# Dual-Band MIMO Antenna with Enhanced Isolation for 5G NR Application
## 1 Introduction  
Nowadays, the fifth generation (5G) communication system, when it is compared with traditional communication systems, has the advantages of high speed, wide frequency bandwidth, low power consumption, and high reliability. Meanwhile, the multiple-input and multiple-output (MIMO) antenna system has attracted extensive attention due to it having lower multipath fading effects, higher channel capacity, and an increased transmission rate than those of the single antenna communication system. However, there are still many challenges in designing high-isolation and compact-size MIMO antennas, such as the mutual coupling between the antenna elements [1].  
Therefore, it is a critical problem in the design process of the MIMO antenna to reduce mutual coupling. Many scholars have presented various decoupling techniques to reduce mutual coupling between antennas [2; 3; 4; 5; 6; 7]. In [2], an ultrawideband (UWB) MIMO antenna achieved high isolation values that are larger than 20 dB for the whole operating band by slotted stubs. A coplanar waveguide-fed MIMO antenna with high isolation values was proposed by adding a double Y-shaped branch [3]. In [4], by introducing a creative un-protruded multi-slot (UPMS) isolating element, a four-port MIMO antenna array was proposed to reduce the mutual coupling between the antenna elements further. In [5], each antenna element was placed orthogonally to achieve the good isolation of 45 dB at the band of 36.83-40.0 GHz. It is reported in [6] that a combination of a defected ground structure (DGS) and electromagnetic band gaps (EBG) can be adopted to ease mutual coupling. In [8], the mutual coupling of a four-port MIMO antenna has been reduced by the orthogonal orientation of radiating elements rather than any decoupling structures.  
Furthermore, it is also challenging to design a multi-band MIMO antenna to meet substantial wireless communication bands because the same isolation method may not work at different frequency bands. Recently, several dual-band MIMO antennas have been proposed [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. In [9], a dual-notched four-element MIMO antenna with dual-bandcharacteristics has been proposed by introducing gap sleeves and an H slot. It covers the frequency bands of 3.3-4.1 GHz and 8.2-8.6 GHz. A dual-band MIMO antenna was proposed in [10] by implementing an L-shaped feeding strip, a parasitic rectangle strip, and a modified Z-shaped radiating strip. In [13], an eight-element MIMO antenna was designed to meet the requirement of the 5G mobile terminals. A quad-element MIMO antenna with E-shaped and G-shaped stubs was presented to achieve two resonances at 1.5 GHz and 2.45 GHz, respectively in [18]. In [20], a MIMO antenna array was realized which can cover the 5G NR Bands n77/n78/n79 and WLAN 5 GHz band.  
In this paper, a two-port MIMO antenna with high isolation and dual-band characteristics is presented. The antenna consists of two identical three-quarter circular patches which are placed symmetrically on the top of the substrate, with the size of 41 \(\times\) 30 \(\times\) 1.59 mm\({}^{3}\). The antenna generates two frequency resonances by introducing two L-shaped branches and two step-shaped gaps in the radiators. The working bands can cover n7 (2.5-2.69 GHz), n38 (2.57-2.62 GHz), n41 (2.496-2.690 GHz), and n79 bands (4.4-5 GHz). The measured isolation is larger than 20 dB. Other MIMO antenna parameters are presented to analyze the performance, such as the S-parameters (reflection and transmission coefficients), radiation patterns, ECC, DG, and efficiencies.