A Design of MIMO Prototype in C-Band Frequency for Future Wireless Communications-1

The main challenges of MIMO design for future wireless communication is the size reduction which leads to more mutual coupling. In this paper, a proposal has presented of MIMO with four antenna elements prototype each one having a dual polarized patch rectangular microstrips. To mitigate the impact of mutual coupling, a twin of annular parasitic of a rectangular shaped and opened rib has been placed in front of each microstrip and over a circular ring slot in the ground. The advantage of such ground slot is to obtain a regular radiation pattern distributed around device body. The four elements with eight ports have integrated on 67×139 mm2 PCB with FR-4 dielectric layer. The single antenna has simulated by using CST.STUDIO 2019 resulting an operated frequency of 6.23 GHz with a band of 850 MHz (5.81-6.66) GHz at -10 dB. The fabrication MIMO system prototype has tested to show its measurement results which are match the simulated results. All results of the MIMO prototype have operated at C-band frequency which is very important for future wireless applications.


Introduction
As it is known, the multiple-input-multiple-output (MIMO) antenna technology characterized with mitigation the multipath interference and capacity enhancement [1]. In the future pivotal wireless technology, the massive MIMO antenna (MA-MIMO) has realized fifth generation (5G) which usually ranged with minimum number started with 6 up to 100 elements or more in a single device [2]. In order to integrate a smartphone with desired quality of MIMO system, many challenges are existed [3]. Because of integrated multi elements in one space leads to more than one challenges like small size, mutual coupling and easy fabrication. To address such requirements, microstrip antenna design technique is specified with ease of fabrication, light weight and low profile [4]. Also, with exploiting the advantage of dual polarization ports of increasing the frequency spectrum can eliminate the huge size of single ports of MIMO antenna system [5]. On the other hand, the isolation can be achieved with adding a parasitic structure or modifying in the ground structure [6]. Furthermore, many researches proposed an antenna mobile operated with 5G applications in order to improve wave propagation [7][8][9][10][11]. However, some of researches deal without dual polarization which result in a huge size while others obtained narrow bandwidth, meanwhile, most of them not obtained a desired mutual coupling. In [12] propose a 5G smartphone applications with multi-ports but concerned with mutual coupling not qualified. In addition, [13] proposed MIMO antenna system with dual polarized ports which obtained a mutual coupling about -20dB and a band width of 600 MHz at -10dB and operated in sub-6 band which qualified for 5G smartphone applications. In this research, as a reflected to [13] antenna structure to improve the bandwidth and mutual coupling with lesser size less size as well as it operated in C-band frequency which is a promising band for 5G applications. Also, to ensure the reliability of the proposed model, a fabrication part will be provided and a comparison between results of simulation and measurement will be applied.

MIMO Antenna Design
The design is started with a proposed of a single antenna element having a size of 30×30 mm 2 as shown in Fig. 1. It comprises of three layers; the first one is the patch layer which contained a dual polarized feeding line of 50 Ω microstrip connected to power source through SMA connector. Such layer contains a twin of annular parasitic with a rectangular shape opened one of its ribs placed in front each microstrip. Placing such parasitic in this manner in order to improve the isolation between the neighboring ports. The second layer is representing the separation between patch and ground layers having a height of 1.6 mm and made of FR-4 with permittivity 4.4 and thermal conductivity of 0.025. The third layer acts as a ground layer included a circular ring in a bottom of the parasitic mentioned in the patch layer. Furthermore, all dimensions detailed in Table 1 which are marked in Fig.1.

Results and Discussions
The simulation part is applied using CST.STUDIO 2019 to obtain its performance in term of return losses Snm, mutual coupling Sjk, radiation pattern, mean effective gain (MEG), channel capacity loss (CCL), envelop correlation coefficient (ECC) and diversity gain (DG). Also, the fabrication part of MIMO antenna system has been integrated using Proto Mat S100 Laser and Electronics machine which is belong to Iraqi Ministry of Science and Technology / Electronic Manufacturing Department. In order to test the prototype of proposed MIMO antenna, has done in the laboratory of the same office mentioned above by using an Agilent N5247A vector network. The fabrication MIMO model is shown in Fig.3 Fig.4 which shows the operating bandwidth is 850MHz (5.81-6.66) GHz at -10 dB with resonance frequency of 6.23 GHz. Such simulation shows that the return losses which represented as S1,1 and S2,2 has obtained of -20 dB at such operating frequency. On the other hand, the isolation between the adjacent ports which marked as S1,2 and S2.1 has approached -45 dB.

The performance of MIMO antenna
In order to confirm the capability of the MIMO antenna proposal diversity, the following parameters have been calculated and analyzed:

Envelop correlation coefficient and diversity gain
The aim of calculating the ECC is to measure the correlation between antenna elements. In order to achieve high diversity between MIMO elements, ECC (ϼe) value must be < 0.5 which can be calculated using the radiation pattern of the far field as follow [14].
where Ƒ "⃗ ! ( , ∅) is the MIMO antenna's far field property after exciting all ports. Also, Fig. 7 shows the ECC analysis over frequency which appears less than 0.001.
On the other hand, the DG is another important parameter which must be > 9 dB and can be calculated related to ECC as [14].
Also. Fig.8 displays the analysis of DG which approach 10 dB for all ports.

Channel capacity loss
In order to evaluate the performance of MIMO antenna system, CCL must be taken in account. It is worth to mention, CCL neither increased by incrementing the bandwidth nor the transmitted power in the conventional system [15]. It just depends on the number of antenna elements with a specific assumption [15]. Also, CCL value must be < 0.4 bits/s/Hz and can be calculated as [16].

Mean effective gain
The antenna gain is measured statistically in the mobile environment by the MEG. It is defined as 'the ratio of the average power received at the antenna to the sum of the average power of the vertically and horizontally polarized waves receives by isotropic antenna " [17]. Fig. 10 shows the MEG which is less than -3 dB. Also, it can be calculated as [17].
where N is the number of antenna element. In order to show the proposed MIMO system performance, Table 3 shows abbreviate compression in term of bandwidth (BW), ECC, Snm Sjk between the currently proposal and other previous references. However, the proposed MIMO system operates in C-band which is the bandwidth of future communication applications.

Conclusion
In this research, a proposed MIMO prototype with four elements that each has a dual polarized port. A method of applying the parasitic rectangular is to investigate low mutual coupling. Also, a slot of circular ring has etched in the ground in reverse of the gabs made in such parasitic in order to achieve uniformly radiation pattern distribution. The proposed model has fabricated and tested to evaluate its performance. The results show that such design has operated at C-band frequency (5.81-6.66) GHz at -10 dB. It is worth to mention that we achieve a matching result in both simulation and measurements for in the visual and integrated of operating frequency.