# Slotted Circular-Patch MIMO Antenna for 5G Applications at Sub-6 GHz
## 1 Introduction  
In the past decade, advanced wireless systems have revolutionized our daily lives, making technology an integral part of connecting and communicating [1]. This astounding evolution has enabled us to enjoy higher data rates, allowing seamless streaming, gaming, and browsing experiences. However, maximizing bandwidth is crucial for optimizing transmission rates [2]. The rapid growth in demand for internet speed and mobile data presents critical challenges for modern wireless systems [3]. Fifth-generation (5G) technology is designed to meet these demands, allowing countless users to connect simultaneously, significantly expanding network capacity, and delivering breathtaking data rates in gigabits per second (Gbps). Furthermore, 5G reduces latency to milliseconds and offers cost-efficient solutions for service providers [4]. To facilitate this revolutionary communication standard, the International Telecommunication Union (ITU) has two identified essential microwave bands, the sub-6 GHz band, and millimeter-wave (mm-wave) [5]. Fading and low signal-to-noise ratios remain hurdles for mm-wave communication. Key sub-6 GHz bands for 5G encompass N77 at 3.7 GHz, N78 at 3.5 GHz, and N79 at 4.7 GHz [6]. Sub-6 GHz operates across three distinct frequency bands: the Low Band (sub-1 GHz) for extensive coverage, the High Band, featuring cutting-edge millimeter-wave (mm-wave) technology that delivers exceptional capacity and the Mid Band (sub-6 GHz), which strikes an optimal balance between coverage and capacity; and the sub-6 GHz band is particularly powerful, as it effectively confronts the challenges of both capacity and coverage [7; 8; 9]. This makes it not just an option, but a cornerstone for the future of connectivity, ensuring that we stay connected in an increasingly digital world [10]. Fifth-generation technology relies heavily on MIMO (multiple-input multiple-output) and array antennas to enhance coverage and increase data transmission rates, facilitating beamforming and allowing a single signal to reach its destination through multiple paths, ensuring strong and reliable connectivity for users. MIMO antennas, which utilize multiple transmitting and receiving antennas, are becoming increasingly popular in WLAN applications [11]. They significantly enhance channel capacity and data rates without demanding additional spectrum or power, even in complex multipath fading environments. Therefore, implementing MIMO antennas in limited spaces is a superior solution for boosting transmission capacity and overcoming the challenges posed by multipath fading, ultimately paving the way for a more connected and efficient future [12]. In ref. [13], an antenna array consisting of eight elements with a total size of \(136\times 68\) mm\({}^{2}\), based on an inverted-F antenna, is identified as suitable for 5G mobile applications. The array coverage for the bands n77, n78, and n79 has an ECC of less than 0.1. In ref. [14], the antenna design is integrated with cognitive radio, benefiting from the orthogonal arrangement of its elements, which significantly enhances port isolation. While the use of reflectors also improves isolation, it does come with the trade-off of increasing the overall size of the antenna system. In ref. [15], a compact C-shaped antenna measuring 20 mm \(\times\) 15 mm has been designed for WLAN and 5G applications. Although its small size is advantageous for 5G, its scattering characteristics measured at \(-6\) dB fall short of industry standards, especially with evolving regulations such as IEEE 802.11 ax. The insufficient gain of this antenna renders it unsuitable for 5G wireless requirements. Therefore, it is essential to prioritize the design of antennas that achieve a sufficient gain to meet the demands of next-generation communication systems. In ref. [16], the eight-element antenna array, measuring 136 \(\times\) 68 mm\({}^{2}\) and based on an inverted-F design, issuitable for 5G mobile applications. The array covers the n77, n78, and n79 bands, with an ECC of less than 0.1.