MIMO Antenna Array Configuration for Minimum Mutual Coupling

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 5, Issue 6, June 2015)

418

MIMO Antenna Array Configuration for Minimum Mutual

Coupling

Adham Mohamed Gamal

1

, A.M. M. Allam

2 1

Teaching Assistant, Faculty of Information Engineering and Technology, German University in Cairo, Cairo, Egypt. 2_{Professor, Faculty of Information Engineering and Technology, German University in Cairo, Cairo, Egypt.}

Abstract- A design of trapezoidal MIMO antenna with a decreased mutual coupling is proposed. The system is designed to operate in the Wi-Fi band (5 GHz) with a bandwidth of 110 MHz. Two different spatial configurations for the same antenna as a basic unit are demonstrated to show the effect of the new configuration. The first configuration has

dimensions of 18.6*64.4*1.554 mm3 _while _the ₂nd

configuration has 33.6*64*1.554 mm3. The 2nd configuration succeeded to decrease the mutual coupling from 6dB to 14dB as a minimum and from 14dB to 27dB as a maximum. There is a perfect match between the measured and the simulated results for the new configuration.

Keywords-- Wi-Fi, MIMO, mutual coupling, DGS. I. INTRODUCTION

Every day, there is a demand for faster bit rate communication techniques. As it’s noticed, starting from 1985, wireless communication systems evolved from 1G to 2G and from 2G to 3G and 4G [1]. All of these systems were invented to cope up with the need of people’s demand. Multiple-input Multiple-output (MIMO) antennas are the solution to the gigabit wireless communication systems [2]. MIMO antennas can increase the capacity of the system by increasing the number of the antennas without any need for an extra frequency spectrum or power [3]. The high potential of MIMO antennas is confirmed by their usage in many different wireless standards such as WLAN [4], WiMax [5], Long Term Evolution (LTE) [6] and Wifi [7].

One main constrain in using MIMO antennas is mutual coupling. One can define mutual coupling as the effect of one of the antennas on the rest of the antennas in the same system. While the antenna is transmitting its power, the other antennas’ receiver is working and will absorb some of this power.

This means that not all the power will be radiated towards the needed receiver. This effect is mutual coupling [8]. Mutual coupling therefore decreases the system’s efficiency and performance in both transmitting and receiving activities.

Many techniques have been studied to prevent mutual coupling between antennas. Parasitic elements [9], DGS designs [10], or electromagnetic gap between the antennas [11] are all examples on these techniques. In this paper, two different configurations for a four antenna MIMO system are proposed to reduce the mutual coupling between the two designs from 6dB to 14dB as a minimum and from 14dB to 27dB as a maximum.

II.ANTENNA DESIGN

The two configurations are implemented on a Rogers RO4350 substrate with a relative permittivity of 3.66, tangential loss of 0.004, and thickness of 1.524, and the two configurations share the same antenna element as a unit cell for the MIMO system. The thickness of copper to make the ground and the antenna is 0.015mm. The antenna

takes the shape of a trapezoidal as demonstrated in Figure

1. All the dimensions of the antenna are listed in Table 1.

For configuration 1, it is made of four symmetrical trapezoidal antennas with a gap of 0.4 mm between each antenna. A gap of 2 mm is left from the four corners. The configuration has total dimensions of 18.6*64.4*1.554

mm3. Each antenna is fed by a 50 Ω matched coaxial cable.

The coaxial cable is positioned to be centered in each antenna with respect to the x-axis and with a separation of

10.2 mm between the feeding point and the base of the

trapezoidal antenna. The 1st configuration is demonstrated

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International Journal of Emerging Technology and Advanced Engineering

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Figure 1 Antenna element in the proposed structures (a) Front view (b) Back view

Table 1

Dimensions of trapezoidal antenna

L (mm)

W1(mm)

W2(mm)

14.6

14.8

10.2

Figure 2 (a) MIMO configuration 1 (in simulation)

Figure 2 (b) MIMO configuration 1 (after fabrication)

For configuration 2, it is consisted of four antennas. The

1st and the 4th antenna are symmetrical and facing each

other. The 2nd and the 3rd antenna are a 90o rotated version

of the 1st and the 4th antenna. There is a small gap of 0.4 mm is left between the most two far points in each antenna.

