# A Compact Quad-Element UWB-MIMO Antenna System with Parasitic Decoupling Mechanism
## 1 Introduction
This system has low peak gain and efficiency in comparison. Moreover, important parameters like TARC and CCL are left unevaluated. In [22] a two-element U-shaped MIMO configuration for handheld devices is presented. The antenna achieves the required isolation with a decoupling slot introduced in the ground plane. However, the proposed design operates in the lower UWB band only with much larger size. The efficiency of antenna is 60% and certain MIMO performance parameters are also left uncalculated. Another two element UWB-MIMO antenna with band-notching is presented in [23]. The T-shaped stub in the ground plane provides necessary isolation between antennas. This design has an isolation of 15 dB with the much larger size in comparison. The TARC and CCL values are not evaluated as well. A four-element MIMO antenna is presented in [24]. The proposed system operates in LTE band with sufficient MIMO performance. This system has much larger size with the minimum isolation level of 10.5 dB. Moreover, the efficiency and gain are also low. Another four-element annular slot-based MIMO antenna is designed for UWB applications in [25]. The antenna system is meant for cognitive radio platform and operates up to 7.65 GHz. This presented system has low isolation level of 13 dB with larger size and low performance in terms of efficiency and gain comparatively. Likewise, in another study, reported in [26], a four-element MIMO system for LTE band 1 and 4 is discussed. This system achieves isolation of 10 dB in limited frequency band of 1.77 GHz-2.51 GHz. Another semi-elliptical two-element UWB-MIMO antenna is reported in [27]. This design has larger size along with the isolation level of 18 dB. The MIMO performance is evaluated in terms of ECC and CCL. Four monopoles based MIMO antenna is discussed in [28]. The design possesses the band rejection ability because of band-stop design insertion on the ground plane. The structure acts as an LCband-stop filter and rejects the wireless local area network (WLAN) band. However, this system has isolation of 17 dB with larger size. The design in [29] has four quasi-self-complementary (QSC) antenna elements. This system possesses polarization diversity but with much larger size comparatively. The isolation level of 20 dB is achieved but certain MIMO performance parameters are left unevaluated. In this work, a highly compact semi-elliptical shaped quad-element UWB-MIMO antenna is presented. The antenna possesses two distinct mechanisms for coupling reduction. The ground plane is modified by introducing defects as well as stubs have been attached to the ground plane to reduce coupling between side-by-side elements. A parasitic decoupling structure is designed and placed on the back side of substrate which offers multiple resonances in order to ensure wideband decoupling for across elements, these structures provide isolation better than 20 dB in the entire frequency band. Diversity performance of the proposed MIMO antenna is established by evaluating the performance parameters like envelope correlation coefficient (ECC), total active reflection coefficient (TARC), channel capacity loss (CCL) and diversity gain (DG). The rest of the paper is organized as follows: Section 2 describes the proposed design in detail followed by Section 3, in which the isolation mechanisms for mutual coupling reduction are discussed. The simulated and measured results of the proposed MIMO antenna are discussed in Section 4. A comprehensive comparison of the proposed design is presented in Section 5 and the paper is concluded in Section 6.