E-shaped multiple-input-multiple-output (MIMO) microstrip antenna systems operating in WLAN and WiMAX bands (between 5 and 7.5 GHz) are proposed with enhanced isolation features. The systems are comprised of two antennas that are placed parallel and orthogonal to each other, respectively. According to the simulation results, the operating frequency of the MIMO antenna system is 6.3 GHz, and mutual coupling is below −18 dB in a parallel arrangement, whereas they are 6.4 GHz and −25 dB, respectively, in the orthogonal arrangement. The 2 × 3 matrix of C-shaped resonator (CSR) is proposed and placed between the antenna elements over the substrate, to reduce the mutual coupling and enhance the isolation between the antennas. More than 30 dB isolation between the array elements is achieved at the resonant frequency for both of the configurations. The essential parameters of the MIMO array such as mutual coupling, surface current distribution, envelop correlation coefficient (ECC), diversity gain (DG), and the total efficiency have been simulated to verify the reliability and the validity of the MIMO system in both parallel and orthogonal configurations. The experimental results are also provided and compared for the mutual coupling with simulated results. An adequate match between the measured and simulated results is achieved.
The current wireless communication systems have to fulfill the demands such as high data rates, increased capacity, high quality, and high reliability for different applications. Multiple-input-multiple-output (MIMO) systems provide the suitable technology for these requirements without the necessity of additional bandwidth or transmit power by spreading multiple antennas, with sufficient element spacing, the correct number of elements, and appropriate array geometry or topology [
Various methods have been studied and analyzed for suppression of mutual coupling and improvement of the isolation between the antennas, including the neutralization technique [
In this work, two E-shaped MIMO antenna systems 41 × 100 mm2 are presented. The mutual coupling is reduced by placing the EBG metamaterial structure consists of a 2 × 3 matrix of C-shaped resonator (CSR) between the antenna elements. The same 2 × 3 CSR matrix has been placed and examined between the two antennas, in both cases when the two antennas are in a parallel or orthogonal configuration. The CSR’s effective permittivity and permeability are also provided and obtained by the use of Nicholson-Ross-Weir (NRW) method [
Since the goal is to deal only with the mutual coupling mechanism, a simple MIMO system consisting of two E-shaped monopole antennas has been chosen to demonstrate the capability of the approach in this paper. The antennas are placed in parallel and operating between 5 and 7.5 GHz as shown in Figure
(a) The proposed MIMO system and (b)
Dimensions of the parameters of the MIMO antenna system.
Parameter | Ws | Ls | W1 | W2 | Wf | L1 | L2 | L3 | Lf | a | b | c | c1 | g | d |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dimension (mm) | 100 | 41 | 19 | 11 | 3 | 11 | 20 | 11 | 10 | 9 | 11 | 10 | 2 | 2 | 20 |
A prototype of this antenna system is shown in Figure
Fabricated antenna.
Comparison between the simulated and the measured
Figure
The unit cell of CSR structure. (a) 3D view of the unit cell. (b) The equivalent circuit of the CSR.
Dimension of unit cell.
Parameter | L | Rin | Ro | g | h |
---|---|---|---|---|---|
Dimension (mm) | 8 | 1.8 | 3 | 3.6 | 1.6 |
(a) 2 × 3 matrix of MTM-CSR, (b) effective permittivity, (c) and effective permeability.
It is obvious that the placement of a sheet of resonators of infinite extension between the two antennas is not possible. Therefore, the designed structure consisting of a 2 × 3 matrix of CSR is studied. To investigate the characteristic properties such as the effective permittivity and effective permeability of this matrix, the Nicolson-Ross-Weir (NRW) method is used [
The sign is chosen such that
In this section, the proposed decoupling technique is utilized to enhance the isolation between MIMO antennas by placing a 2 × 3 matrix of the CSR structure as shown in Figure
(a) MIMO system antennas with CSR matrix structure. (b) Simulated
The prototype of the antenna system is printed as shown in Figure
Fabricated antenna.
Comparison between measured
Comparison between simulated and measured
The influence of decoupling can be observed by visualizing the surface current plots on the antenna array without CSR matrix structure; strong surface current can be observed on the patch antenna on the right-hand side when the patch antenna on the left is excited as shown in Figure
Surface current distribution (a) without CSR matrix structure (b) and with CSR matrix structure.
The ECC is calculated for MIMO system, without and with MTM-CSR matrix structure and is plotted in Figure
ECC of MIMO system with and without MTM-CSR matrix structure.
The higher the value of diversity, the better the isolation and vice versa. As observed from Figure
Diversity gain of the MIMO antenna with and without MTM-CSR matrix structure.
Simulated total efficiency of whole antenna system with and without MTM-CSR matrix structure.
The performance of a MIMO system can also be improved by using the antennas with the same gain in a given direction, but with orthogonal polarization, which is known as polarization diversity. In order to study this concept, the antenna elements are placed such that they have orthogonal polarizations as given in Figure
The proposed MIMO system in orthogonal configuration, (a) without CSR matrix structure, (b) and with CSR matrix structure.
Figure
Comparison between simulated
Figure
Fabricated antenna in orthogonal configuration (a) without and (b) with CSR structure.
Comparison of the measured
Comparison of the simulated and measured
By analyzing the surface current distribution, the mechanism of CSR matrix structure can be more understandable. The mutual coupling between the two antenna elements in a MIMO system is related to the current direction that flows on the surface of the antennas. If the current flows in the same direction on the adjacent sides of both the antennas, the mutual coupling increases. Similarly, if the currents are in the opposite direction, the induced mutual coupling is suppressed. The surface waves can clearly be observed in Figure
Surface current distribution (a) without and (b) with CSR matrix structure.
To evaluate the orthogonal MIMO system performance in terms of correlation and reliability, the ECC and DG were calculated using (
ECC of orthogonal MIMO system with and without MTM-CSR matrix structure.
DG of the orthogonal MIMO system with and without MTM-CSR matrix structure.
Figure
Simulated total efficiency of a whole antenna system with and without MTM-CSR matrix structure.
To examine the influence of the decoupling structure on the radiation pattern of the antennas, the left patch antenna was excited, and the right patch was terminated with 50 Ω impedance in Figures
Radiation pattern of the E-shaped MIMO antenna system in parallel configuration: (a) E-plane and (b) H-plane.
The radiation pattern of the E-shaped MIMO antenna system in the orthogonal configuration: (a) E-plane and H-plane for the left and (b) E-plane and H-plane for the right E-shaped antennas.
In this work, two E-shaped antenna MIMO systems suitable for WLAN and WiMAX applications with enhanced isolation are proposed. The motive of this work is to suppress the mutual coupling between the antenna elements using metamaterial structure. This MIMO antenna system resonates at 6.3 GHz with an impedance bandwidth of 34.42% in the range of frequencies from 5–7.5 GHz. The coupling reduction has been achieved by printing a 2 × 3 MTM-CSR matrix between array elements. This structure can block the surface waves inside the substrate of the antenna and guide them in another direction. The mutual coupling in the E-shaped MIMO antenna system is investigated with and without the 2 × 3 CSR matrix in both configurations, parallel and orthogonal antenna arrangement. The isolation of MIMO antennas with 2 × 3 MTM-CSR decoupling structure is shown to improve about 12 dB, whereas it is improved in orthogonal arrangement around 6 dB and maintaining the overall mutual coupling less than −20 dB in the operating band.
The authors declare that they have no conflicts of interest.