This gap is left to make the design as compact as possible and to make sure that the separation between the center of any two antenna to be not more than λ/2 to negate

any grating loops. The design is demonstrated in Figure 3.

All the dimensions regarding the 2nd configuration are

illustrated in Table 2.

Figure 3 (a) MIMO configuration 2 (in simulation)

Figure 3 (b) MIMO configuration 2 (after fabrication)

Table 2

Dimensions of configuration 2

L1(mm)

L2(mm)

W1(mm)

W2(mm)

6

4.6

7.4

III. RESULTS AND MEASUREMENTS

The 1st configuration is simulated using CST simulation

tool. As it is a symmetric configuration, one can find that antennas, number 1 and number 4, and, number 2 and number 3, will have the exact same s-parameters.

W1

L

W2

L2

L1

W1 W2

1

2

3

4

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International Journal of Emerging Technology and Advanced Engineering

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Antenna 1 and antenna 2 have a resonance frequency of approximately 5.1 GHz with a bandwidth of 150 MHz. Antenna 3 and antenna 4 have the same response as antenna 2 and antenna 1 simultaneously. This 100 MHz shift in the frequency was from the effect of each antenna

on the resonance frequency of the other. Figure 4 discusses

the simulated and the measured results.

As mentioned before, there is a mutual coupling between the 4 antennas, which allows part of the power to be transferred from any antenna to the rest of the antennas

affecting the total efficiency of the MIMO system. Figure

5 and Figure 6 illustrate the effect of antenna 1 and

antenna 2 simultaneously on the rest of the system

(Measured and Simulated). Table 2 has the value of the 1st

configuration s-parameters at frequency 5 GHz.

Figure 4 Reflection coefficient for antenna 1 and antenna 2 in configuration 1 (Measured and Simulated)

From table 2, one can find that there is a mutual coupling of 6 dB as a min and 13.9 dB as a max.

The 2nd configuration is simulated using CST simulation

tool. It is also a symmetric configuration, so antenna number 1 and 4 share the same s-parameters and antenna number 2 and 3 share the same s-parameters. The four antennas have a resonance frequency of 5 GHz with a

bandwidth of approximately 110 MHz. Figure 7

demonstrates the s-parameters of antenna number 1 and antenna number 2 (Simulated and Measured).

Figure 5 Scattering parameters at 5 GHz frequency of the 1st_antenna

in the 1st_{configuration (Measured and Simulated)}

Figure 6 Scattering parameters at 5 GHz frequency of the 2nd_antenna

in the 1st_{configuration (Measured and Simulated)}

Table 2

S-parameters of antennas 1 and 2 in configuration 1

S21

-6dB

S12

-6.1dB

S31

-13dB

S32

-9.8dB

S41

-13.9dB

S42

-12.9dB

Figure 8 and Figure 9 illustrate the effect of antenna 1 and antenna 2 simultaneously on the rest of the system

(Simulated and Measured). Table 3 has the value of the 2nd

configuration s-parameters at frequency 5 GHz.

Figure 7 Reflection coefficient for antenna 1 and antenna 2 in configuration 2 (Measured and Simulated)

As noticed from Table 3, the mutual coupling between the 4 antennas has greatly decreased from 6 dB as a minimum to 14.2 dB and from 13.9 dB as a maximum to 26.7 dB.

Figure 8 Scattering parameters at 5 GHz frequency of the 1st_antenna

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Figure 9 Scattering parameters at 5 GHz frequency of the 2nd_antenna

in the 2nd_{configuration (Measured and Simulated)}

Table 3

S-parameters of antennas 1 and 2 in configuration 2

S21

-20.4dB

S12

-20.5dB

S31

-26.5dB

S32

-13.9dB

S41

-18dB

S42

-26.7dB

In Figure 10 and Figure 11, the electric field of the 1st antenna in configurations 1 and 2 respectively in demonstrated.

Figure 10 The electric field of the 1st_{antenna in the 1}st_{configuration}

Figure 11 The electric field of the 1st_{antenna in the 2}nd_{configuration}

As one can notice from figure 10 and 11, the effect of antenna number one on the rest of the system has decreased

from the 1st configuration to the 2nd configuration.

