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

Metamaterials offer the freedom to manipulate the polarization states of light at the subwavelength scale. However, previous designs generally made use of single resonance and, hence, only function for one specific incident polarization within a single-frequency band. In this paper, we present a dual-band metasurface based on a concentric rectangular arrangement to control the polarizations of both linearly and circularly polarized electromagnetic waves. We show that, on the one hand, the proposed metasurface structure can convert the polarization of linearly polarized waves to the cross direction in two broad frequency bands 4.40–5.30 GHz and 9.45–13.60 GHz. On the other hand, this design can also reflect circularly polarized waves without changing the handedness in two frequency bands 4.47–5.35 GHz and 9.57–13.57 GHz. For both cases, average polarization conversion efficiency larger than 86% was obtained in our experiment. The numerical simulations reveal that the dual-band cross-polarization coupling results from the strong electric and magnetic resonances between the top and bottom layers. The ultrathin polarization converter experimentally realized in this paper provides an important stepping stone for the future design of other dual-band metasurface devices and is believed to be extendable to higher frequency regimes.

Ultrathin Dual-Band Metasurface Polarization Converter

A novel and high-performance four-element ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna is proposed in the paper. The proposed antenna is designed by using a novel integration technology of the symmetric layout, orthogonal structure, four-directional staircase-shaped decoupling, and multi-slit and multi-slot techniques. The mutual couplings among the antenna elements are significantly reduced by introducing the symmetric orthogonal and separated four-directional staircase-shaped structure. Furthermore, the antenna size is effectively miniaturized, and its impedance bandwidth is broadened by using a two-sided symmetric layout, partial and defected ground structure, the decoupling structure, and multi-slot and multi-slit techniques. Therefore, the antenna has the low-profile structure and a small dimension of 39mm $\times 39$ mm $\times 1.6$ mm. Moreover, the proposed antenna achieves triple band-notched characteristics by embedding different type slots and slits on the square radiating elements, default ground structure, and the decoupling structure, respectively. As a result, the antenna obtains the wider bandwidth of 2.30–13.75 GHz with the notched bands of 3.25–3.75 GHz, 5.08–5.90 GHz, and 7.06–7.95 GHz. The three notched bands are good in agreement with the existing interference bands of WiMAX (3.3–3.7 GHz), WLAN (5.15–5.875 GHz), and ${X}$ -band (7.1–7.9 GHz), respectively. In addition, the proposed antenna also has a lower mutual coupling ( $\eta _{\mathrm {mux}}> -3.0$ dB), stable gain, and quasi-omnidirectional radiation patterns at the entire impedance bandwidth. Therefore, a good tradeoff of the performance is obtained for the proposed antenna. The proposed antenna can be a good candidate for UWB-MIMO wireless communication applications, and especially for portable UWB-MIMO systems.

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Compact UWB-MIMO Antenna With High Isolation and Triple Band-Notched Characteristics

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

Terahertz absorption modulator with largely tunable bandwidth and intensity

Ultra-wideband (UWB) antennas with wide rectangular notched-band are proposed in this paper. The rectangular notched-band design is realized by placing dual mushroom-type electromagnetic-bandgap (EBG) structures on the CPW feeding line. The patches of the EBGs are laid on the back size of the substrate and dead against the CPW feeding line. Shorting pins are utilized to connect the patch and the feeding line. Then, the CPW feeding line operates as ground plane for the EBGs. By tuning the two resonant frequencies of the EBG structures and making them merge with each other, a rectangular notchedband is achieved. Research also found that design methodology has the ability to tune the width and the frequency of the rectangular notched-band by adjusting the EBG parameters. Then, three cases of rectangular notchedband designs to reject the wireless local-area network (WLAN, 5.150-5.825 GHz) interference band and the X -band downlink satellite communication band (660 MHz, 7.10-7.76 GHz) were designed by using different EBG parameters. The design methodology can also be utilized to design rectangular notchedband for other interference bands efficiently.

