portugal, within the Mobile Systems group. In 2014, ˜ Issa received a prestigious FCT fellowship for his postdoctoral research. The evaluation of the first triennium of this scholarship was given an excellent rating by the evaluators. He is now a senior researcher at the Instituto de Telecomunicac¸oes, Aveiro (Portugal), working with European-funded ˜ research projects, while leading technical activities in antenna design for ENIAC ARTEMOS (2011–2014), EUREKA BENEFIC (2014–2017), CORTIF (2014–2017), GREEN-T (2011–2014), VALUE (2016–2016), THINGS2DO (2014–2018), H2020-ITN-SECRET (2017–2020), POSITION-II(2018-2021), and Moore4Medical (2019–2023). Some of these projects have been successfully concluded, and some are still ongoing. He is currently working on (5GWAR) Novel 5G Millimetre-Wave Array Antennas for Future Mobile Handset Applications as Principle Investigator. Issa is an Honorary Visiting Research Fellow in Faculty of Engineering and Informatics, University of Bradford, UK. Issa is the Guest Editor for the Special Issue of *Electronics* "Recent Technical Developments in Energy Efficient 5G Mobile Cells"; the Special Issue on "Recent Advances in Engineering Systems Journal (ASTESJ)"; Guest Editor for the Special Issue of *Electronics* "Recent Advances in Antenna Design for 5G Heterogeneous Networks"; and Guest Editor for the Special Issue of *Electronics* "Unable RF Front-End Circuits for 5G and Beyond: Design, Challenges and Applications". Issa has around 160 high-impact publications in international conferences, journal papers, and books/book chapters, with a Google Scholar h-index of 20.

**Raed A. Abd-Alhameed** (Senior Member, IEEE) is currently a Professor of electromagnetic and radiofrequency engineering with the University of Bradford, UK. He is also the Leader of radiofrequency, propagation, sensor design, and signal processing in addition to leading the Communications Research Group for years within the School of Engineering and Informatics, University of Bradford. He has many years' research experience in the areas of radio frequency, signal processing, propagations, antennas, and electromagnetic computational techniques. He has published in over 800 academic journals and conference papers; in addition, he has co-authored six books and several book chapters, including seven patents. He is a principal investigator for several funded applications to EPSRCs and the leader of several successful knowledge Transfer Programmes, such as with Arris (previously known as Pace plc), Yorkshire Water plc, Harvard Engineering plc, IETG Ltd., Seven Technologies Group, Emkay Ltd., and Two World Ltd. He has also been a co-investigator in several funded research projects. He is the chair of several successful workshops on energy-efficient and reconfigurable transceivers: Approach toward Energy Conservation and CO2 Reduction, which addresses the biggest challenges for the future wireless systems. He is a co-editor for the MDPI journal *Electronics* since June 2019; in addition, he was a Guest Editor of the *IET Science, Measurements and Technology* journal since 2009. He has been a Research Visitor of Wrexham University, Wales, since 2009. He is a fellow of the Institution of Engineering and Technology and a fellow of the Higher Education Academy and a Chartered Engineer.

**Abubakar Sadiq Hussaini** MSc, PhD, received his Post Graduate Diploma (PGD) in Electrical/Electronic Engineering from Bayero University, Kano, in 2005; received his MSc in Radio Frequency Communication Engineering from the University of Bradford (UK) in 2007; and received his PhD in Telecommunications Engineering from the University of Bradford, UK, in 2012. He is a member of IEEE, IET, ENF, and the Optical Society of America; has contributed to numerous publications; and is involved in European and CELTIC research projects. His research interests include radio frequency system design and high-performance rf-mems tunable filters, with specific emphasis on energy efficiency and linearity; renewable energy (wind, solar, hydropower, ocean energy, and geothermal), smart grids, batteries, electric vehicles, and power plants and equipment; robotics, nanotechnology, the Internet of Things (IoT), artificial Intelligence (AI), embedded systems, virtual reality, and augmented reality; radio frequency identification (RFID), software defined radio, cognitive radio (CR), radio resource managemen<sup>t</sup> (RRM), mobile for development (mHealth or Mobile Health, mAgric or AgriTech, Mobile Money, and Mobile for Humanitarian Innovation), and optical communications. He has successfully attained several European projects, amongs<sup>t</sup> which are the projects MOBILIA (2009–2011), ARTEMOS (2011–2014), CORTIF (2014–2016), THINGS2DO (2014–2018), and 5GWAR (2019–2021). He is a TPC member and a reviewer for many international conferences and journals. He is a gues<sup>t</sup> editor for an *IET Science, Measurement & Technology* Special Issue, as well as a gues<sup>t</sup> editor for a Special Issue of MDPI's *Electronics*—"Microwave and Wireless Communications".

