# Compact Quad-Port MIMO Antenna with Ultra-Wideband and High Isolation
## 1 Introduction  
With the development of modern wireless communication systems, high data transmission rates and large channel capacities have become hot topics of interest for researchers. The two bottlenecks limiting the increase in data transmission rate are the limited available spectrum resources and the multipath fading of the channel, while ultra-wideband technology (UWB) [1] shows strong resistance to multipath fading and high-speed transmission capability, while having the advantages of strong penetration capability, low power spectrum density, accurate positioning, etc., and is regarded as the optimal information carrier for short-range transmission and high-speed wireless connectivity. Multi-input multiple-output (MIMO) technology can be well realized with large channel capacity and high spectrum utilization. Without increasing the transmitting power and occupying the channel bandwidth, MIMO technology achieves a significant increase in channel capacity and high transmission rate by placing multiple antennas at the transmitter and receiver simultaneously and using spatial diversity technology and the multipath transmission effect of the channel to make the channel capacity of the system proportional to the number of antennas. UWB MIMO technology [2], on the other hand, combines UWB technology with MIMO technology to take advantage of both, achieving large channel capacity and high data transmission rate while avoiding multipath fading effects. UWB MIMO antennas have also become a research hot spot for scholars in recent years. The most important concern for UWB MIMO antennas is the low coupling of the antennas. With the rapid development of modern mobile communications, mobile terminals and wireless devices are becoming increasingly miniaturized and integrated. Along with the miniaturization of UWB MIMOantennas, the physical distance between antenna elements will become closer and the degree of mutual coupling will rise, leading to stronger and stronger electromagnetic interference that will cause the antenna to fail to function properly. Therefore, how to improve inter-cell isolation is a central issue in UWB MIMO antenna research. Common approaches of MIMO antenna to improve isolation include designing decoupling structures [3; 4], opening slits [5], loading metamaterials, frequency selective surfaces [6], electromagnetic bandgap structures [7; 8; 9], introducing neutralization lines [10; 11; 12; 13] and parasitic cell decoupling [14], as well as using defective ground structures [15; 16] and polarization diversity techniques [17; 18; 19]. The two-port MIMO antenna in [3] uses a \"T\"-shaped decoupling structure between cells and successfully increases the isolation to more than 24 dB, but the bandwidth is only 2.33-6.71 GHz and the size is too large. The two ports MIMO antenna in [5] work at 3.1-10.6 GHz and achieves good isolation of 31 dB by etching the center slot on the ground plane, but the antenna size is 47 mm \(\times\) 93 mm \(\times\) 1.6 mm. In the MIMO antenna in [10], through the operation of polarization diversity and docking floor, the average isolation degree of the antenna in the working range is greater than 26 dB, and the working bandwidth is extended to 2.1 GHz to 20 GHz, but the ECC of the antenna is low and the size is too large. The four-port MIMO antenna in [12] has a compact structure and can operate from 3-13.5 GHz. The antenna adopts an orthogonal layout structure, but the isolation degree is very low and the ECC only meets \(<\)0.4. In summary, the antennas in the aforementioned references suffer from complex structures, limited isolation effects, and large sizes. In this paper, we design a compact four-port MIMO antenna with high isolation and ultra-wideband. By changing the antenna feeding method and introducing \"T\" shaped metamaterial, the impedance bandwidth is extended and a UWB MIMO antenna covering 1-17 GHz is designed.