Figure 12 (a, b, c) illustrates the 1st configuration 1st antenna’s far field response in 3D, and polar form.

In the XZ plan, the antenna has a main lobe magnitude

of 5.2 dBi with a direction of 28o. The angular width is

88.7o. In the YZ plan, the antenna has a main lobe

magnitude of 4.08 dBi with a direction of 6o. The angular

width is 109.6o.

Figure 12 (a) Far field response for antenna number 1 in the 1st

configuration in 3D

Figure 12 (b-c) Far field response for antenna number 1 in the 1st configuration in XZ plan and YZ plan respectively

Figure 13 (a, b, c) illustrates the 2nd configuration 1st antenna’s far field response in 3D, and polar form.

In the XZ plan, the antenna has a main lobe magnitude

of 6.38 dBi with a direction of 34o. The angular width is

98.5o. In the YZ plan, the antenna has a main lobe

magnitude of 5.74 dBi with a direction of 6o. The angular

width is 85.8o.

Figure 13 (a) Far field response for antenna number 1 in the 2nd

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International Journal of Emerging Technology and Advanced Engineering

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Figure 12 (b-c) Far field response for antenna number 1 in the 2nd

configuration in XZ plan and YZ plan respectively

From figure 11 and figure 12, an enhancement to the radiation efficiency is observed. A slight change in the main lobe magnitude has occurred.

IV. CONCLUSION

Two configurations have been implemented for a four MIMO antenna system to work in the WiFi band. Mutual coupling has been greatly decreased from 6 dB to 14 dB with a difference of 8dB as a minimum and from 14 dB to 27 dB with a difference of 13 dB as a maximum. The far field response of the two systems is approximately the

same in the shape with an enhancement in the 2nd

configuration in the radiation efficiency.

REFERENCES

[1] Amit Kumar, Dr. Yunfei Liu, and Dr. Jyotsna Sengupta, Divya, “Evolution of Mobile Wireless Communication Networks: 1G to 4G”, International Journal of Electronics & Communication Technology, VOL. 1, December 2010.

[2] AROGYASWAMI J. PAULRAJ, DHANANJAY A. GORE, ROHIT U. NABAR, Aswathy K. Sarma, Chinnambeti Raviteja, and HELMUT BÖLCSKEI, “An Overview of MIMO Communications—A Key to Gigabit Wireless”, PROCEEDINGS OF THE IEEE, VOL. 92, February, 2004.

[3] D. Tse and P. Viswanath, Fundamentals of Wireless Communication, Cambridge University Press, New York, 2005. [4] Pawel Kulakowski, and Wieslaw Ludwin, “THE CAPACITY

EVALUATION OF WLAN MIMO SYSTEM WITH MULTI-ELEMENT ANTENNAS AND MAXIMAL RATIO COMBINING”, the 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2006. [5] E.T. Tchao, K. Diawuo, W.K. Ofosu ,and E. Affum, “Analysis of

MIMO Systems used in planning a 4G-WiMAX Network in Ghana”, International Journal of Advanced Computer Science and Applications, VOL. 4, 2013.

[6] I.F. Akyildiz, D.M. Gutierrez-Estevez, and E.C. Reyes, “The evolution to 4G cellular systems: LTE-Advanced”, Physical Communication 3, 2010.

[7] Andrea Goldsmith, “Designing Reliable Wi-Fi for HD Delivery throughout the Home”, Quantenna Communications, March, 2009. [8] J. Daniel, “Mutual coupling between antennas for emission or

reception application to passive and active dipole,” IEEE Trans. Antennas Propagat. , VOL. 22, pp. 347-349, March, 1974.

[9] O.F. Ahmed, R.S. Ghoname, and A. A. Zekry “Mutual Coupling Reduction of MIMO Antennas using Parasitic Elements for Wireless Communications”, International Journal of Computer Applications, VOL 62, January, 2013.

[10] Ahmed, M.I. , Sebak, A. , Abdallah, E.A. , and Elhennawy, H., “Mutual coupling reduction using defected ground structure (DGS) for array applications”, Antenna Technology and Applied Electromagnetics (ANTEM), 2012.

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