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

In this article, a novel slot-array defected ground structure (DGS) for decoupling microstrip antenna array is proposed. The slot-array DGS is etched surrounding each antenna element on the ground plane and parallel to the radiating edges of each antenna element. The decoupling mechanism is elucidated via an equivalent circuit model and the coupled current field analysis, which reveals slot-array DGS has the spatial band-stop characteristic and changes the direction of the partially coupled current, respectively. Both characteristics of the slot-array DGS contribute to mutual coupling reduction. Three practical design examples of applying slot-array DGS to single-linearly polarized (LP), dual-LP, and compact circularly polarized (CP) antenna array are given to illustrate the design process and considerations. The simulated and measured results show that about 50 dB isolation enhancement is obtained by using the slot-array DGS when the edge-to-edge spacing between CP antenna elements is 0.057 wavelength. Additionally, a wheel-shaped absorber based on the electromagnetic loss material is designed and fabricated to reduce the backward radiation caused by slot-array DGS. The absorber has an absorptivity of more than 95% in the frequency range of 1.2–1.35 GHz and suppresses the backward radiation over 12.5 dB in the plane phi = 0° and 16.1 dB in the plane phi = 90° without deteriorating other antenna performances.

A Novel Slot-Array Defected Ground Structure for Decoupling Microstrip Antenna Array

Four grounded coplanar waveguide (GCPW)-fed open-slot antennas conjoined into a cross shape with a central circular slot are presented for $4\times4$ fifth-generation (5G) multi-input multi-output (MIMO) operation in 3.3–4.2 GHz. The four conjoined slot antennas have a 1 mm low profile (about $0.01\lambda $ at 3.3 GHz) and a compact size of 42 mm $\times42$ mm (about $0.46\lambda \times 0.46\lambda $ at 3.3 GHz) to provide four wide operating bands (about 24% centered at 3.75 GHz) with antenna efficiency better than 40%. The conjoined circular slot leads to enhanced operating bandwidth of the four slot antennas and good decoupling thereof. Owing to the 1 mm low profile, the conjoined slot antennas are promising to be integrated within the dielectric back cover of the 5G terminal device, thereby decreasing the antennas’ volume inside the device. Details of the GCPW-fed conjoined slot antennas for 5G-MIMO operation are presented.

Low-Profile Wideband Conjoined Open-Slot Antennas Fed by Grounded Coplanar Waveguides for $4\times4\,\,5$ G MIMO Operation

A 24 GHz Industrial, Scientific, and Medical (ISM)-band Doppler radar antenna with high isolation is proposed for moving target sensing applications. Both transmit (Tx) and receive (Rx) antennas consist of 2 × 2 microstrip patch antenna arrays. High isolation of the proposed antenna is achieved by inserting periodic Jerusalem cross slot array between Tx port and Rx port, and thus the surface wave from Tx port to Rx port is significantly suppressed. The −10 dB reflection coefficient bandwidth of the proposed antenna is 510 MHz, and the isolation between Tx port and Rx port is higher than 36.7 dB over the 24 GHz ISM band. The simulated half-power beamwidths are 58.53° for E -plane and 47.54° for H -plane. Experimental results show that high isolation can lead to improve Doppler radar performance. The proposed antenna can be a good candidate for contact-free monitoring of the heart rate and respiration rate in healthcare applications.

A 24 GHz ISM-Band Doppler Radar Antenna With High Isolation Characteristic for Moving Target Sensing Applications

Multiple-input multiple-output (MIMO) antennas have been a mainstream technology in fifth generation (5G) communications However, strong mutual coupling between both adjacent and nonadjacent elements is inevitable, especially among highly integrated devices In this paper, a slit embedded mushroom electromagnetic bandgap structure (EBG) is primarily proposed to suppress the propagation of surface wave between antenna elements Subsequently, complementary split-ring resonators (CSRRs) are periodically arranged in two sides of the ground to steer the surface wave By effectively utilizing the unusual electromagnetic property of EBG and CSRR to manipulate the propagation of surface wave, the mutual coupling between antenna elements has immensely alleviated Finally, an H-shape defected ground structure (DGS) is introduced to reinforce decoupling effect In order to validate the feasibility of the design principle, a prototype of the proposed antenna has been fabricated and measured Measured results demonstrate that the decoupling concept in this paper is reasonable and approximately 12 dB reduction of mutual coupling is realized

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Enhancing MIMO Antenna Isolation Characteristic by Manipulating the Propagation of Surface Wave

Decoupling of a wideband circularly polarized (CP) conformal array is studied by a single-negative (SNG) metamaterial (MTM) isolator. The conformal array consists of two wideband CP antennas, which are assembled on an arched carrier, and a wideband SNG structure is placed between the antenna elements for coupling reduction. The SNG structure consists of four wheel-shaped units concentrically. The SNG structure is bianisotropic, and its constitutive parameters are tailored to form a wide isolation band. By using the proposed wideband SNG structure, the mutual coupling between the conformal antenna elements is reduced up to 25 dB at 4.5 GHz, and the mutual coupling is improved in the 3.9–5.05 GHz band. In addition, the loading of the SNG structure has little effect on the axial-ratio bandwidth. Besides, the gain and radiation patterns are improved. This research provides some basis in the decoupling of the conformal array.

