Resonant cavity antennas (RCAs) are suitable candidates to achieve high-directivity with a low-cost and easy fabrication process. The stable functionality of the RCAs over different frequency bands, as well as, their pattern reconfigurability make them an attractive antenna structure for the next generation wireless communication systems, i.e., fifth generation (5G). The variety of designs and analytical techniques regarding the main radiator and partially reflective surface (PRS) configurations allow dramatic progress and advances in the area of RCAs. Adding different functionalities in a single structure by using additional layers is another appealing feature of the RCA structures, which has opened the various fields of studies toward 5G applications. This paper reviews the recent advances on the RCAs along with the analytical methods, and various capabilities that make them suitable to be used in 5G communication systems. To discuss different capabilities of RCA structures, some applicable fields of studies are followed in different sections of this paper. To indicate different techniques in achieving various capabilities, some recent state-of-the-art designs are demonstrated and investigated. Since wideband high-gain antennas with different functionalities are highly required for the next generation of wireless communication, the main focus of this paper is to discuss primarily the antenna gain and bandwidth. Finally, a brief conclusion is drawn to have a quick overview of the content of this paper.

In this article, a millimeter-wave (mmW) leaky-wave antenna (LWA) with unidirectional beam scanning (UBS) capability based on the Fabry–Perot cavity (FPC) structure is proposed for fifth-generation (5G) applications. The proposed antenna consists of a main radiating element, a metallic wall, a ground plane, and a single-layer partially reflective surface (PRS). To improve the UBS performance, a single radiating element is designed such that it provides a tilted beam with good radiation performance over the scanning angles. The metallic wall is used close to the radiating element as a reflector to suppress the propagation in undesired directions. A general design guide is given by a theoretical analysis of the FPC structures using the ray-tracing method to formulate the beam-steering functionality versus frequency over desired predetermined angles. The PRS layer is designed without any tuning elements to provide the beam scanning over the angles of $10^\circ < \theta < 45 ^\circ $ and the frequency range of 24–30 GHz. The proposed Fabry–Perot cavity antenna (FPCA) structure is fabricated and the measurement results show a beam steering from 19° to 54° over the frequency band from 24 to 30 GHz, which is in good agreement with the calculated results from the theoretical analysis. A maximum measured gain of 14.5 dBi at 24 GHz is achieved. The proposed antenna can be used in the 5G mobile communication systems and 5G base station antennas (BSAs), where beam steering with high-gain and low fabrication cost features over the mmW spectrum is required.

In this paper, a new wideband resonant cavity antenna (RCA) with circular polarization is proposed. A wideband circularlypolarized (CP) crossed-dipole antenna acts as the main radiating element inside the cavity. The dipole is designed based on self-complementary structures and utilizes several parasitic patches and posts to obtain wideband circular polarization and impedance characteristic. The incorporation of the proposed antenna with a new broadband thin singlelayer dual-sided partially reflective surface (PRS) designed based on the complementary structure contributes to the improvement of the antenna gain in a broad bandwidth. The fabrication of the PRS on both sides of a thin single laminate substrate reduces thickness of the PRS compared with conventional multilayer PRS structures, in which the layers are separated by a gap distance. The measured results, which are in agreement with the simulations, show that the proposed antenna has a maximum measured gain of 12.5 dBic, while the 3-dB gain bandwidth, and 3-dB axial ratio bandwidths are 39% and 43.37%, respectively. The measured results demonstrate the wideband functionality of the proposed antenna.

In this paper, a technique to obtain a high-gain multi-beam antenna for millimetre-wave (mmW) fifth-generation (5G) base stations (BSs) is proposed using the Fabry–Pérot Cavity (FPC) structures. A thin single metallo-dielectric layer is used as the partially reflective surface (PRS) layer to enhance the radiation performance of the FPC antenna and provide an off-axis pencil beam. As the feeding structure illuminating the PRS layer, a cylindrical cavity is designed using a right-angle-type semi-waveguide (RATSW) structure. An array of coaxial probes are used to obtain beam switching at the azimuth plane by exciting different probes. To confirm the functionality of the proposed FPCA, a prototype is fabricated with five coaxial probes to generate five radiation beams; however, the structure can be extended to provide more antenna beams by using higher number of probes. The cavity structure of the proposed antenna is fabricated by the three-dimensional (3D) printing technology as an easy manufacturing process. The measurement results show that multiple switchable beams with high antenna gain are obtained over the desired frequency bandwidth of 27-30 GHz. The maximum measured gain is 19 dBi at 27.5 GHz when a single probe is excited. A conceptional scenario for the potential applications of the proposed antenna in the 5G BSs is presented.

This paper presents a high-gain leaky-wave antenna based on a resonant cavity (RC) structure with the capability of unidirectional beam scanning over predefined frequencies and desired beam angles. Frequency scanning is achieved by implementing a properly-designed single-layer partially reflective surface (PRS) based on a simple ray tracing-based design procedure. In the proposed structure, two techniques are combined to make the beam scanning unidirectional. First, a metallic wall is placed at a proper distance from the radiator to suppress the antenna propagation at undesired directions. Second, a feed antenna with a tilted beam is designed as the main radiating element inside the cavity to radiate over desired beam angles. A prototype of the proposed antenna is fabricated and measured demonstrating a beam scanning from $12^{o}$ to $46^{o}$ over the frequency band from 25 to 31 GHz. Moreover, a maximum gain of 15.4 dBi is achieved at 26 GHz on the elevation plane with tilted angle of $22^{o}$ . The proposed leaky-wave antenna is suitable for various wireless applications, in particular the $5^{th}$ generation of cellular networks and small cell base station antenna (BSA) applications.

In this paper, a dual band circularly polarized (CP) antenna is proposed. The antenna consists of a radiator placed above a slotted reflector at a certain distance resulting in significant improvement of the front to back ratio (FBR). The antenna has a 10-dB impedance bandwidth of about 36 % (1.17-1.69 GHz), with a 3-dB axial ratio bandwidth (ARBW) of 8 % (1.21-1.31 GHz) in the lower band and 11.7 % (1.53-1.72 GHz) in the upper band, which makes the antenna suitable for global positioning system (GPS) applications. The FBR of the antenna is about 20 dB over the frequency bands of interest.