## *Editorial* **Recent Advances in Antenna Design for 5G Heterogeneous Networks**

**Issa Elfergani 1,2,\*, Abubakar Sadiq Hussaini 3, Jonathan Rodriguez 1 and Raed A. Abd-Alhameed 2**

> 1 Mobile Systems Group, Instituto de Telecomunicações, 3810-193 Aveiro, Portugal; jonathan@av.it.pt

2 School of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK; R.A.A.Abd@bradford.ac.uk


Fifth-generation will support significantly faster mobile broadband speeds, low latency, and reliable communications, as well as enabling the full potential of the Internet of Things (IoT). This will open up the possibility for new services, such as tactile communications, smart manufacturing and cities, and enhanced broadband connectivity. Over the last years, industries and the academic community both have implemented tremendous efforts to bring these systems into the real world. In fact, the 5G system has been tested and deployed in several countries; however, this upgrade only exploits a small portion of the huge potential of 5G system. This is because the system still uses the sub-6 GHz band, i.e., the lower microwave frequency ranges between 3.1–3.55 and 3.7–4.2 GHz in the United States, 3.4–3.8 GHz in Europe, 3.3–3.6 and 4.8–5.0 GHz in China, and 3.5 GHz in South Korea [1,2]. Although some advancements have been accomplished (mostly in the software part and using existing 4G infrastructure) to improve the service, the transmission rates are still limited by the narrow bandwidth. In fact, when proposing 5G systems, both industries and academics agreed on the use of millimeter wave (mm-wave), i.e., with the operating frequency in the range of 24 GHz to 40 GHz [2,3] for significantly large bandwidth (large bandwidth is equivalent to large data rate). Unfortunately, the mm-wave has still not been deployed in the current 5G system. One of the main reasons is that the antenna, i.e., the end hardware part in the communication device that is used to transmit the electromagnetic signal, still does not fulfil the strict requirement of a 5G application.

The main topic articles in this issue highlight the recent advances in designing antenna systems for 5G heterogeneous networks. This Special Issue features 11 papers that present novel antenna designs synthesis along with effective approaches in order to improve the overall antenna performance. These 11 papers are concisely described as follows.

Yuxiang Tu et al. [4] give an overview and an introduction to microstrip filters, antennas, and filtering antennas (filtennas). Then, performance comparisons between the key and essential structures for these aspects are presented and discussed. Furthermore, a comparison between several RF reconfiguration techniques, current challenges, and future developments is presented and discussed in this review. Among several reconfigurable structures, the most efficient designs with the best attractive features are addressed and highlighted in this paper to improve the performance of RF and MW front-end systems.

