###### Abstract  
In this article, the compact, ultra-wideband and high-gain MIMO antenna is presented for future 5G devices operating over 28 GHz and 38 GHz. The presented antenna is designed over substrate material Roger RT/Duroid 6002 with a thickness of 1.52 mm. The suggested design has dimensions of 15 mm \(\times\) 10 mm and consists of stubs with loaded rectangular patch. The various stubs are loaded to antenna to improve impedance bandwidth and obtain ultra-wideband. The resultant antenna operates over a broadband of 26.5-43.7 GHz, with a peak value of gain \(\sim\)8 dBi. A four-port MIMO configuration is achieved to present the proposed antenna for future high data rate devices. The MIMO antenna offers isolation \(<\)-30 dB with ECC of \(<\)0.0001. The antenna offers good results in terms of gain, radiation efficiency, envelop correlation coefficient (ECC), mean effective gain (MEG), diversity gain (DG), channel capacity loss (CCL), and isolation. The antenna hardware prototype is fabricated to validate the performance of the suggested design of the antenna achieved from software tools, and good correlation between measured and simulated results is observed. Moreover, the proposed work performance is also differentiated with literature work, which verifies that the suggested work is a potential applicant for future 5G compact devices operating over wideband and high gain. widespread antenna; compact size; high gain; Ka-band; 5G +
Footnote †: journal: Applied  
good services. These demands need an antenna to operate over wideband and offer high gain along with radiation efficiency [7, 8]. In the case of the MIMO antenna system, the MIMO parameters are also important to be considered. The mutual coupling, envelop coding coefficient (ECC), and diversity gain (DG) and many others are key parameters, which will be analyzed [9, 10]. In literature, the researcher has present a number of monopole antennas operating over 28 GHz [11, 12, 13, 14, 15, 16, 17, 18, 19], and various MIMO systems operational over 28 GHz for wireless communication systems [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. The author of the selected literature reported in [11, 12, 13, 14] presents the antenna with compact size and simplified geometry. The setback of these designs is that the report work has a lack of measured results or low gain or radiation efficiency. The antenna reported in [15, 16, 17, 18, 19], offers high gain and has simplified geometry. The demerit of these works is narrow operational bandwidth or large size. The requirement of 5G devices is the antenna having a compact size, low profile, and simplified geometry along with wideband and high gain. According to the requirements of the 5G communication system, the MIMO antenna, which offers a high number of gains, is reported in literature. In [19], the MIMO antenna with improved isolation is proposed by inserting electromagnetic band gap (EBG) between MIMO elements. The reported four-port MIMO antenna operates at 28/38 GHz with a bandwidth of 4 GHz and 2 GHz. Although the antenna is operational over high gain and broadband, it has complex geometry due to inserting EBG cells. A two-port MIMO antenna with simple geometry is reported in [21]. The geometry consists of defected ground structure (DGS) to improve the performance. This work has the demerit of a narrow bandwidth of 1.1 GHz. Another work that offers high gain and wideband is reported in [22]. The antenna consists of eight ports, which lead the reported work with large dimensions. Compact and wideband antenna operating on mm wave application is reported in [23]. The antenna has a compact size of 24 mm \(\times\) 20 mm \(\times\) 1.85 mm and operates over a wideband of 33-44 GHz, but a setback of low value of ECC (that is 0.1). A wideband and high-gain antenna with a bandwidth of 22-50 and peak gain of 15 dBi is reported in [24]. The antenna has complex geometry, as array is adopted to achieve the high gain. In [25], an antenna is presented for the 5G communication system, which offers a bandwidth of 27.1-28.1.