Keywords

1 Introduction

The MIMO technique enhances channel capacity and signal transmission. The incorporation of a number of antenna elements in both the transmitter as well as the receiver results in greater channel capacity. The presence of a number of paths in between the transmitter and the receiver ensures multipath propagation. The drawback of this multipath propagation is that it produces signal fading which is controlled by spatial diversity. Spatial multiplexing can be applied in the system. MIMO is the main technique used in advanced wireless communication systems, such as 4G and 5G, etc. The range and robustness of the whole system is also augmented but at the same time this increases its complexity. In a MIMO antenna, it can be seen that in a single beam array, capacity increases even in the presence of high interference and high correlation between multipath signals, whereas in multi-beam arrays there is a decrease in capacity compared to normal antenna arrays. One of the biggest hurdles in MIMO antenna technology is mutual coupling among the antenna elements in the case of small sizes of antenna. In this case, the mutual coupling is reduced by taking an L-shaped lot in the ground plane. MIMO antennas with a miniature size have incredible future scope for use in variable portable devices as per users [1,2,3,4].

2 Experiments: Antenna Design

We have designed a MIMO antenna with two symmetrical planar monopole antennas that are separated by a distance of 9 mm, placed within the compact area of 35 × 22 mm2. We have used Rogers as a substrate with a dielectric constant, \( \in_{r} \) of 3.5, a loss tangent \( \delta \) of 0.002 and a thickness of 1.6 mm.

In Fig. 1, two symmetrical monopole antennas designed with a square-shaped radiator of 8 mm are shown. The technique used for feeding both ports is microstrip feed. Impedance matching is effortless in the case of microstrip feed compared to other techniques. It is reliable and easy to construct. The ground plane has an L-shaped slot. Using the dimensions given in Table 1, the antenna is designed in Ansoft HFSS software [5,6,7,8] (Fig. 2).

Fig. 1
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Geometry of the MIMO antenna

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Table 1 Dimensions of the MIMO antenna (in mm)
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Fig. 2
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Design of the MIMO antenna in a software interface using an L-shaped slot in the ground plane

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In HFSS software, when we apply the radiation far field to the airbox in terms of software that enfold the antenna, the direction of propagation of radiation is observed as in Fig. 3.

Fig. 3
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Radiation far field applied in the software interface

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2.1 Derivation and Explanation

All the parameters used in the geometry of the MIMO antenna are calculated using the following formulas:

Some waves travel in other substrates as well as in air, so an approach effective dielectric constant concept is introduced. The value of the effective dielectric constant, \( \in_{reff } \) is given by:

$$ \in_{reff} = \frac{{ \in_{r} + 1}}{2} + \frac{{ \in_{r} - 1}}{2}\left[ {1 + 12\frac{h}{W}} \right] $$

where h: thickness of the antenna, W: width of the patch

Fringing effects increase the electrical length of the microstrip patch of the antenna. This makes the dimensions appear larger. Let the length of the patch be L and the length travelled by electric field be 2∆L. Thus, the effective length is given by [9]

$$ L_{eff} = {\text{L}} + 2\Delta {\text{L}} $$

The empirical values when pertained in the formula, imply that:

$$ \frac{\Delta L}{h} = 0.412\frac{{( \in_{reff} + \,0.3)\left( {\frac{W}{h} + 0.264} \right)}}{{\left( { \in_{reff} - \,0.258} \right)\left( {\frac{W}{h} + 0.8} \right)}} $$

For better radiation efficiency, the width of the radiator must be calculated by:

$$ {\text{W}} = \frac{c}{{2f_{r} }} \sqrt {\frac{2}{{ \in_{r} + 1}}} $$

The actual length of the patch is given by:

$$ {\text{L}} = \frac{1}{{2f_{r} \sqrt { \in_{reff} } \sqrt {\upmu_{0} \in_{0} } }} - 2\Delta L $$

2.2 Calculations

The resonant frequency is taken as 4.4 GHz and accordingly the width is calculated first. Once the width (W) is calculated, we can easily get all other parameters by using the formulas mentioned above.

$$ {\text{W}} = \frac{{3 \times 10^{8} }}{2 \times 4.8}\sqrt {\frac{2}{3.5 + 1}} = 22.2\,{\text{mm}}\, \approx \, 22{\text{mm}} $$

In this design, the ground plane is cut into L-shaped so that the mutual coupling between the antenna elements can be controlled even when the size is miniaturized. The fabrication cost is estimated to be low because of the use of microstrip feed. All design parameters are calculated and implemented in the structure.

