# Design of Multi-Mode Antenna Array for Use in Next-Generation Mobile Handsets
## 1 Introduction  
With the rapid evolution of wireless communications, the 5G network has received a great deal of attention from both academia and industry, with many reported efforts and research outputs [1; 2; 3]. Significant improvements will be made in different areas, including the data rate speed and resolution, mobility, latency, etc. Multiple-input-multiple-output (MIMO) technology with multiple antennas is a promising technology to obtain the requirements of 5G communications [4; 5; 6]. To date, \(2\times 2\) MIMO systems have been successfully employed for 4G mobile networks, and a larger number of antenna elements is expected to be applied for 5G communications [7; 8]. The 5G system is predicted to possess an aggregate data rate 1000 times faster than 4G, and it has better link reliability. Thus, compared with the 4G MIMO antenna systems, at least six to eight antenna elements are integrated intoa mobile terminal for 5G massive MIMO to provide good diversity and multiplexing gain [9]. This can enhance the channel capacity and link system reliability [10; 11]. The greater number of antennas could make it more resistant to intentional jamming and interference. Through spatial diversity and spatial multiplexing, larger channel capacity and better communication reliability can be achieved. Therefore, the multi-antenna system is much more capable of resisting multipath fading and improving data throughput [12]. The 5G network also needs fundamental technologies to enable small cells, beamforming, full duplexing, MIMO, and millimeter-wave (MM-Wave). For sub-6 GHz 5G cellular communications, LTE band-41 (2.6 GHz), band-42 (3.5 GHz), band-43 (3.7 GHz), and band-47 (5.8 GHz) are the main important candidate frequency bands [13]. Due to the available radio frequency (RF) circuit and test system, 2.6 GHz LTE can be considered as a default for future mobile communications, and it has recently attracted a great deal of interest. Besides this, 3.4-3.8 GHz (LTE band 42/43) is also recognized by many countries as a first step in demonstrating 5G systems. To further support more potential sub-6 GHz frequency bands, LTE band 47, which is also known as the wireless wide area network (WLAN) operation band, can be considered for 5G massive MIMO antenna design [14; 15]. Several smartphone antenna designs with MIMO systems have been proposed recently [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. However, all of these designs either cover only a single-band operation frequency or use a few antenna elements with large sizes which could occupy a huge space of the mainboard. The 5G handset antenna designs introduced in [16; 17; 18; 19; 20; 21; 22; 23; 24] only cover a single frequency band. In [25; 26; 27; 28; 29], dual-band or wideband arrays are proposed to support two 5G spectrums. Only a few handset antennas with tri-band function are reported in [30; 31; 32] for handset applications. However, these antennas have double/quad antenna elements or do not cover important bands such as 2.6 GHz. In this study, we introduced a new MIMO antenna with eight-element planar-inverted F antenna (PIFA) elements which, unlike the reported designs, can cover multi-frequency bands simultaneously. In addition, due to the large number of radiators, the proposed handset antenna can be applied for massive MIMO communication [33]. Furthermore, the proposed handset antenna generates the polarization diversity characteristic, which supports both vertical and horizontal polarizations [34; 35; 36]. The modified PIFA radiation elements of the design are employed at four corners of the printed circuit board (PCB) to operate at three different frequencies covering the LTE 2600, 42/43, and 47 operation bands. The proposed PIFA array system operates at three different bands--2.6, 3.6, and 5.8 GHz--of sub-6 GHz 5G cellular networks. It exhibits good properties in terms of the fundamental characteristics and could be used in future handsets.