MIMO Antenna Performances on Microstrip Antenna with EBG Structure for WLAN Applications

A dual microstrip MIMO antenna with Electromagnetic Bandgap (EBG) structures presented. EBG structures proposed in order to reduce the coupling between elements .Simulated scattering parameters with and without EBG structures compared. An evaluation of MIMO antenna characteristics is presented, with the analysis of the mutual coupling, correlation coefficients, total active reflection coefficients (TARC), capacity loss and channel capacity using Computer Simulation Technology (CST) Microwave Studio Software. The proposed antenna is a good candidate for WLAN practical applications.


Introduction
Recently, MIMO antenna system generated great interest for researchers to improve reliability and enhanced channel capacity in wireless communication fields [1].The challenge in designing MIMO antenna is the mutual coupling between antenna elements.The mutual couplings exist due to the finite spacing between elements.Higher antenna efficiencies and lower correlation coefficients can be achieved with lower mutual coupling [2].Previous work that related with reduction of mutual coupling techniques studied in [3][4][5][6][7][8][9][10][11].Authors in [3] proposed T-Shape slot impedance transformer with isolation over 19.2 dB and 22.8 dB at 2.4-2.48GHz and 3.4-3.6GHzrespectively with correlation coefficients less than 0.5.Mutual coupling lower than -28 dB over WLAN band achieved in [4], by introducing an additional non radiating folded shorting strip between element and ground plane.Two packed antennas with coupling element is reported in [5] to obtain mutual coupling more than <-15 dB, with edge to edge separation is 0.04λ.The use of neutralization line technique proposed in [6], and minimization of mutual coupling less than -20 dB is achieved.Other approaches to reduce mutual coupling and enhanced isolation of the MIMO antenna, such as the use of soft surface [7], inserting slits on ground plane [8], defected ground structures (DGS) [9,10] and using two metamaterial inspired small printed monopole antennas [11] also have been discussed.
The incorporation of electromagnetic band gap (EBG) structures is an attractive idea in antenna community.This is because the electromagnetic band gap (EBG) structure has ability to reduce mutual coupling as well as increase the MIMO antenna performance [12,13].
This paper presents a planar H-shaped antenna with EBG structure, operating at 2.45 GHz applications.The S-parameters result for the proposed antenna with and without EBG structures are compared.In addition, the MIMO antenna performances in term of mutual coupling, correlation coefficients, total active reflection coefficients (TARC), capacity loss and channel capacity are analyzed.

A. Antenna Array
The configuration and parameters of microstrip antenna array are shown in Fig 1 .The distance between antenna elements is 0.36λ at 2.45 GHz.The dimension of proposed antenna array listed in Table 1.

B.Electromagnetic Bandgap (EBG) Structure
EBG structure with high impedance surfaces are realized with periodic arrangement of metallic element.The design of EBG structure is illustrated in Fig 2 .A center patch is connected to four folded bend metallic connecting lines.Folded bend acts as connecting bridge that plays important role in determining band gap position.The connecting bridge provides the inductance while the gap within the cell and the gap within adjacent cell provide capacitance.Thus, the characteristic result of the EBG structure similar to that in [12].The dimension of the EBG structure listed in Table 2.

C.Antenna array with EBG structure
Fig 3 show the proposed antenna array design with EBG structure in front and back view.EBG structure placed in between antenna elements.The integration of EBG structures in microstrip antenna array is quite attractive for their ability to suppress surface wave and reduce the mutual coupling.The purpose of EBG structure is to enhance the performance of antenna array and reduce mutual coupling between them.

B.Envelope Correlation Coefficient
To evaluate the capabilities of the MIMO antenna, envelope correlation coefficient is an important criterion to be presented.Envelope correlation coefficient can be computed using far-field radiation An improvement of the ECC can be seen after the EBG structure was applied in Fig 6and it fulfils the characteristic of diversity p e < 0.5 [8].Therefore, the proposed antenna is suitable candidate for WLAN applications.

C.TARC and Capacity Loss
The simulated TARC and capacity loss of the proposed antenna with and without EBG structure shows in Fig 7 and Fig 8, respectively.Next, to show the performance of the MIMO system, the correlation coefficient, TARC, capacity loss and channel capacity are presented in Table 2.The results show that the microstrips with EBG structure have lower loss of capacity and better performance of TARC and correlation coefficient.The channel capacity of microstrip with and without EBG structure is 5.589bits/s/Hz and 5.768 bits/s/Hz at 2.45 GHz, respectively, as illustrated in Fig 9 .Table 2 :Simulated results for correlation coefficient, TARC and capacity loss at 2.45 GHz.

4.Conclusion
A two-elementmicrostrip MIMO antenna for covering 2.45 GHz WLAN applications presented.EBG structure applied to meet the requirement of MIMO in term of low mutual coupling parameter.Simulated result shows that the proposed antenna design improved the mutual coupling <-20 dB.Further, the correlation coefficient, TARC, capacity and capacity loss have been analyzed for antenna with and without EBG structure.Therefore, the proposed antenna is an ideal option for WLAN applications.

Fig 4
Fig 4 shows the simulated S-parameters result of the proposed antenna with and without EBG Structure.It can be seen that an improved isolation of -3 dB is achievable against proposed antenna without EBG structure.The mutual coupling of the proposed antenna design improved form -16.08 dB to -19.25dB after the EBG structure employed between the elements.The comparison between simulated and measured results plotted in Fig 5.

Table 1 :
[14]nna array dimension Table2: EBG structure dimension pattern and using S-parameters.In this case, envelope correlation expressed using s-parameters by using following equation[14]