Coupling Reduction for a Wideband Circularly Polarized Conformal Array Antenna With a Single-Negative Structure

The previously proposed resonance-based reflector (RBR) antennas have the merits of wide impedance bandwidth, low-profile, and unidirectional pattern; however, their front-to-back ratio (FBR) bandwidths are narrow. In this paper, a wideband RBR is proposed for FBR bandwidth enhancing antenna with a low profile. The proposed RBR consists of a metallic ring with three stubs printed on a substrate. It has a ±90° in-phase bandwidth of 63.57% (3.23-6.24 GHz) and its reflection magnitude values range from -0.02 to -11.5 dB within the in-phase band (-90° <; φ <; 90°). Compared with the existent RBRs, the proposed RBR achieves a greater reflection magnitude within the in-phase band, which would be critical for FBR enhancement of an antenna. The proposed RBR is applied to a ring-loaded teardrop-shaped bowtie antenna to form unidirectional radiation pattern. The measured -10-dB impedance bandwidth of the proposed antenna is 3.48-6.12 GHz (55.00%), while FBR values are larger than 10 dB within the whole impedance band and the maximum FBR value is up to 20.4 dB. The peak gain is bigger than 7.3 dBi within the whole impedance band. There is a reasonable agreement between the simulated and measured results. The profile of the overall unidirectional antenna is 0.15λ
 L
 at the lowest operating frequency, and the RBR has the same size as the bowtie antenna. Compared to the previous RBR antennas, the proposed RBR antenna has wider FBR bandwidth. The operating band of the antenna is suitable for 5G system applications.

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A Low-Profile and Wideband Unidirectional Antenna Using Bandwidth Enhanced Resonance-Based Reflector for Fifth Generation (5G) Systems Applications

A dual-band gyrotron traveling wave amplifier (Gyro-TWT) can reduce the size, cost, and weight of a transmitter in dual-band radar and communication systems. In this paper, a dual-mode input coupler for K/Ka dual-band gyrotron traveling wave amplifier (Gyro-TWT) is designed. This structure is composed of two different types of input couplers, one is the coaxial input coupler for the Ka-band TE2,1 Gyro-TWT and the other is a Y-type input coupler for the K-band TE1,1 Gyro-TWT. For reducing the backward wave of the TE2,1 mode reflecting into the Y-type input coupler to influence the operating bandwidth, a Bragg reflector with a strong mode selective characteristic is inserted between these two couplers, which can make the reflection coefficient of the TE2,1 mode better than −1 dB and the phase matched in the whole bandwidth, and the transmission coefficient of the TE1,1 mode can reach better than −1 dB. Based on the simulation results, the −1 dB bandwidth of the Ka-band TE1,0□-TE2,1◯ mode input coupler reaches 3.32 GHz and the −1 dB bandwidth of K-band TE1,0□-TE1,1◯ mode input coupler reaches 3.15 GHz. The designed dual-mode input coupler has the advantages of broad bandwidth and low loss and can be well used in dual-band Gyro-TWTs.

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Design of a Dual-Mode Input Structure for K/Ka-Band Gyrotron TWT

Alternating projection method combined with Fast Fourier Transform technique solves uniform linear array pattern synthesis effectively, but it's not valid for non-uniform linear array pattern synthesizing. In this paper, the weighted least mean squares method as a reverse operator optimizing element excitations is introduced in an alternating projection method to formulate the weighted alternating projection method. The weighted alternating projection method can synthesize arbitrary antenna array patterns with excitation constraints. This method is employed to optimize non-uniform linear array patterns with low side lobe level and specified beam-forming. The detailed examples, including a comparison with the the results simulated by the commercial 3D electromagnetic field simulation software CST, are presented and illustrate that the proposed method has better performance in lower side lobe level and beam-forming and accelerates convergence.

Kun-shan Mo

Papers

Coupling Reduction for a Wideband Circularly Polarized Conformal Array Antenna With a Single-Negative Structure

A Low-Profile and Wideband Unidirectional Antenna Using Bandwidth Enhanced Resonance-Based Reflector for Fifth Generation (5G) Systems Applications

Design of a Dual-Mode Input Structure for K/Ka-Band Gyrotron TWT

Weighted Alternating Projection Method Synthesizing Nonuniform Linear Array Pattern