There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The intensive research in the fifth generation (5G) technology is a clear indication of technological revolution to meet the ever-increasing demand and needs for high speed communication as well as Internet of Thing (IoT) based applications. The timely upgradation in 5G technology standards is released by third generation partnership project (3GPP) which enables the researchers to refine the research objectives and contribute towards the development. The 5G technology will be supported by not only smartphones but also different IoT devices to provide different services like smart building, smart city, and many more which will require a 5G antenna with low latency, low path loss, and stable radiation pattern. This paper provides a comprehensive study of different antenna designs considering various 5G antenna design aspects like compactness, efficiency, isolation, etc. This review paper elaborates the state-of-the-art research on the different types of antennas with their performance enhancement techniques for 5G technology in recent years. Also, this paper precisely covers 5G specifications and categorization of antennas followed by a comparative analysis of different antenna designs. Till now, many 5G antenna designs have been proposed by the different researchers, but an exhaustive review of different types of 5G antenna with their performance enhancement method is not yet done. So, in this paper, we have attempted to explore the different types of 5G antenna designs, their performance enhancement techniques, comparison, and future breakthroughs in a holistic way.

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Fifth Generation Antennas: A Comprehensive Review of Design and Performance Enhancement Techniques

A mutual coupling reduction strategy that employs meta-structures is introduced for wideband, dual-polarized, high-density, planar, and patch antenna arrays The meta-structures consist of two types of resonators: grounded capacitively loaded loops (GCLLs) and $\pi$ -shaped elements By incorporating the meta-structures into the array configuration, the isolation levels between adjacent radiating elements in both the E- and H-planes orientations are improved by as much as 715 dB The surface current distribution behaviors of the array with only the GCLLs and with only the $\pi$ -shaped elements are investigated thoroughly to explain the mutual coupling reduction mechanisms A proof-of-concept array was constructed and tests were performed that validate the reported design principles and simulation results

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Mutual Coupling Reduction Using Meta-Structures for Wideband, Dual-Polarized, and High-Density Patch Arrays

Strong mutual coupling between both adjacent and nonadjacent elements is unavoidable, especially when antenna array is densely arranged. In this paper, an efficient T-shaped decoupling network is primarily proposed to decouple closely spaced two-element E-plane antenna array based on network analysis and implementation deduction. Subsequently, it is extended to a three-element one by combining with a fine transmission line and further generalized to a multielement linear one. All the accordant simulation and measurement results demonstrate that as compared to the coupled arrays, impedance matching, radiation patterns, and scanning characteristics of the decoupled arrays are evidently improved through adding the decoupling network, while mutual coupling between every two elements is greatly reduced. Moreover, such combined decoupling network is effective in the H-plane array and monopole antenna array, implying its huge potential for large-scale array and various arrays.

An Efficient Decoupling Network Between Feeding Points for Multielement Linear Arrays

A compact and broadband rectifying antenna (rectenna) which consists of a novel slotted antenna and an efficient rectifying circuit is presented for wireless power transfer at LTE-2300/2500 band. Three different broadband antennas have been investigated and compared. It is found that the newly designed antenna which combines the annular slot radiation patch and a slotted ground plane (GP) has the widest bandwidth and the best matching characteristic. A new broadband dual-stub matching network is proposed to maximize the transmission of wireless power and enhance the reliability of the rectifier when the input power and load varies. In addition, the ground of the rectifier is directly connected to the GP of the antenna for compactness and low energy loss. The proposed designs have been simulated, fabricated, and tested. The results indicate that the proposed receiving antenna performs well at LTE band with a wide bandwidth from 2 to 3.1 GHz. The maximum efficiency of the proposed rectenna reaches 70% at 2.5 GHz under the input power of 5 dBm. Furthermore, the rectenna remains at a relatively efficient power conversion when the load varies in a wide range from 2 to 20 $\text{k}\Omega $ under different low input power, which is of great significance when the ambient radio frequency(F) signal is weak. Therefore, the rectenna is potentially applicable to a wide range of low-power electronic devices by harvesting RF energy at LTE band.

An Efficient Broadband Slotted Rectenna for Wireless Power Transfer at LTE Band

The active impedance of each element in a phased array changes substantially with scan angle due to mutual coupling. Therefore, a key challenge in the design of wide-angle scanning phased array is to achieve wide-angle impedance matching (WAIM). In this communication, an efficient decoupling network is developed to reduce the mutual coupling between adjacent elements for this purpose. A complete scattering matrix analysis of the decoupling network is given and the design procedure is summarized. A ${1} \times {16}$ linear microstrip phased array has been designed and fabricated for experimental verification. Good agreement is obtained between measured and simulated results. The measured mutual coupling between adjacent elements is reduced to lower than $- {35}\; \text{dB}$ at the center frequency. Due to the reduced mutual coupling, the array can experimentally scan to 66° with a gain reduction of only 3.04 dB.

