# Review of Recent Phased Arrays for Millimeter-Wave Wireless Communication
## 3 Discussion  
Although mm-wave technology is being considered overall for future 5G wireless communication systems, there remains the need for improvement and changes in current mm-wave architectures and the expected commercial designs to be used in mm-wave cellular communication networks [4; 6; 9]. A few basic improvements are required with respect to the deployment and implementation of mm-wave cellular networks. Sophisticated antenna array designs are proposed to improve the performance of wireless communication systems in terms of high gain and coverage area. Because there is a trade-off between the gain and bandwidth, phased arrays structures offer an increased angular beam-scanning range along with high gain [70]. Various types of phased-array structures have been developed in the last few years. For the BS array antenna, there remains a relaxation of the space factor for designing complicated phased-array designs with increased spatial beam-scanning. However, in the case of MS antennas, it is more demanding to implement phased arrays in cellular handsets because of the limited space, user mobility, and user effects. Therefore, in this article, we discussed in detail the design constraints and implementation challenges of mm-wave phased arrays for 5G mobile terminals (MSs) and access terminals (BSs). To obtain the directional fan-beam radiation patterns in both the vertical and horizontal directions, two separate phased arrays positioned on the top and bottom of the chassis of a cellular handset were demonstrated in [10]. In [49], 3D beam-scanning was achieved by employing a folded 3D structure using three subarrays at the top position, but there is a limitation in terms of the implementation of the 3D array structure in the cellular handset. To address this limitation, 3D coverage was achieved in [50] using the surface waves of three identical slotted subarrays by switching the main beam direction to separate regions. Each subarray behaves similar to a phased array to tilt the beam to a separate region. To address the polarization mismatch [7; 46] and incurred losses owing to the mobility of the user and various kinds of motions experienced by the mobile terminals at different angles, multi-polarized, i.e., horizontally polarized and vertically polarized antennas are combined in a subarray to achieve the diversity gain, while enhancing the efficiency of the transmission and reception at mm-wave frequencies [47]. To achieve polarization diversity along with the enhanced bandwidth, printed horizontally polarized and vertically polarized quasi-Yagi-Uda antennas were implemented [51]. Enhanced beam-scanning is achieved by combining horizontally polarized and vertically polarized antennas in an array using the side-by-side placement of these subarrays in a cellular handset [44; 47]. Because of the constraints of space and position in cellular handsets, much research has been performed on the structure of mm-wave phased arrays with different handset positions [28; 37; 41; 50]. Most of the phased arrays for 5G mobile terminals are single-layer substrate planar phased arrays to better fit into the chassis of cellular phones [48; 49; 50; 42]. Further research efforts have been attempted on multilayer mesh-grid phased arrays [7] and SIW phased arrays [37]. Although most research has been performed on low-cost substrate boards [71; 72], such as PCB-based phased arrays, there is little compliance with the metallic frame casing of modern cellular devices [56; 57]. However, the study in [41] introduced a set of two eight-element cavity-backed slotted subarrays on both sides of the metallic casing of a cellular device. The study in [40] illustrates the degree to which the human body is vulnerable to electromagnetic fields (EMF) owing to the designed phased-array antennas in terms of the specific absorption rate (SAR). The authors in [40] successfully reduced the SAR to 0.88 W/kg by using slotted subarrays on the top and back of the metallic casing of a cellular handset.