# A Wideband Eight-Port MIMO Antenna with Reduced Mutual Coupling for Future 5G mm-Wave Applications
## 1 Introduction  
The growing customer demand for higher-speed mobile internet data poses a challenge for network providers. In today's digital landscape, where many applications rely on cloud or online services, there is a pressing need for uninterrupted, low-latency data connectivity, a demand that 5G technology aims to fulfill [1, 2]. Presently, numerous countries worldwide are in the process of implementing 5G technology to cater to the needs of their customers. The International Telecommunication Union (ITU) estimated that by 2021, approximately 4.9 billion people worldwide, constituting roughly 60% of the global population, would be using the internet. However, as of 31 December 2021, the number of internet users globally reached 5.2 billion. Currently, 4G LTE (Long-Term Evolution) is facing bandwidth saturation levels owing to its limited capacity and growing number of users. The importance of advancing antenna technology for 5G's mm-wave technology has become extremely critical in the recent past. To address the challenges posed by atmospheric and propagation losses, 5G mm-wave systems require the use of wide-band antennas with high gain. There have been numerous study initiatives to improve patch antenna gain and bandwidth designed for mm-wave applications [3, 4, 5, 6, 7]. Employing mm-wave frequencies in evolving 5G network systems has immense potential to significantly increase channel capacity compared to existing wireless networks, especially when combined with MIMO technology. Hence, the interest in mm-wave MIMO antennas is raising user demands for high throughput, requiring close spacing of antenna elements for miniaturization. Nonetheless, designing mm-wave antennas with MIMO applications poses a challenge for antenna engineers due to the unneeded mutual coupling betweenantenna's components. To tackle this challenge, various techniques in recent years have been extensively described to reduce mutual coupling between ports [8; 9; 10; 11; 12]. Recently, a plethora of MIMO antenna designs have been provided in the literature to achieve different frequency bands, including WiMAX, WiFi, WLAN, Ku band, K band, X band, UWB and mm-wave applications [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. In article [18], an eight-port antenna with UWB application is introduced. Two C- and T-shaped slots in a 5G metal frame are used to optimize the bandwidth from 3.3-6 GHz with 18 dB enhanced isolation. The 150 \(\times\) 75 \(\times\) 7 mm\({}^{3}\)-size antenna achieves a total efficiency of 40-90% with envelope correlation coefficient (ECC) \(\leq\) 0.05. A four-port PIFA MIMO antenna with vast bandwidth is presented to cover 5G NR bands in 5G smartphones [19]. The antenna pair is 30 \(\times\) 7 mm\({}^{2}\) in size, achieving a 3.3-7.5 GHz bandwidth. The PIFA MIMO antenna achieves > 10 dB isolation with ECC \(\leq\) 0.05. A four-element, eight-port diversity antenna is presented for 5G handheld cellular IoT applications [20]. The dimension of the proposed antenna is taken as a modern mobile set. The radiating elements are placed at the corners of two feeding ports each. The antenna works at a bandwidth of from 2.4 to 3.8 GHz, with isolation \(\leq\) 13 dB. The secured ECC is <0.03, with 3.2-5 dBi peak gain. A compact 3D eight-port antenna array is examined in [21]. The array consists of eight radiating elements, in which four radiating elements are configured in planar configuration, and the other four are placed perpendicular to it. The antenna obtains an enhanced isolation of 15 dB from 3 to 11 GHz bandwidth. The designed 50 \(\times\) 50 \(\times\) 25 mm\({}^{3}\) dimension antenna is printed on Roger TMM4 laminate. An eight-element, fifth-generation antenna is designed for 5G smartphone applications in [22]. The 150 \(\times\) 75 \(\times\) 7.8 mm\({}^{3}\)-size proposed antenna achieves 3.2-6 GHz frequency with an isolation of \(\leq\)12.6 dB, an efficiency of 38-83% and ECC \(\leq\) 0.31.