# A Compact Quad-Element UWB-MIMO Antenna System with Parasitic Decoupling Mechanism
## 1 Introduction  
In recent years, data rate and channel capacity issues have been expansively investigated, since high throughput is imperative for modern wireless devices [1]. The unlicensed ultra-wideband (3.1 GHz-10.6 GHz) spectrum is considered promising for short range wireless systems that require high data rate. However, power restriction on license free UWB spectrum reduces its benefits for its applications in modern wireless systems [2]. Since multiple-input multiple-output (MIMO) technology is known to offer better data-rate for same channel thus the concept of incorporating UWB with MIMO antenna system offers a potential solution to enhance data rate [3]. The resulting UWB-MIMO antennas provide greater throughput and better channel capacity. Despite of various advantages achieved with MIMO antennas in modern communication systems, some challenges are also associated with them. Placement of multiple antennas in a close vicinity with a shared ground plane introduces strong mutual coupling among antennas resulting in reduced diversitygain. Thus, one of the main challenges is to maintain isolation among antennas by reducing their mutual coupling. Recent literature reports a number of techniques to achieve isolation among multiple radiating elements [4]. These techniques include stub loading [5; 6], defected ground structure (DGS) [7], neutralization lines [8], frequency selective surfaces (FSS) [9], metamaterial isolator [10], patterned grounds [11], meander line resonator [12], decoupling structure and parasitic elements [13], etc. Another challenge in UWB-MIMO antennas is to incorporate multiple antennas in limited space as per the miniaturization demands of radio frequency (RF) front-ends of modern antenna systems by keeping mutual coupling at a desired minimum level. The literature reports extensive research work on UWB-MIMO antennas [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. A four-element UWB-MIMO antenna for portable wireless applications is reported in [14]. The stepped slot antenna is used as a single radiating element. Isolation level of 22 dB is achieved with this arrangement. However, no specific decoupling mechanism is used for isolation. Another quad-element UWB-MIMO with band notching is presented in [15]. The staircase shaped decoupling and defected ground structure provides the isolation level of 22 dB. This antenna is designed for portable wireless systems. The MIMO antenna design in [16] proposes two symmetric half-slot radiating elements. A Y-shaped slot introduced in the ground plane provides isolation at lower frequencies of UWB by increasing the current path between the two ports. This system has compact dimensions but isolation level of 15 dB is achieved only at lower frequencies of UWB band. Another two-element UWB-MIMO notched band antenna reported in [17] has a T-shaped stub in the ground plane that helps in achieving 20 dB isolation level. A four element UWB-MIMO is presented in [18]. The orthogonal arrangement of four monopoles provides an isolation level of 15 dB as well as the polarization diversity. However, the isolation level of 15 dB with a larger size in comparison is achieved. Moreover, the MIMO performance parameters like total active reflection coefficient (TARC) and channel capacity loss (CCL) are left unevaluated with antenna efficiency of 70%. Another four-port UWB-MIMO system is presented in [19]. The system has four circular monopole antennas with defected ground plane and electromagnetic bandgap (EBG) structures. This system has much larger antenna dimensions and offers the isolation level of 17.5 dB. Split Ring Resonator (SRR) based quad-element MIMO antenna is proposed in [20]. This antenna configuration is designed for LTE, WiMAX and Bluetooth. However, the isolation of only 14 dB is achieved with a much larger size. In another study a two-element UWB-MIMO antenna is presented [21]. The two quarter elliptical ring resonators achieve the isolation of 15 dB with a comparatively large antenna size.