# Metamaterial Based Design of Compact UWB/MIMO Monopoles Antenna with Characteristic Mode Analysis
## 1 Introduction  
The multiple input multiple output system is all about the channel capacity, and it depends on bandwidth and signal to noise ratio. The low power requirements of the UWB system have earned significant attention in wireless high data rate [1]. The Federal communication commission has dispensed an unlicensed frequency spectrum of 3.1-10.6 GHz for ultra-wideband (UWB) frameworks [2]. One of the factors that affect the output of the UWB system is the limited channel capacity. To serve more users with better performance within the limited available frequency: bandwidth and transmission power are critical topics in the wireless communication industry [3]. To have a better performance, some have sought to allocate the operable spectrum dynamically, called spectrum collaboration [4], or increase the operable scope by operating at a millimeter frequency range proposed for a future fifth-generation (5G) [5]. Examples of these technologies are multiple-input-multiple-output (MIMO) methods [6; 7] and full-duplex radio [8]. MIMO system is a morepopular choice because multiplexing efficiency can be vastly raised without offering more frequency spectrum channel capacity [9]. MIMO techniques present a solution to overcome the channel capacity, antenna physical limits, and mutual coupling challenges [10]. Deployment of multiple antennas increases channel capacity and transmission range [11]. MIMO systems promise an increase in channel capacity without offering an additional spectrum of frequency and transmitted power. Many factors could undermine the system performance, especially in compact devices, because of many constraints imposed on MIMO antenna design due to some limitation in terms of area and electromagnetic interaction between the MIMO elements termed as mutual coupling. When several radiating pieces are situated close to each other, the fields generated by one antenna changes the current distribution on the different radiating elements. Therefore, each MIMO element's impedance matching and radiation pattern are assigned based on the presence of other components [12]. This electromagnetic interaction between the antenna elements subverts the performance of the MIMO system. All the radiating features must be in a single aperture, making it a single multiport antenna design problem. The traditional array design concept that a single antenna element can be designed and then assembled into an array topology does not apply to compact devices. Some researchers use a multiport decoupling or matching network to reduce the antennas' mutual coupling effect [13]. It is clearly shown that MIMO radiating elements should have more than one port, and it is desirable to have those ports being isolated and well-paired (for the fact that no matching network is connected). However, different applications will have other properties when it comes to design, like the base stations where the antenna's size is not a problem in terms of the space. This implementation will be more comfortable. But for a much spacelimited application, this is not practical. For reduction the electromagnetic interactions and ample space between the radiating elements, some researchers used various innovative techniques known as defected ground structure [14; 15; 16], parasitic elements [17], neutralization line [18], and electromagnetic bandgap (EBG) configurations [19]. The modal analysis of the MIMO antenna helps to see some good thoughts regarding antenna arrays. The MIMO antenna designers cannot avail from this understanding without seeing how the modes influence the antenna's performance parameters [20]. The use of modal analysis for antenna design and analysis was investigated in recent literature [21; 22]. All this research work has significantly advanced the capability and application of characteristic mode theory. The linearity and orthogonality of the characteristic mode are potentially useful for MIMO system design.