TELKOMNIKA Telecommunication Computing Electronics and Control

Karrar Shakir Muttair, Oras Ahmed Shareef, Ahmed Mohammed Ahmed Sabaawi, Mahmood Farhan Mosleh Department of Computer Techniques Engineering, College of Technical Engineering, The Islamic University, Najaf, Iraq Department of Computer Engineering Techniques, Electrical Engineering Technical College, Middle Technical University, Baghdad, Iraq Department of Electronics Engineering, College of Electronics Engineering, Ninevah University, Mosul, Iraq


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Design of multiple-input and multiple-output antenna for … (Karrar Shakir Muttair) 35 developing many antennas with small size and low coupling specifications being used widely in telecommunication long term evolution (LTE) and digital tone code squelch (DTCS) [7]. For example, a multilayer millimeter-waves (mm-Waves) antenna with MIMO technology has been recently proposed in [8], operating at 28 GHz. The investigation of this design showed high efficiency and gain that meet the requirements of 5G applications. Another MIMO antenna design [9] has produced a dual-band notch as a U-shape. The proposed antenna operates within a frequency range of (3.0 to 11 GHz). This range of frequencies is used in UWB applications, where isolation of high data rates is one of the demands.
Moreover, in [10] there is more focus to adjust the dimensions of the proposed MIMO antenna where band-notched fits the 5G smart mobile applications. Even though, in [11], [12] a high isolation MIMO antenna has been proposed with a specific sharp notched band that is utilized in UWB filtering applications. In this type, a MIMO antenna with two-element is used to enhance the isolation feature and to create the notch in C-band (3.62 to 4.77 GHz). In contrast, in [13] 5G antenna has been proposed as dual-band MIMO, which is designed with four antennas operating at (3300 to 3600 MHz) and (4800 to 5000 MHz). The investigation has shown achieved isolation that is (<12 dB) for smart-phone communications. Finally, in [14] an antenna for UWB-MIMO has been suggested, where an asymmetric layout with including an integration technology for wireless communication applications. The notched bands of that design are (3.25 to 3.75 GHz), (5.08 to 5.90 GHz), and (7.06 to 7.95 GHz), aiming to achieve multiple band-notched characteristics for wider impedance bandwidth.
In this paper, a compact single element and MIMO antennas are designed for modern wireless communications. The proposed design in this work covers the frequency range of (3.5 to 10 GHz). Simulations were performed by computer simulation technology (CST) software. The rest of the paper is organized as follows: section 2 presents the proposed design of single and MIMO antennas. The simulation results of a single antenna have been presented in section 3. While the simulation results of a MIMO antenna with comparing the proposed antenna in this work with other antennas in literature have been presented in section 4. Finally, the conclusion is summarized in section 5 with proposing suggestions for future works. Figure 1(a) shows the geometry and its related dimensions for the proposed antenna. The proposed antenna is printed on a relatively thin FR-4 substrate (22×24 mm 2 ) with a dielectric constant of 4.3. The ground plane covers the entire backside of the designed structure. In contrast, the back view is shown in Figure 1(b). The substrate top patch has a size of 20×9 mm 2 and is fed by a 2 mm wide strip line. A substratum bottom patch is merely a ground plane.

MIMO antenna
The MIMO antenna is designed using a Sierpinski-based fractal geometry with a partial rectangular ground plane. The self-similarity characteristics of the fractal geometry are employed in this work to achieve wide bandwidth microstrip patch antennas (MPA). The MIMO antenna is designed on the FR4 substrate with a small size of (22 × 56 mm 2 ) as shown in Figure 2 and the ground plane length (9 mm) and the feed line width (2 mm). It is suitable for various applications within the S-band and the lower part of the C-band such as wireless LAN, radar systems, mobile handsets, Bluetooth, global positioning system (GPS), and microwave devices. The resulting wideband response is reached to be within (1 to 10 GHz) frequency range.

Reflection coefficient (S11)
The reflection coefficient curve of the proposed single antenna is shown in Figure 4. It exhibits an acceptable resonance within the frequencies range of 3.3 to 9.5 GHz, indicating two resonances at 3.9 GHz and 7.5 GHz with reflection coefficient values are -65.994 dB and -46.441 dB respectively. In addition, we have noticed that the antenna operates at a wide bandwidth from 3.3 to 9.5 GHz, and this gives importance to the use of this antenna in various advanced communications applications that depend on broadband.

Voltage standing wave ratio (VSWR)
The VSWR curve of the proposed single antenna is shown in Figure 5. It has an acceptable value over the range of frequencies between 3.9 GHz and 7.5 GHz. Therefore, we noticed that the value of the VSWR is less than 2 at the frequencies from 3.3 to 9.5 GHz, and this indicates that the antenna gives a good and stable performance.

