## 1 Introduction  
The ultra-wideband (UWB) protocol is a wireless communication system used for high transmission rates over an ultra-wide frequency band with low-power requirements [1; 2]. UWB technology has received significant attention since 2002, when the Federal Communications Commission (FCC) regulated its utilization for commercial, unlicensed applications over the frequency band of 3.1-10.6 GHz in the United States [3; 4]. This technology faces some challenges, such as multipath fading, channel capacity, and the bit error rate [5]. MIMO technology, which makes use of multiple antennas, provides a solution for the multipath effect and increasing the channel capacity without the need for an additional frequency spectrum or transmitted power [6]. However, many factors hinder the system performance, especially in compact and small devices, due to multiple constraints faced during MIMO-antenna design, such as the antenna dimensions and the induced mutual-coupling effect between the elements. Indeed, the main design challenges are the antenna size, inter-port isolation, impedance bandwidth, and diversity performance [7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Mutual coupling is the electromagnetic interaction between adjacent antenna elements in MIMO or array antennas. Strong mutual coupling causes poor port isolation, radiation pattern deformation, antenna efficiency and gain reductions, as well as high correlations between the elements [17]. Increasing the distance between the antenna elements in different ways is commonly used to decrease the mutual-coupling effect; however, the compact overall size of wireless devices limits this approach [18; 19; 20]. In [21], orthogonally placed MIMO elements were used to reduce the transfer of energy between the polarized elements without a size increase. The use of decoupling networks is one of several other techniques reported in the literature to enhance the mutual coupling between the radiating elements; indeed, a \(-28.24\) dB decoupling coefficient was achieved by introducing a decoupling network in the defected ground plane of a compact, four-port, wrench-shaped MIMO antenna in [22]. In [23], a coupling coefficient of \(-25\) dB was achieved using electromagnetic-band-gap (EBG) structures printed between the radiators of a two-port MIMO antenna. The effect of the number of EBG cells on the mutual-coupling reduction is analyzed in [24]. In [25], the decoupling increased from \(7.81\) dB to more than \(20\) dB over a large bandwidth via the introduction of the neutralization line without affecting the gain or radiation pattern; this technique was also used in [26; 27; 28]. L-shaped and F-shaped stubs were introduced in the defected ground to produce multiple resonances and enhance the isolation between the radiating elements in [29; 30], respectively. A reverse coupling was created by introducing parasitic elements in [31], resulting in mutual-coupling reduction. In [32; 33], the isolation enhancement was achieved by introducing slots and slotted strips into the defected ground plane. In [34], nine circular-shaped complementary splint-ring resonator (CSRR) slot structures were etched into the defected ground plane with the aims of reducing the mutual coupling and controlling the frequencies. In [35], CSRR structures were used as radiating elements and achieved a \(52\%\) size reduction compared to the conventional circular radiator, with a less than \(-30\) dB coupling coefficient over the entire frequency band. Further structures that make use of this technique are given in [36]. In [37], a band-stop filter-based isolation method was implemented to restrict the mutual coupling between the MIMO-antenna elements. The suggested decoupling approach produces isolations of more than \(30\) dB for two- and four-element antennas. In this paper, a two-element MIMO antenna with a compact size is proposed for UWB applications. The antenna consists of two pentagonal radiators with parasitic elements fed by \(50\)\(\Omega\) microstrip lines placed symmetrically on the substrate.