# A Compact Ultra-Thin 4 \(\times\) 4 Multiple-Input Multiple-Output Antenna
## 1 Introduction  
The tendency of rapid development of fifth-generation (5G) communication technology requires antennas with high data rates and low volume. Multiple-input multiple-output (MIMO) technology provides a promising solution to obtain a higher message transmission data rate than single-input single-output (SISO). Since the 3.5 GHz (3.3-3.8 GHz) band was initially deployed as one of the mobile phone spectrums, significant efforts have been made to exploit MIMO antennas operating at the 5G 3.3-3.8 GHz band for smartphones [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. One thorny issue encountered in the antenna design is effectively accommodating as many antenna elements as possible in a space-restricted smartphone. In a MIMO system, the inevitable mutual magnetic coupling between any two radiating elements increases with decreasing distance. Numerous decoupling schemes have been presented to enhance antenna performance, such as the utilization of auxiliary decoupling branches [1; 2; 3; 4], defected ground structure (DGS) [5; 6; 7; 8], neutralization lines [9; 10], lumped components [11; 14], and orthogonal modes [12; 13; 14].  
Recently, numerous MIMO smartphone antennas have been reported [12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. By integrating two antenna elements in a block and taking appropriate approaches to attenuate the mutual coupling, an antenna pair (AP) block was designed. The radiating elements of traditional smartphone antennas are were along two edges, and the APs system could reserve more space for 2G/3G/4G antennas integrated within a mobile handset due to its smaller dimension. A novel tightly arranged AP [12] was designed to construct a 4 \(\times\) 4 MIMO system. By placing two antenna elements in an orthogonal mode in an AP block, a predominant isolation performance larger than 17 dB within 3.4-3.6 GHz was achieved. In ref. [15], a four-element self-decoupled AP system operating in 3.4-3.6 GHz was presented, and the mutual coupling in an AP block decreased to 17 dB by sharing onecommon grounding branch. A dual-band four-element AP system for 5G handsets was represented in ref. [16]. In ref. [17], a connecting line was inserted between two closed-located antenna elements in an AP block to weaken the mutual coupling in a wideband slot antenna. An acceptable isolation performance superior to 10.5 dB was maintained over the 3.3-5 GHz band.  
Almost all lateral boards of the abovementioned APs systems exceed 6 mm, which is a great challenge to the realization of an ultra-thin smartphone. In ref. [18], a four-port MIMO AP system operating in the 5G n78 band (3.4-3.6 GHz) was studied and presented. Two APs were positioned in the center of the sideboard, and a decoupling (DC) stub was utilized further to improve the port impedance matching and isolation performance. Preferable isolation of around 17.5 dB between port 1 and port 2 was obtained in an AP block. The integral dimension of the proposed MIMO AP system was 150 \(\times\) 75 \(\times\) 6 mm\({}^{3}\); 1 mm height reduction on the lateral board was realized compared with the aforementioned designs [15; 12; 16; 17].  
In this article, a minimized four-element AP system functioning in the 5G n78 band (3.4-3.6 GHz) for ultra-thin smartphones was provided. The proposed four-element MIMO antenna was made up of two APs, which were printed at the center of two sideboards. A T-shaped open-ended slot was utilized to attenuate the mutual coupling and improve the port impedance matching. The explored AP obtained a relatively small area of 18 mm \(\times\) 4.2 mm, which is smaller than many reported APs [15; 12; 13; 14; 16; 17; 18; 19; 20]. The measured \(-\)10 dB bandwidth of the fabricated AP system is capable of covering the desired band, and ideal isolation performance superior to 17 dB was also maintained. Numerous indicators were computed to evaluate the MIMO performance, such as ECC, DG, TARC, and ME. Remarkable coherence between the simulated and measured curves was observed.