# Highly Compact 4 \(\times\) 4 Flower-Shaped MIMO Antenna for Wideband Communications
## 1 Introduction  
The evolution of wireless technologies aims for high data rates with improved channel capacities; a communication system with a frequency range exceeding 500 MHz is known as ultra-wideband (UWB) technology [1]. The UWB spectrum has unlicensed bandwidth of 7.5 GHz from 3.1 to 10.6 GHz, which is offered by the federal communication commission (FCC) [2]. One of the main problems in UWB communications is the multipath effect which degrades the overall performance of the system, and this effect can be mitigated by using the multi-input multi-output (MIMO) technology [3, 4]. Moreover, by designing the MIMO antenna, the needs of UWB technology can be met since the MIMO technology has great advantages, which in turn will be reflected positively on the UWB system. Antenna radiator isolation is considered one of the main key factors that affect the diversity characteristics of MIMO antenna. The isolation should be as large as possible to reduce the capacity losses of the channel and to improve its diversity gain. In order to increase the isolation, the radiators should be well separated. Nevertheless, the electrical distance between antenna radiators is decreased by increasing the number of these radiators in size-constrained devices such as mobile terminals, which in turn reduces the isolation between radiators [5]. Consequently, the small isolation between radiators causes a reduction in antenna efficiency and throughput of the MIMO system due to the correlation between the different channels. In order to increase the isolation between the radiators in the MIMO system, symmetrical decoupling/isolation structures are used without affecting compactness [6; 7]. The decoupling structures reflect and/or absorb radiation from the radiators and prevent interference with the radiations [1; 6]. Various MIMO antennas are reported in many works of literature, which are aimed to achieve compact size with improved impedance and diversity performances using various techniques to achieve higher isolation between antenna radiators [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. No isolation structures are implemented in the reported antenna in [8], and this, in turn, resulted in poor isolation between the four circular elements. In [9], eight elements slot antenna based on a stepped impedance resonator (SIR) is designed and fabricated for dual-band operation. The antenna is printed on an FR4 substrate without any isolation structures. The experimental and simulation results are in good agreement, but the isolation between antenna elements is not high enough, although it occupies a larger size. A four-port MIMO with a wide axial ratio is presented in [10] to achieve a frequency band from 5.37 GHz to 11 GHz with circularly polarized radiation. Although, a much smaller size is needed since the antenna occupies an area of 50 \(\times\) 50 mm\({}^{2}\). The proposed UWB MIMO antenna in [11] succeeded in achieving the desired impedance characteristics, but it did not succeed in achieving the required compact size and the lower values of _ECC_. A multiband MIMO antenna is introduced in [12] to be operated at the lower and the upper bands of WLAN, but the isolation is poor in both bands as well as the _ECC_ value is significant in the upper band. The reported antenna in [13] failed to obtain the reduced size where it has an occupied area of 40 \(\times\) 80 mm\({}^{2}\) for two radiators only. In [14], the designed antenna is utilized for WLAN and WiMAX applications using 4-port MIMO slot antennas operating at 2.46 GHz and 3.5 GHz with a larger size of 60 \(\times\) 60 mm\({}^{2}\) and an improved diversity characteristic. The radiators are distributed in an orthogonal manner on both sides of the substrate in order to enhance the isolation between radiators reaching values above 24 dB and 26 dB in both bands. The authors in [15] designed and fabricated an ACS-fed 4-port MIMO antenna for UWB applications operating from 2.7 GHz to 11 GHz with band stop behavior at the WiMAX frequency band.