# Design of Wideband Decoupling Antenna Array for 5G Smartphones at N77/N78/N79/WLAN 5 GHz Bands
## 1 Introduction  
The information needs of human society's production and life have accelerated the development and improvement of mobile communication systems. Since 1973, mobile communication systems have gone through analog telecommunication systems, SISO, MIMO, and massive MIMO, and the current fifth-generation mobile communication system (5G) pursues the characteristics of wide bandwidth, large capacity, high speed, low latency, and high reliability. The 5G frequency bands are mainly divided into a sub-6 GHz band (FR1) and millimeter-wave band above 24 GHz (FR2) [1]. The sub-6 GHz band (FR1) has better coverage area and smaller propagation loss than the millimeter-wave band (FR2), but it is not as good as the millimeter-wave band in terms of transmission speed and security, and the spectrum resources are relatively tight. However, due to the serious path loss and expensive deployment cost of millimeter-wave technology, the sub-6 GHz band is more practical at this stage. The foregoing suggests that multiple-input multiple-output (MIMO) technology makes the application of the sub-6 GHz band in 5G a reality due to its feature of effectively increasing the channel capacity of the communication system in a rich scattering environment [2]. According to the requirements of 5G-related protocols, sub-6 GHz mobile terminal MIMO systems require at least 6-8 antenna elements. However, integrating multiple antenna elements within a compact mobile terminal will result in significant mutual coupling between antenna elements. Mutual coupling leads to decreasedantenna isolation and increased envelope correlation coefficient, and then channel capacity is reduced and communication quality deteriorates [3]. Therefore, how to design broadband high-isolation terminal antennas with compact size has received increasing attention. Previously proposed decoupling techniques mainly include loaded parasitic elements (PEs) [4; 5; 6], defective ground structures (DGSs) [7; 8; 9], neutralization lines (NLs) [10; 11; 12], diversity technique [13; 14; 15], common ground plane higher-order mode [16], self-curing decoupling technique [17; 18], and decoupling network [19]. These techniques have been proven to be effective in solving the mutual coupling problem of traditional MIMO antennas. However, when applied to eight-element antenna arrays and broadband antenna decoupling, they face problems such as decoupling only 2-3 elements or part of the frequency range. In recent years, some decoupled antennas covering sub-6 GHz n77 (3300-4200 MHz), n78 (3300-3800 MHz), n79 (4400-5000 MHz) have been proposed [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. The orthogonal pattern dual-antenna shared radiator design is used in [20] to achieve 3.3-5.0 GHz impedance bandwidth (\(-6\) dB) with a high isolation of 21 dB. This decoupling is so cleverly designed that it does not require any external decoupling structure, but its bandwidth coverage is small. In [22], by placing eight antenna elements orthogonally at the four corners of the model and decoupling them with a 7.8 mm short neutralizing line, the dual-band impedance bandwidths of 3.1-3.85 GHz and 4.8-6 GHz are achieved. This design is very compact, and it cleverly utilizes the space in the four right-angle positions of the smartphone, but the broadband performance is less than ideal and does not fully cover the n77 and n79 bands. In [23], an ideal impedance bandwidth of 3.3-5.6 GHz was obtained by coupling the radiating branch with a T-shaped feeder branch, and a high isolation of 15 dB was obtained by introducing a T-shaped neutralizing line structure. However, the antenna unit and decoupling structure in this design almost completely occupy the two longer-side bezels.