Wide-Angle Scanning Phased Array Using an Efficient Decoupling Network

This letter proposes a low-profile circularly polarized metasurface antenna with a wide axial-ratio beamwidth (ARBW). The proposed antenna consists of an array of square metallic metasurface cells that are excited by a microstrip line through four cross slots. The feeding structure can be seen as a symmetrical multiple-feed structure. Thus, ${E_\varphi }$ and ${E_\theta }$ can be nearly equal in amplitude with a phase difference of 90° over a wide angle range. A wide ARBW is obtained. With a size of 0.93 λL  × 0.93 λL  × 0.024 λL ( λL is the wavelength in free space at the lowest operating frequency), the proposed antenna achieves an impedance bandwidth of 17% and an AR bandwidth of 14.5%. Moreover, the ARBW is more than 205°.

A Low-Profile Circularly Polarized Metasurface Antenna With Wide Axial-Ratio Beamwidth

In this paper, we propose a low-profile ultra-wideband (UWB) antenna for wireless body area networks. The proposed antenna is low-weight and easy to produce by printed circuit board manufacturing. Optimized shape of the radiation patch and shorted top-loading patches are introduced to broaden the bandwidth and reduce the profile of the antenna. The height of the antenna is $0.05\lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at the lowest operating frequency. The simulated and measured results show that an enhanced impedance bandwidth of about 162% in the range of 2.5 to 24 GHz (S11 < −10 dB) is achieved. Besides, the influence of the human body to the proposed antenna is minimal. The time-domain behavior of the low-profile UWB antenna is tested, and the results show a satisfactory performance in transmitting and receiving pulse signals.

A Low Profile UWB Antenna for WBAN Applications

A low-profile broadband dual-circularly-polarized metasurface antenna is proposed. The antenna consists of a modified cross-slot coupling antenna with a lattice of 4 × 4 square metallic patches. A characteristic mode analysis is used to investigate the operating mechanism of the metasurface antenna. Then, the desired modes are chosen and excited by the modified slots. The modified slot introduces a new resonance to achieve broadband characteristic. Two orthogonally placed microstrip lines are used to achieve both right-hand circular polarization and left-hand circular polarization in the same frequency band. The final prototype, with an overall size of 100 mm × 100 mm × 6 mm (0.71 λ 0× 0.71 λ 0 × 0.04 λ 0 at 2.15 GHz) is measured. The measured results show that the proposed antenna has an axial-ratio bandwidth of 31.3% and an impedance bandwidth of 52.5%.

A Low-Profile Broadband Dual-Circularly-Polarized Metasurface Antenna

A low-profile dual-polarized metasurface antenna with high isolation and low cross-polarization is proposed in this communication. The proposed metasurface consists of an array of $4\times 4$ patches. Through characteristic mode analysis (CMA), a pair of degenerate modes is identified to achieve the desired radiation features. By modifying the patches of the metasurface, the strongest mode current distributions of the two modes are moved from the center to the edge of the metasurface. One feed port can efficiently excite only one particular mode, and the other mode cannot be excited simultaneously. Each of the modes are differentially fed using two Y-shaped microstrip lines via two slots. As a result, both high isolation and low cross-polarization can be obtained. The simulation and experimental results show that the proposed antenna achieves not only a low profile of $0.05\lambda _{0}$ (where $\lambda _{0}$ is the free-space wavelength at the center resonant frequency) but also a broad impedance bandwidth of 36% and a high isolation of 53 dB. In particular, the enhanced cross-polarization discrimination (XPD) is lower than −40 dB at the broadside, lower than −33 dB within ±30° of the main lobe, and lower than −28 dB within ±60° of the main lobe.

Deqiang Yang

Papers

Wide-Angle Scanning Phased Array Using an Efficient Decoupling Network

A Low-Profile Circularly Polarized Metasurface Antenna With Wide Axial-Ratio Beamwidth

A Low Profile UWB Antenna for WBAN Applications

A Low-Profile Broadband Dual-Circularly-Polarized Metasurface Antenna

High Isolation and Low Cross-Polarization of Low-Profile Dual-Polarized Antennas via Metasurface Mode Optimization