# Slotted Circular-Patch MIMO Antenna for 5G Applications at Sub-6 GHz
## 1 Introduction
This design achieves an efficiency rate of 70%. In ref. [17], a four-MIMO array design for 5G mobile systems measures 150 mm by 75 mm and operates across three bands in the sub-6 GHz range. This design achieves an efficiency rate of 60%. Furthermore, reference [18] introduces an innovative transmission-line-based decoupling method that significantly boosts isolation between co-polarized elements by 16-20 dB for MIMO antenna arrays operating at 2.4 GHz. As we move toward a fully integrated 5G infrastructure, including indoor access points, we must uphold 5G connection standards, particularly in indoor environments where signal levels are inadequate, as highlighted in reference [19]. Most contemporary solutions employ dual-polarized antennas, which offer orthogonal and low-correlated radiation patterns--critical characteristics for enhancing the performance of MIMO antennas. In ref. [20], the author has proposed a groundbreaking miniaturized MIMO design that operates at 3.2 GHz to 3.9 GHz, covering the WiMAX, LTE, Wi-Fi, and cognitive radio applications, and with overall dimensions of 60 \(\times\) 120 \(\times\) 1.56 mm\({}^{3}\), based on FR4 substrate. Its design cleverly positions the biasing circuitry and Microstrip feeding lines on the top layer, while the bottom layer--the reference ground plane--features expertly engineered pentagonal radiating slots. This closed slot-line design on the lower substrate effectively responds to reactive loading. The decision to employ a pentagonal slot line stems from its exceptional impedance matching, outstanding radiation qualities, optimal size, and ease of reactive loading. Moreover, integrating a defective ground structure (DGS) significantly elevates the isolation between adjacent antennas, ensuring superior performance and reliability in complex environments. A triple-port antenna designed with a monopole structure and two orthogonal dipoles is discussed in reference [21]. The increasing demand for MIMO capabilities is essential for enhancing data rates in 5G indoor base stations, necessitating antennas that provide high isolation and low ECC. Additionally, reference [22] presents a four-port wideband antenna operating in the 3.3 to 5 GHz range, while reference [23] introduces a four-port cavity-backed antenna for MIMO applications, evaluating the coupling between the ports. MIMO bandwidth ranges from 1.55 to 6 GHz, achieving gain ranging from 4 dBi to 10 dBi. A compact, two-port dual-circularly polarized MIMO antenna for WLAN applications is presented in ref. [24], and the gain is in good agreement with simulation results. Tapered-width stubs and different-width slot structures have been adopted to enhance the isolation and enlarge the impedance matching bandwidth, offering gain of 8 dBi over a narrow band of 2.4 to 2.5 GHz. In ref. [25], a concept of circular polarization agility is presented with two antennas operating in the sub-6 GHz band. Two cylindrical and ring DRA prototypes with dual-port MIMO on ROGER 5870 of octagonal shape operate at bandwidths of 3.15-3.9 GHz and 3.12-3.9 GHz, respectively, offering 7.3 dBi gain. The circular design in ref. [26] uses ROGER R04003C and features four ports. Both measurements and simulations show good agreement, with an impedance range from 1.27 GHz to 6 GHz, demonstrating high gain. A 4 \(\times\) 1 array of optimized leaf-shaped radiating elements is presented in ref. [27], with bandwidth variations ranging from 3.43-3.95 GHz dB and 4.61-4.99 GHz. Gain variations are around 6.56 dBi across the working bandwidth. Pattern diversity and SD-MIMO designs with good bandwidth, minimal mutual coupling, and acceptable MIMO parameters have been proposed. In ref. [28], four patches are elliptical. This design operates at three bands: 3.0-3.8 GHz, 3.8-5.0 GHz, and 5.0-5.5 GHz, with a maximum gain of 7.8 dB.