# A Flower-Shaped Miniaturized UWB-MIMO Antenna with High Isolation
## 1 Introduction  
Ultra-wideband (UWB) technology has been extensively applied in areas including short-range communications, radar, location, and tracking due to its extraordinarily low transmission power and high data speed [1]. The employment of multiple-input, multiple-output (MIMO) technology in a UWB wireless communication system enhances the signal-to-noise ratio and data capacity of the communication system by enabling multiplexing, hence improving the overall performance of the system [2]. The performance of a UWB MIMO system is influenced by factors such as bandwidth, isolation, and other functional characteristics. In recent years, researchers have used a variety of ways to improve the performance of UWB antennas for MIMO systems. In [3], an antenna designed on a \(34\times 34\times 1.6\) mm\({}^{3}\) FR4 substrate was proposed, with L-shaped and C-shaped slots embedded in each radiator and an electromagnetic bandgap (EBG) structure loaded near the microstrip feeding line, thereby allowing the antenna to exhibit triple-band slot characteristics, resulting in a bandwidth of 2.5-12 GHz. Here, in addition to the EBG structure being used to enhance the gain of the UWB antennas, the frequency selection surface (FSS) method will also have a significant gain-enhancing effect. Reference [4] presented a method for enhancing UWB antennas with FSS, which minimized power loss in the undesired transmission area of the antenna and blocked possible interference from undesirable and wasteful radiation, in order to obtain constant gain. Additionally, ref. [5] utilized FSS to separate and effectively isolate the antenna elements. A compact UWB antenna printed on a Rogers RO4003 substrate with a size of 30 \(\times\) 31 mm\({}^{2}\) for personal communication and with Bluetooth capability was proposed in [6], where the UWB characteristics were achieved by employing a conventional cylindrical radiating patch and an improved partial ground plane. Through the operation of a small resonator with capacitors, the antenna could also work in the Bluetooth band. In addition, a slit resonator was integrated in the radiating bulk to prevent interference in the WLAN band, resulting in a band-notched characteristic. In [7], sound isolation between antenna components was available throughout the UWB by adding a vertical stub and an H-slot in the ground plane. In [8], the combination of the ground stub on the bottom layer and the EBG structure between the two rectangular patches on the top layer led to a remarkably low mutual coupling between the two radiating patches. This antenna design possessed a compact size of 26 \(\times\) 31 mm\({}^{2}\) and displayed a frequency range of 3.1-11 GHz. To increase the VSWR bandwidth, a bending and defective ground plane for the basic radiator was proposed in [9]. A longer ground stub was also installed to increase the bandwidth to meet current automotive needs. However, it had a larger size of 42 \(\times\) 24 mm\({}^{2}\). Progressively, as described in [10], two homogeneous, semi-circular radiating elements with a synchronous stepped elliptical structure and an I-shaped ground structure were developed to generate strong isolation and a broad bandwidth between 1.99 GHz and 10.02 GHz. A fence-style structure and an L-shaped parasitic branch were placed on the ground to enhance the impedance bandwidth and isolation at low frequencies, as prescribed in the literature [11]. In [12], a flower-shaped radiator was utilized to boost the isolation of the MIMO elements, and the isolation was further improved by placing a swastika-shaped stub on the ground to achieve a return loss of S11 (\(<\)\(-\)10 dB) and isolation of S12 (\(<\)\(-\)18 dB) on an FR4 substrate of 40 \(\times\) 40 mm\({}^{2}\), capable of covering the whole UWB spectrum (3.1-14 GHz). Furthermore, the authors of [13] proposed a UWB-MIMO antenna with four suppression bands and a T-shaped stepped stub on the back ground for achieving 3-11 GHz impedance bandwidth and \(-\)15 dB isolation.