_applied_  
_sciences_  
_Article_  
Design of an Integrated Sub-6 GHz and mmWave MIMO  
Antenna for 5G Handheld Devices  
[PERSON] 1, [PERSON] 2, [PERSON] 3, [PERSON] 4, [PERSON] 5, [PERSON] 6,* 6, [PERSON] 7, [PERSON] 8,* 9 and [PERSON] 9,* 10  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/article/pdf/10.10](https://www.mdpi.com/journals/article/pdf/10.10)]([https://www.mdpi.com/journals/article/pdf/10.10](https://www.mdpi.com/journals/article/pdf/10.10)). spectrum, which has the advantages of wider bandwidth capable of providing higher data rates in comparison to sub 6 GHz bands [3, 4]. The electromagnetic (EM) waves radiating at the mmWave spectrum may face various issues, including multipath fading and atmospheric absorption losses [5, 6, 7]. To deal with these wireless network limitations, the researcher in [8, 9, 10], suggested MIMO and antenna arrays to be one of the solutions which can overcome the limitations faced by mmWave band spectrums. The antenna arrays are suitable to achieve high gain, while MIMO technologies can help us achieve higher capacity with high data rates and multiple Gbps throughput [8, 9, 10]. However, the isolation between multiple antennas may affect the device performance; thus, to design MIMO antenna arrays with high isolation, high gain and wider bandwidth, researchers had to adopt various isolation reduction techniques [11, 12, 13]. On the other hand, handheld devices, including cellular phones, operate using various band spectrums; therefore, single or multiple antennas operating in the spectrums are required. The solution to design a single antenna for multiple bands could not be an optimal solution to handle this problem, due to the different requirements associated with different bands [14]. For an instance, the mid-band for 5G and globally allocated band spectrum for 4G require a compact antenna with wideband characteristics. Moreover, low gain characteristics are not an issue for this band, due to lower power losses in wave propagation. Contrary to this, the mmWave band requires a wider operational bandwidth and high gain with a broadside radiation pattern [15]. These requirements lead to the designing of integrated antennas on the same board for cellular systems. Recently, an enormous amount of work is dedicated to the design of integrated antennas [16, 17, 18, 19, 20, 21, 22, 23]. An integrated 4G/5G MIMO antenna system having a two-elements 4G MIMO antenna and an eight-element array antenna for 5G applications is proposed in [16]. The proposed work covers globally allocated Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) band for 4G and the sub-6 GHz band for 5G applications. A similar work, containing a 12-element MIMO antenna for 4G and 5G bands is reported in [17]. The designs reported in [16, 17] have lower gain and radiation efficiency and the designs support only the lower frequency bands. A multilayered 4G and 5G integrated antenna are proposed in [18]. The antenna offers a peak gain of 4 and 8 dBi in the 4G and 5G bands, respectively. However, no solution for 5G MIMO applications is proposed. Another interesting work consisting of printed F type MIMO antenna for 4G as well as 5G application is proposed in [19]. The paper lacks in reporting MIMO performance analysis for the 5G band and the proposed 4G antenna also has low gain. A conformal integrated antenna for smartphone applications is proposed in [20]. Researchers achieved a high gain of 9 dBi at 5G band using the metamaterial loading technique, while still having a lower gain of 2.15 dBi at a lower frequency band. Moreover, the work proposed in [21] offers good performance parameters including high gain and wide bandwidth at both targeted bands of 4G and 5G. MIMO configuration for the 4G and 5G designs is not being investigated.