Input impedance (Z11)
The impedance (Z11) of the proposed single antenna is shown in Figure 6. It indicates a good impedance matching at frequencies range (3.3 to 9.5 GHz) because the input impedance of the antenna will be close to the characteristic impedance of the feed line that equals 80 Ω. In addition, we noticed that the lower the antenna's impedance value, the antenna performance will gradually decrease, as we noticed at the frequencies from 1.21 to 3.3 GHz.

Gain versus frequency
The gain over the range of frequencies (1.5 to 7.5 GHz) is shown in Figure 7. It is clearly seen that the gain is rapidly increasing from 1.5 to 7.5 GHz then gradually increasing to reach its maximum value of 1.8 dB at the frequency of 7.3 GHz. While the minimum gain value is 0.25 dB at the frequency of 1.45 GHz. The S11 and S22 curves of the proposed two-port MIMO antenna are shown in Figure 8. We can note that for the frequencies are 3.3 GHz and 9.5 GHz, the value of S11<-10 dB for the same frequencies.
While the value of S22<-10 dB too for both ports meet the return loss requirement.

Mutual coupling
The plot in Figure 9 shows the S12 and S21 curves for the proposed two-port MIMO systems. It is clearly noted that both S12 and S21<-14 dB for 3.9 GHz and both S12 and S21<-21 dB for the 7.5 GHz. So, the two ports are almost independent of each other and the mutual coupling value between the two antennas is very low, which is preferred in most modern applications. Figure 9. The mutual coupling (S12 and S21) curves for the two-port MIMO systems

VSWR
In Figure 10 the plot displays the corresponding VSWR for the two antennas in the composition of the proposed MIMO. It is obvious that VSWR1=1.07 and VSWR1=1.03 for the 3.9 GHz and 7.5 GHz frequencies and VSWR2=1.08 and VSWR2=1.03 for the 3.9 GHz and 7.5 GHz frequencies, which are less than 2 indicating an improved matching condition. Therefore, since the values of the parameter VSWR are much less than the value of the normal orientation which is 2, the antenna gave a good and independent performance between the ports in the proposed MIMO configuration.

Input impedance (Z11, Z12, Z21, and Z22)
The impedances (Z11, Z12, Z21, and Z22) of the proposed MIMO antenna are shown in Figure 11. It indicates a good impedance matching at frequency range (3.3 to 9.5 GHz) because the input impedance for the antenna will be close to the characteristic impedance of the feed line that equals 75 Ω. Also, we noticed that the impedances (Z12 and Z21) between the two ports are very low and this indicates that there are no influences between the ports so that each port is independent in performance over the other.

Gain versus frequency
The gain values in dB of the proposed MIMO antenna are shown in Figure 12. The gain within the range of frequencies (1 to 10 GHz), it is shown that the gain jumps rapidly from 1 to 10 GHz then gradually increases to a maximum value of more than 2.6 dB at the higher frequency of 10 GHz. Therefore, it is clear that the antenna gives various values of gain at all frequencies, and this gives preference to the antenna for uses in various applications of modern wireless systems.

Envelope correlation coefficient (ECC)
The coefficient of correlation and the gain in diversity for the two antenna arrays have also been investigated in this work. The formula for the ECC with S-parameters is (1) [15].
Where ρ is the ECC for a MIMO antenna and S11, S12, S21, S22 are the MIMO system S-parameters.
The ECC curve versus frequency is shown in Figure 13. At the frequencies of 3.5 GHz and 10 GHz, it is found that the value of the ECC is less than 0.002 and such value is very low, which is preferred due to the fact that the ECC for a MIMO antenna should be less than 0.05. Therefore, based on the values of the  Figure 13, the performance of the proposed MIMO antenna in this paper is stable from the frequency 3.5 to 10 GHz, and each antenna element in the MIMO configuration operates completely independently without one influence on the other.

Comparison with previous works
The proposed design in this work was compared with other works presented in the recent literature as listed in Table 1. In this comparison, we focused on the most important parameters that determine the performance of the proposed antenna. These parameters are the antenna size, the frequencies at which the antenna operates, the isolation performance between the ports, the diversity gain values, and the values of the ECC parameter. It can be seen in Table 1 that the antenna proposed in this work is superior to other antennas in all parameters for various aspects.

CONCLUSION
The design of a functional MIMO system along with the design criteria identified a new methodology. The system operates at frequencies of 3.9 GHz and 7.5 GHz using realistic antennas based on CST Studio Suite. We evaluated and analyzed various parameters of the MIMO and found that the antennas in the MIMO system work independently of each other, which is a required prerequisite for the design of MIMO systems. However, MIMO systems provide improved efficiency and this requires complex design and it is important to take care of the problems associated with shared coupling, otherwise, they cause immense conflict, as well as high system designing costs. Finally, the proposed antenna in this work showed better performance and characteristics when compared with other works in literature. As future work, more elements will be added, investigated, and implemented to prove the suitability of this design for 5G mobile systems and other wireless communication networks.