# Quad Element MIMO Antenna for C, X, Ku, and Ka-Band Applications
## 1 Introduction  
Multiple input multiple output (MIMO) antenna technology is an indispensable option for wireless transmissions to meet more data needs and increase spectral efficiency. MIMO antenna configurations for 4G will also be an important system for enabling 5G [1]. This 5 G communication must promote two frequency bands: the sub-6 GHz band and the mm-wave band. Focused fast transmission, shorter latency, and increased bit rate are achievable in mm-wave bands due to spectrum commitment. However, mm-wave communication experiences congestion and loss. Therefore, there should be diversified integrated antenna architectures that can provide a good frequency band [2]. Implementation of these systems seems difficult due to antenna dimensions and less separation of antennas that generally operate in the microwave band [3]. The proposed MIMO design in [4] has four antennas out of which two elements are operating at 3.46 GHz and the other two at 17 GHz. In this design, no elements are operating in the mm-wave 5G band. In [5], a quad-element inverted F-shaped antenna and a planar-connected array with a dimension of 100 mm \(\times\) 60 mm \(\times\) 0.76 mm are presented. This design has been fabricated on an RO-4350 substrate, and the proposed structure has an operating frequency of 2.1 GHz and 12.5 GHz. In [6,7], multi-layer MIMO antenna systems have been introduced with three elements and five elements. The design covers the mm-wave 5G band, but both fail to cover 5G sub-6 GHz and WLAN bands. The literature [8] consists of an eight-element MIMO antenna system in which four elements operate in the 2.4-5.6 GHz band and the other four in the 23-30 GHz frequency band. In this design, the detected ground structure is implemented to lessen the mutual coupling among elements. In [9], an eight-element integrated MIMO antenna for sub-6 GHz 5G band and mm-wave band has been proposed. Out of the eight elements, four elements operate in the sub-6 GHz 5G band (i.e., 2.38-4.13 GHz and 5.04-6.12 GHz), and the other four elements operate in the mm-wave band (i.e., 22.28-29.28 GHz). The antenna design presented in [10, 11, 12] works in both the sub-6 GHz 5G band and mm-wave band; however, they do not cover the WLAN/Wi-Fi band. A miniaturized four-port MIMO antenna system has been developed in [13] for sub-6 GHz 5G band and WLAN applications. The introduced design has been operating at frequencies 2.6, 3.5, 4.8, and 5.8 GHz. A dual-band dual-port high isolation trident-shaped MIMO antenna system with an operating frequency band from 2.99 to 3.61 GHz and 4.53 to 4.92 GHz has been proposed in [14], which is operating in the sub-6 GHz 5G band only and doesn't cover the mm-wave band. The literature reported in [15,16] operated a dual frequency band under the sub-6 GHz band and the operating frequency was 3.4-3.6, 4.76-5.04, and 3.95-4.04 GHz, respectively. On the other end, the literature reported in [17, 18, 19] only covers the mm-wave 5G band, which is a bandwidth of 1.23 GHz at 28 GHz and 1.06 GHz at 38 GHz, 26-30 GHz, 36-41.5 GHz, 26.65-29.2 GHz, and 36.95-39.05 GHz, respectively. Additionally, the literature [20] consists of a flower-shaped radiating patch where two rectangular-shaped slots in the ground plane provide a wide band of 2.83-14.0 GHz. In [21], a lotus-shaped radiating element with DCS in-ground and the addition of slits provides a 5G: n7/n78 wireless band with higher isolation (i.e., >16 dB). This article consists of three sections. The first section covers the details of antenna geometry. The second section covers the detailed analysis of different diversity characteristics parameters like ECC, DG, \(\left|MEG_{i}-MEG_{j}\right|\), TARC, CCL, and CC. The operating bands investigated by this structure are 3-15.5 GHz, 26.4-34.3 GHz, and 36.1-48.9 GHz, which are referred to as OFB1, OFB2, and OFB3, respectively. The minimum port isolation and ECC over the bands OFB1, OFB2, and OFB3 are observed as (15.1 dB, 0.11), (22 dB, 0.035), and (22.37 dB, 0.04), respectively.