# Review of Recent Phased Arrays for Millimeter-Wave Wireless Communication
## 2 Millimeter-Wave Antenna Array for 5G Communication
### Millimeter-Wave Antenna Arrays for 5G Mobile Terminals
#### 2.1.2 Multi-Polarized Multilayer Phased-Array Antennas for 5G Mobile Terminals  
To improve the diversity gain in 5G mobile terminals, the use of multi-polarized antenna arrays is a desirable solution to overcome the polarization mismatch problem. A simple demonstration dual-polarized mm-wave antenna with good isolation is reported in [43]. To achieve dual polarizations at 28 GHz, the topology of a side-by-side arrangement of horizontal and vertical Yagi-Uda antennas is used in [44]. In [45], an aperture antenna array with a dual-polarized unidirectional pattern through a back cavity on multilayer PCB technology is proposed. A multi-polarized mm-wave antenna array configuration for 5G mobile terminals was demonstrated in [46]. The proposed work is the continuation of the previously reported work of [PERSON] et al. The reported research focuses on losses incurred by the polarization mismatch to enhance the mm-wave transmission and reception efficiency. At mm-wave frequencies, it is complicated to transmit high-power energy because of well-understood propagation and absorption losses. The 5G wireless communication link budget estimation has become more inflexible owing to real-life constraints such as limited battery life. Because mobile antennas integrated inside the mobile terminals face different angular motions, polarization mismatches between transmit (Tx) and receive (Rx) antennas have become an important loss factor for mm-wave cellular communication. To overcome the polarization mismatch loss factor, two different antenna-element designs based on the antenna array schematic are demonstrated and investigated. A coplanar waveguide-fed horizontally polarized planar Yagi-Uda antenna-element configuration together with a multi-plate antenna-element topology, which excites a vertically polarized electric field, is proposed, as shown in Figure 7a. The 16-element phased-array antenna depicted in Figure 7c is designed by deploying the two linearly polarized antenna elements alternatively along the edge of the mobile terminals with an angular scanning range of \(\pm\)80\({}^{\circ}\). By maintaining the distance at less than 3 mm, an isolation of more than 40 dB was achieved between the horizontally polarized and vertically polarized multi-antenna elements. The two sets of 16-element phased-array antennas on the opposite corners at the top and bottom sides of the cellular device provide maximum spherical coverage and polarization diversity. An antenna array with a maximum height of 0.8 mm was fabricated using 10-layer FR-4 lamination. The dielectric constant and tangent loss at 28 GHz are determined to be 4.2 and 0.09, respectively. Figure 6: (**a**) Three-dimensional (3D) illustration of proposed antenna element. (**b**) Electric fields of proposed antenna’s modes: top view (**left**) and bottom view (**right**). (**c**) Geometry of proposed eight-element phased-array antenna. (**d**) S-parameters of proposed phased-array antenna. (**e**) 3D radiation patterns of proposed phased array in all modes. (**f**) 3D view of antenna and MMPX feeding structure. (**g**) Simulated and measured 3D radiation patterns at different scanning angles at 25, 27, 29, and 31 GHz. (**h**) Overview of the layers in the antenna structure. (**i**) Prototype of the phased-array antenna. (Redrawn from [42])A multi-polarized antenna array that integrates the horizontally and vertically polarized quasi-Yagi antennas together into a single area was implemented and demonstrated in [47] for 5G mobile terminals, achieving polarization diversity and size reductions. To achieve polarization diversity along with the beam-scanning capability, dual-polarized quasi-Yagi-Uda antennas for both the corner edges and a lateral array design were addressed, as illustrated in Figure 8a. The proposed array antenna was implemented on a three-layer structure on a Rogers4003 substrate with a total thickness of 1.93 mm, which has a dielectric constant of 3.38 and a tangent loss of 0.0027.