Al-Yasir et al. [5] present a compact wide-band microstrip filter antenna design for 2.4 GHz ISM band and 4G wireless communications. The filter antenna has been designed, measured, and studied in three different dielectric substrate materials, which are Rogers RT5880, Rogers RO3003, and FR-4. The analysis was performed by using CST microwave studio software. A performance comparison for the designed filter antenna with different dielectric substrate materials and heights is presented and discussed using the same design configuration. The results obtained from each design indicate that the most suitable

**Citation:** Elfergani, I.; Hussaini, A.S.; Rodriguez, J.; Abd-Alhameed, R.A. Recent Advances in Antenna Design for 5G Heterogeneous Networks. *Electronics* **2022**, *11*, 146. https://doi.org/10.3390/ electronics11010146

Received: 17 December 2021 Accepted: 27 December 2021 Published: 4 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

characteristics for a specific application can be achieved by using Rogers RT5880 dielectric substrate material. The structure is printed on a compact size of 0.32 λ0 × 0.30 λ0, where λ0 is the free-space wavelength at the center frequency. The designed filter antenna with the achieved performance can find different applications for a 2.4-GHz ISM band and 4G wireless communication.

Iqbal et al. [6] propose an isolation enhancement of two closely packed multiple-input multiple-output (MIMO) antenna systems using a modified U-shaped resonator. The modified U-shaped resonator is placed between two closely packed radiating elements resonating at 5.4 GHz with an edge to edge separation distance of 5.82 mm (λ◦/10). Through careful adjustment of parametric modelling, the isolation level of −23 dB among the densely packed elements is achieved. The coupling behavior of the MIMO elements is analyzed by accurately designing the equivalent circuit model in each step. The antenna performance is realized in the presence and absence of decoupling structure, and the results show the negligible effects on the antenna performance apart from mutual coupling.

Dahri et al. [7] investigate a dual-resonance asymmetric-patch reflectarray antenna with a single layer for 5G communication at 26 GHz. The asymmetric patch is developed from a square patch by tilting its one vertical side by a carefully optimized inclination angle. A progressive phase range of 650◦ is acquired by embedding a circular ring slot in the ground plane of the proposed element for gain improvement. A 332-element, center-feed reflectarray is designed and tested, where its high cross polarization is suppressed by mirroring the orientation of asymmetric patches on its surface. The asymmetric-patch reflectarray offers a 3 dB gain bandwidth of 3 GHz, which is 4.6% wider than the squarepatch reflectarray. A maximum measured gain of 24.4 dB was achieved with an additional feature of dual linear polarization. A simple design with a wide bandwidth and a highgain asymmetric-patch reflectarray make it suitable for use in 5G communications at high frequencies.

Altaf et al. [8] study an antenna design for a UWB MIMO antenna system with an isolation of −20 dB. The UWB response is achieved by designing a stepped-shaped antenna while the isolation is improved by using slotted stubs etched in the ground plane within the radiating elements. The system is fabricated on an FR4 substrate. The circuit theory is used to model the circuit model and compared the results with the EM model. The simulated, measured, and the circuit model results are in good agreement. It is found that ECC and DG for the proposed system are 0.15 and 9.85 dBi, respectively. The CCL is less than 0.06 bps/Hz for the whole operating bandwidth.

Muhammad et al. [9] present a compact rectifier, capable of harvesting ambient radio frequency (RF) power. The total size of the rectifier is 45.4 mm × 7.8 mm × 1.6 mm, designed on FR-4 substrate using a single-stage voltage multiplier at 900 MHz. GSM/900 is among the favorable RF energy-harvesting (RFEH) energy sources that span over a wide range with minimal path loss and high input power. The proposed RFEH rectifier achieves measured and simulated RF-to-dc (RF to direct current) power conversion efficiency (PCE) of 43.6% and 44.3% for 0 dBm input power, respectively. Additionally, the rectifier attained 3.1-V DC output voltage across a 2-k Ω load terminal for 14 dBm and is capable of sensing low input power at −20 dBm.