2.3 Simulation and Results

The reflection coefficient of the MIMO antenna is shown as:

In Fig. 4, the value of the reflection coefficient, \( S_{11} \) for this MIMO antenna can be seen as below −10 dB for a wide range of frequencies, i.e. 2.4–8 GHz for both ports but at 3 GHz and 7.7 GHz it made deep cuts close to −37.5 dB and −62.5 dB respectively. It can be observed that the port-1 output (colored red in the figure), gives better results compared to the port-2 output (in grey). Since there are two active ports, we get the return loss for both ports. In case of the MIMO antenna, when a signal is transmitted port-1 gives a better response at 3 GHz and 7.7 GHz while port-2 remains below −10 dB throughout the bandwidth.

Fig. 4
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Return loss \( S_{11} \) plot of the MIMO antenna

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In Fig. 5, it can be seen that \( S_{12} \) is below −20 dB for a range of frequencies of 2.8–11 GHz and the maximum deep cut is of −45 dB at 10 GHz. This implies that the mutual coupling between the antenna elements is less and is controlled.

Fig. 5
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Plot of \( S_{12} \) for the MIMO antenna

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In Fig. 6, it can be seen that the VSWR for this particular MIMO antenna is less than 2 for the whole bandwidth. It is almost 1.6 which is desirable for an antenna and indicates that the impedance matching is good (Figs. 7 and 8).

Fig. 6
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VSWR for the MIMO antenna

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Fig. 7
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2-Dimensional gain for the MIMO antenna

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Fig. 8
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3-Dimensional gain for the MIMO antenna

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The maximum gain is pragmatic at approximately 2 dB. This is seen in the two-dimensional plot and the three-dimensional figure.

In Fig. 9, the radiation pattern in the far field is shown. It is an omni-directional pattern, one which is desired for the MIMO antenna.

Fig. 9
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Radiation pattern for the MIMO antenna design

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In the MIMO antenna, an omni-directional pattern is desirable since it is directional in one plane and isotropic in the other. The antenna is expected to be omni-directional and this will affect its application too (Fig. 10).

Fig. 10
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Smith chart representing the path from port-1 to port-2

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In the Smith chart, the centre represents 50 Ω. The aim of the designer is to reach to the centre of the Smith chart in the construction of the matching network, and this has been achieved in our design.

2.4 Discussion and Analysis

In the above experiment, a MIMO antenna is designed with the help of two symmetrical planar monopole antennas. The ground plane is cut into an L-shaped slot so that the mutual coupling which is undesirable in a MIMO antenna can be minimized. The dielectric material used as the substrate, i.e. Rogers, is easily available and hence easy to fabricate. The structure when simulated, gives \( S_{11} \) below −10 dB for a wide frequency range of 2.4–8 GHz and at 3 GHz and 7.7 GHz it made deep cuts close to −37.5 dB and −62.5 dB respectively. We take −10 dB as a reference point because it indicates that 90% of the radiation coming from antenna is radiated and only 10% is reflected back. We have observed that \( S_{11} \) made deep cuts up to −62.5 dB and this is highly desirable. If this structure is fabricated then even with a significant amount of fabrication error, it can be expected that \( S_{11} \) will have desirable values in the manufactured antenna. The plot of \( S_{12} \) represents power from port 2 delivered to port 1. There is superior amount of isolation between the ports for the MIMO antenna to work properly. The results show that it is below −20 dB over a wide range of frequencies (2.8–11 GHz) and even lower than that for the higher range, i.e. −45 dB at 10 GHz which shows that the mutual coupling is reduced between the antenna elements. The VSWR shows how good the impedance matching is. When the matching is perfect, more power is delivered to the antenna. The value of VSWR is generally greater than or equal to 1, but less than 2. In the graph shown above, the VSWR value is below 2, being at 1.6, which indicates that the antenna has good impedance matching. In respect to this structure, the maximum gain is approximately 2 dB. The radiation pattern at far field shows that it is omni- directional, which is highly desirable for a MIMO antenna. Impedance matching is good and therefore the Smith chart reading shows that the path travels to the center which means 50 Ω. All the parameters are verified and this MIMO antenna can be easily implemented for practical utilization [10,11,12].

2.5 Conclusion

Nowadays wireless communication plays a major role in society. The antenna is the main component of any wireless communication system. The performance of any wireless communication system depends completely on the high performance of the antenna design and implementation. In this MIMO antenna design, a rectangular radiator along with the L-shaped ground plane helps in achieving desirable parameters. The core issue in a MIMO antenna is reducing the mutual coupling when the antenna is of a compact size. This issue has been resolved by making a slotted ground plane. The reflection coefficient is −10 dB over a frequency range of 2.4–8 GHz. The reflection coefficient strikes deeper being −37.5 dB and −62.5 dB at 3 GHz and 7.7 GHz respectively. This makes it usable for various applications in wireless communication such as WLAN and WiMAX etc. All the parameters have been successfully verified by simulating the structure and the design can be fabricated easily.