CPW Fed UWB Antenna by EBGs With Wide Rectangular Notched-Band

An ultrawideband and high-gain circularly polarized (CP) antenna fed by inclined double-Y-shape slot is proposed in this letter. The antenna structure is mainly composed of two patches, an aperture coupling-fed layer and a metallic reflector. The metal reflector is placed under the feeding layer for generating unidirectional radiation and improving the gain of the antenna. Right-hand circular polarization is achieved by adjusting the lengths of the two branches of the Y-shape slot. In order to expand the impedance bandwidth (IBW), two patches are used. By chamfering the radiation patch, good ultra axial-ratio bandwidth is obtained from 3.6 to 6.0 GHz. The measurements show that the IBW is about 71.0% (3.28–6.76 GHz) for VSWR < 2, and the bandwidth of antireflection is about 49.8% (3.61–6.05 GHz). Moreover, the antenna has a stable gain is better than 8.5 dB from 3.2 to 7 GHz. The axial ratio can be kept below 3 dB over the entire half-power beamwidth.

Ultrawideband and High-Gain Circularly Polarized Antenna With Double-Y-Shape Slot

Polarization converter is firstly proposed with the functions of simultaneously converting a linear polarization (LP) and a circular polarization (CP) to its corresponding cross polarizations, and polarization conversion ratio (PCR) of the converter with graphene strips is also controllable in this paper. A reflection-type converter is designed based on a metasurface in the terahertz region. It consists of periodic unit cells with strip-loaded half elliptical rings, a dielectric spacer layer, and a gold layer. The converter with PCR regulating also includes graphene strips on the gold layer. Both the LP and CP incidences can be converted into their cross-polarized ones with good PCR performances in a 2-THz band (2.1–2.9 THz, 32%). The graphene strips on the gold layer that disturbs the gold layer currents can regulate the PCR, typically from 2% to 98% for LP illuminations and from 60% to 98% for CP illuminations by adjusting the chemical potential ${\mu} _{{\text{c}}}$ of the graphene. Also, the physical mechanisms of the polarization conversions were investigated.

A Novel THz Half-Wave Polarization Converter for Cross-Polarization Conversions of Both Linear and Circular Polarizations and Polarization Conversion Ratio Regulating by Graphene

A broadband four-feed circular patch antenna is demonstrated to generate high-purity dual-mode vortex beams carrying orbital angular momentum (OAM) for the first time. The electric field formula of the circularly polarized (CP) component is derived for analyzing radiation characteristics of different OAM modes, such as the CP property, phase and amplitude distributions. Theoretical calculation and numerical simulation results show that the four-feed patch antenna operating in the TMn1 mode can generate ±n OAM waves. A prototype of a four-feed circular TM21-mode patch antenna is fabricated and measured to verify the effectiveness of the theoretical analysis. The proposed antenna consists of a four-feed patch with an air-coupled parasitic patch and a four-way phase-shifting feeding network. Experimental results show that the proposed antenna can generate a left-hand CP OAM with mode l = +2 and a right-hand CP OAM with mode l = −2. The OAM beams are generated in a wide band from 5.50 to 6.50 GHz with a mode purity over 90%. The proposed method is very suitable for the generation of wideband, dual-mode and high-purity OAM beams in the microwave and millimeter wave bands.

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Generation of Broadband High-Purity Dual-Mode OAM Beams Using A Four-Feed Patch Antenna: Theory and Implementation

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.

Xing Jiang

Papers

CPW Fed UWB Antenna by EBGs With Wide Rectangular Notched-Band

Ultrawideband and High-Gain Circularly Polarized Antenna With Double-Y-Shape Slot

A Novel THz Half-Wave Polarization Converter for Cross-Polarization Conversions of Both Linear and Circular Polarizations and Polarization Conversion Ratio Regulating by Graphene

Generation of Broadband High-Purity Dual-Mode OAM Beams Using A Four-Feed Patch Antenna: Theory and Implementation

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