Kamal et al. [10] propose and analyses a novel single-layer multiple-input multipleoutput (MIMO) antenna for fifth-generation (5G) 28-GHz frequency band applications. The proposed MIMO antenna operates in the Ka-band, which is the most desirable frequency band for 5G mm-wave communication. The dielectric material is a Rogers-5880 with a relative permittivity, thickness, and loss tangent of 2.2, 0.787 mm, and 0.0009, respectively, in the proposed antenna design. The proposed MIMO configuration antenna element consists of triplet circular-shaped rings surrounded by an infinity-shaped shell. The simulated gain achieved by the proposed design is 6.1 dBi, while the measured gain is 5.5 dBi. Furthermore, the measured and simulated antenna efficiency is 90% and 92%, respectively. One of the MIMO performance metrics—i.e., the envelope correlation coefficient (ECC)—is also analyzed and found to be less than 0.16 for the entire operating bandwidth.

Bouknia et al. [11] illustrate and examine a theoretical study for the investigation of the electromagnetic field distributions and the input impedance of a printed dipole antenna structure loaded on a uniaxial anisotropic medium. The presentation of the electromagnetic field distributions, for which some examples have been shown here, provides a better understanding of the constitutive parameter (εt, εz, μt, and μz) contributions. Furthermore, the electrical and magnetic uniaxial anisotropy offers more degrees of freedom and further flexibility to realize a good direct matching effect on the input impedance. This shows that the complex media present a grea<sup>t</sup> potential in the design of innovative microwave components. This constitutes a starting point for understanding the behavior of the electromagnetic field in anisotropic and bianisotropic media, and many more interesting results are expected.

Addepalli et al. [12] design and implement UWB multiple-input multiple-output (MIMO) antennas with four and eight elements having connected grounds. The proposed antenna has a modified substrate geometry and comprises a circular arc-shaped conductive element on the top with the modified ground-plane geometry. Polarization diversity and isolation are achieved by replicating the elements, orthogonally forming a plus-shape antenna structure. The modified ground plane consists of an inverted L-strip and semiellipse slot over the partial ground that helps the antenna in achieving effective wide bandwidth spanning from (117.91%) 2.84–11 GHz. Both 4/8-port antennas help to achieve a size of 0.61 λ × 0.61 λ mm2 (lowest frequency), where the 4-port antenna is printed on the FR4 substrate. The 4-port UWB MIMO antenna attains a wide-impedance bandwidth, an omni-directional pattern, isolation of >15 dB, ECC of < 0.015, and an average gain of > 4.5 dB, making the MIMO antenna suitable for portable UWB applications.

Chung et al. [13] present an antenna structure with two branches that can achieve dual-band and broadband bandwidth characteristics. Moreover, the antenna performances were analyzed, validated, and manufactured. Thus, this design is suitable for in-vehicle infotainment and autopilot equipment systems in autonomous vehicle communication systems, including 5G, B5G, 4G, V2X, ISM band of WLAN, Bluetooth, WiFi 6 band, WiMAX, and Sirius/XM radio application.

Chung et al. [14] propose a small-slot antenna system (50 mm × 9 mm × 2.7 mm) for 4 × 4 multiple-input multiple-output (MIMO) on smart glasses devices. The antenna is set on the plastic temple, and the inverted F antenna radiates through the slot in the ground plane of the sputtered copper layer outside the temple. The antenna substrate is made of polycarbonate (PC), and its thickness is 2.7 mm (εr = 2.85, tan δ = 0.0092). The proposed antenna has a peak gain of 4.3 dBi and an antenna efficiency of 85.69% at 5.14 GHz. In addition, it also can obtain a peak gain of 3.3 dBi and antenna efficiency of 82.78% at 6.8 GHz. The measurement results show that this antenna has good performance, allowing future smart eyewear devices to be applied to Wi-Fi 5G (5.18–5.85 GHz) and Wi-Fi 6e (5.925–7.125 GHz).

We would like to take this opportunity to appreciate and thank all authors for their outstanding contributions and the reviewers for their fruitful comments and feedback. Special appreciation should also be paid to the Editorial Board of MDPI's *Electronics* journal for the opportunity to gues<sup>t</sup> edit this Special Issue, and to the *Electronics* Editorial Office staff for their hard and precise work in maintaining a rigorous peer-review schedule and timely publication.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
