# Review of Recent Phased Arrays for Millimeter-Wave Wireless Communication
## 1 Introduction
### Millimeter-Wave 5G Wireless Communication  
Over the past few decades, the continuous development of new generations of communications technology has significantly impacted the daily lives and routines of people and resulted in a constant increase in data traffic and device connections [4; 5; 6; 7]. Different wireless services have been introduced to the market because of the rapid development of wireless communications and mobile networking techniques, which has caused smart devices to become better known, and has prompted a tremendous increase in the data traffic in wireless networks [6; 7]. Owing to the increasing number of users in  
Figure 1: Block diagram of basic phased-array receiver [2].  
Figure 2: Basic architecture of (**a**) passive phased-array architecture, and (**b**) active phased-array architecture [1].  
wireless communications, it is predicted that the number of mobile connections will surpass 100 billion by the year 2020; hence, the Internet of Things (IoT) is becoming a more common concept [5]. The use of currently unused spectra is therefore being highly encouraged because of the increasing need for higher data rates in wireless communications [4; 5; 6; 7; 8; 9; 10; 11]. Millimeter-wave (mm-wave) communications systems have attracted much interest as a next-generation technology and are referred to as the fifth-generation (5G) wireless communication systems, which are expected to be implemented by the early 2020s [5]. The frequency spectrum specified by the International Telecommunication Union (ITU) for 5G communication includes the 3.4-3.6 GHz, 5-6 GHz, 24.25-27.5 GHz, 37-40.5 GHz, and 66-76 GHz bands, while the Federal Communications Commission (FCC) has specified the 27.5-28.35 GHz frequency band for 5G [5; 6; 11].  
The benefit of using mm-waves for wireless communication has been well known for a long time [4; 5; 6; 9; 10; 11; 12; 13]. Compared with 4G, 5G wireless communication systems employ significantly different system performance scales that require data rates of the order of several gigabits-per-second (Gbps), as well as a very high data flow density, millisecond level delay, crowded connections, and enhanced spectral energy and cost factor [5; 11; 13; 14]. It is generally agreed that the signal-to-interference-plus-noise ratio (SINR) reduces considerably owing to extreme free-space loss and blockage experienced by EM waves at high frequencies, particularly in the mm-wave bands [9]. Figure 3 illustrates a typical heterogeneous 5G mobile network scenario. The use of highly directional antennas and their line-of-sight (LOS) propagation can effectively alleviate the signal interference in between the common channels to or from other systems [14]. To achieve this, high-gain directional antennas can be used at both the transmitting and receiving ends, resulting in a significantly enhanced SINR, a reduced Doppler effect, and improved data security, and can be used in long-range mm-wave point-to-point (P2P) communications with an LOS link [6; 8; 10; 11; 13; 15; 16; 17; 18; 19]. The path loss can be reduced by using directional high-gain antennas [8; 9; 20]. However, directional antennas with narrow beams are unsuitable for multiuser mobile communications as they provide only limited spatial coverage [6; 9; 20]. Moreover, directional beams need to be steered either electronically or mechanically to obtain a better substitute link for non-LOS communications [4; 7; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. Therefore, major obstacles for the implementation of commercial mm-wave systems on a large scale are their high cost and compromised performance [9; 10; 25].