# Triple-Band Notched Ultra-Wideband Microstrip MIMO Antenna with Bluetooth Band
## 1 Introduction
Not only does the MIMO network system suppress multipath fading, but it also improves the network channel capacity [6]. Therefore, implementing the UWB antenna with MIMO technology increases the data rate, besides improving the channel capacity of the wireless communication link. The idea of using the MIMO network system is based on using one of the available diversity techniques (i.e., polarization diversity or spatial diversity). Thus, the investigation of the UWB antenna via the MIMO technique is based upon placing and arranging several antennas in a planar shape with different orientations or with a certain separation distance to achieve high multiplying gains [7; 8; 9; 10]. The polarization diversity technique is selected rather than the spatial diversity one to investigate the UWB antenna with the MIMO system because the former introduces a smaller radiator board size than the second one. However, the UWB antenna in the MIMO system suffers from high mutual coupling between the adjacent radiators in the UWB-MIMO system. This is because of the small separated distance between every two adjacent radiators. In order to overcome this problem, the isolation level between the adjacent radiators should be better than a certain level in dB to guarantee alleviation of the interference between the two coupling currents. In [11], the concept of the MIMO capacity and symbol error rate relies on an isolation level between \(-15\) dB and \(-17\) dB in order to overcome the mutual coupling between the radiators. Several techniques have been reported to improve the isolation between the adjacent ports in the UWB-MIMO system [12; 13; 14; 15]. These techniques involve placing stubs between the radiators connected to the ground plane [12], placing neutralization lines [13], usingelectromagnetic bandgap structures (EBGs) [13], and using defective ground structures (DGSs) [15]. Defective ground structures (DGSs) are considered a promising method to improve the isolation between adjacent or different ports. In this paper, four ports of an UWB-MIMO structure were fabricated on a low-cost FR-4 substrate with relative permittivity of \(\varepsilon_{r}=4.5\), a loss tangent of 0.025, and a height of \(h=1.5\) mm. The radiator element used in the proposed MIMO configuration operates from 2.03 GHz to 15.04 GHz with triple rejected bands at the WiMAX band (3.28-3.8 GHz), WLAN band (5.05-5.9 GHz), and X-band (7.78-8.51 GHz). In addition, it supports the Bluetooth band (2.4-2.483 GHz). The proposed radiator is available to operate in FCC-UWB networks. A coupled line with a stub-loaded resonator (SLR), etching two U-shaped slots on the radiator, and an octagon-shaped EBG structure are utilized to achieve the triple band notches at the WiMAX band, WLAN band, and X-band, respectively. The obtained notch bands have very high selectivity at the beginning and the end of the notch bands. The MIMO configuration circuit has one connected ground plane. High isolations were obtained between the four radiators by using the defective ground structure (DGS) technique. The article is arranged as follows. The design of a UWB antenna element is explained in Section 2. In Section 3, the design of the UWB-MIMO antenna is introduced. The results and discussion are given in Section 4. Finally, Section 5 provides the conclusions.