# Slotted E-Shaped Meta-Material Decoupling Slab for Densely Packed MIMO Antenna Arrays
## 1 Introduction  
A compact printed Multiple Input Multiple Output (MIMO) antenna is used in wireless communication and radar application, especially in the ultra-wideband waveform transmission application. It has the advantage of having multiple transmission and reception channels that magnifies the need of it [1]. The MIMO antenna may be a single band MIMO antenna such as a USB dongle MIMO and mobile handset antenna system. In addition, it can be a multi-band MIMO such as multi-band mobile handset and tablet PC MIMO antenna system [2]. The advantages of using a MIMO antenna are increasing the channel capacity, improving the spectrum efficiency, and gaining a more reliable network [3]. The MIMO antenna can be easily fabricated and integrated with small communication systems such as cell phones. For example, an eight MIMO antenna operating at a frequency band from 2.6 to 3.5 GHz was fabricated for 5G mobile applications [4]. One of the methods that increases the channel capacity in the 5G mobile systems is to use a massive MIMO antenna. The large number of the receiving and transmitting elements increases the spectral efficiency and reduces the inter-cell interference [2]. Although the MIMO antenna design and implementation are simpler than the array of antenna fabrication, the cost of simplicity is paid in multipath propagation problemsand the mutual coupling degradation [5]. Mutual coupling is the effect of not isolating the transmitting or receiving elements from each other. It degrades the angle of arrival estimation and increases the signal to interference noise, which has an adverse effect on the channel capacity and bit error rate in digital transmission and reception systems [5]. Mutual coupling happens due to interaction between the system elements or different radiation in free space on surface tracks. Therefore, the problem of massive MIMO antenna fabrication with low mutual coupling is very important and shall be considered [6]. The mutual coupling in a MIMO antenna can be reduced by minimizing the surface current flow using decoupling networks, etched parasitic elements, split rings resonators, electromagnetic bandgap structures, and defected ground structures [5]. Another method to reduce the mutual coupling is using dielectric resonator antenna or meta-materials [7]. A brief study on the mutual coupling effect on MIMO antenna is in [5; 8; 9]. The metamaterial decoupling slab (MTM-DS) was used between antenna elements to reduce the effect of the mutual coupling. In 9-11 GHz frequency band, the average isolation between elements improved 11 dB with an average gain of around 5 dBi [10]. MTM-DS beat the other decoupling methods in enhancing the undesired front-to-back beam ratio and it is simple to be implemented [11]. In this article, a slotted meta-material decoupling slab (S-MTM-DS) with dual reflexes slotted E-shapes extended with an inductive stub is proposed. Its function was validated when located between two microstrip patch antenna elements to reduce the inter-element spacing, the mutual coupling, the return losses, and manufacturing costs due to size reduction. A prototype is simply fabricated in a volume of FR4 substrate material and the frequency-span is measured in the band 8.4:11 GHz. At 10 GHz frequency, the spaces between each transmitting elements are decreased to 0.57 0. When the proposed isolation S-MM-DS is applied, the average isolation among them is measured to be \(-\)36 dB, the operational bandwidth is enhanced to 1.512 GHz, the fractional bandwidth improved to 16.04%, and the return losses are decreased to \(-\)26.5 dB at a center frequency of 9.4 GHz. The design is simple and evacuates the disadvantage of inferior front-to-back proportion, which is recently announced in other decoupling methods. Therefore, this design is suitable for multiple applications that require stringent execution necessities. The paper is organized in the following manner: Section 2 is the methodology of design and the measurements analysis. Section 3 presents the fabrication and validation measurements.