# A Sub-6 GHz MIMO Antenna Array for 5G Wireless Terminals
## 1 Introduction  
The day-by-day increase in the demand for higher data rates and bandwidth has created several issues for the current fourth-generation wireless communication infrastructure [1], as the current wireless communication infrastructure lacks the potential to deliver a higher data rate with a low latency [2]. Thus, the fifth generation has been introduced, which is promising to fulfil the requirement for higher data rates with the desired low latency level [3].  
At the moment, most of the designs reported are single element, which possess quite low gain as compared to the desired level (\(>\)12 dB) required for 5G communication [4, 5, 6, 7, 8]. Likewise, several multiple-port antennas have been presented to improve data rates, but they could not achieve satisfactory gain [9, 10, 11, 12, 13]. In the future, these Sub-6 GHz 5G antennas will be integrated with their predecessors and some other higher-band application antennas on the same printed circuit board (PCB). Thus, the high-level of gain can be quite helpful to efficiently deliver a stronger signal at the user side, instead of congesting the lower frequencies of the spectrum [14]. Along with this, the multiple input multiple output (MIMO) configuration is quite helpful to provide a good channel capacity and data rate as compared to the single port antennas, which is one of the basic challenges in 5G transmission.  
Recently, some MIMO antennas have been reported for operation in the Sub-6 GHz band [15, 16, 17, 18, 19, 20]. Different configurations have been adopted--such as monopole, open-slot,or loop antenna design method to achieve operation in the desired frequency band. As in [15], the size of the proposed design noted is 130 mm \(\times\) 100 mm with an operating band of 5.15-5.925 GHz. The gain obtained ranges from 2.5-4.2 dB within the operating band. Similarly, another antenna with an overall size of 150 mm \(\times\) 75 mm and operational bandwidth of 5.15-5.85 GHz is presented in [16]. The peak gain noted is 4.62 dB. Moreover, an antenna with a size of 136 mm \(\times\) 68 mm is presented in [17]. The proposed design adopted a monopole configuration in order to operate in the band of 5.15-5.925 GHz, while the gain achieved is not discussed. Apart from this, the main drawback observed is a low efficiency in [18; 19; 20], while in [21; 22; 23], the efficiency attained is even below 50%. In [24], the efficiency obtained ranges between 50-82%, while the antenna element is extended to the multi-port configuration over the same PCB with an overall size of 150 mm \(\times\) 75 mm. However, the minimum isolation level observed is \(-\)12 dB, which can be a great cause of signal deterioration on the user side.  
In this paper, we have presented a microstrip circular patch antenna array designed at the 5.5 GHz band with four elements in the array structure using microstrip methods, and then its MIMO configuration for 5G communication applications. The transmission line and quarter wavelength transformation techniques are used for the impedance and phase matching. Moreover, the SAR analysis is also presented for the proposed MIMO configuration. The single element, four-element array, and MIMO antennas are analyzed using CST Microwave Studio. The remainder of the paper is organized as follows: Section 2 presents the design methodology of the proposed antenna. The results and discussion have been presented in Section 3, and Section 4 discusses the SAR analysis. Section 5 concludes the paper and gives recommendations for the future.