antenna array with low mutual coupling for mimo-lte ...international journal of applied engineering...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 12 (2017) pp. 3243-3247

© Research India Publications. http://www.ripublication.com

3243

Antenna Array with Low Mutual Coupling for MIMO-LTE Applications

Eduardo Rodríguez Araque1, Ezdeen Elghannai2, Roberto G. Rojas3 and Roberto Bustamante4

1Foundation Universitary Cafam (Unicafam), Bogotá, Colombia. 2Ohio State University, USA, 3MIT Lincoln Laboratory, USA, 4University of Los Andes, Bogotá, Colombia.

1ORCID: 0000-0003-0229-7920

Abstract

In this work we present a compact MIMO array platform that

operates in the 2.6 GHz for Long Term Evolution (LTE) band

and wireless communication systems. The array consists of

four compact patch antennas on a dielectric substrate with total

dimensions of 125 mm 62.5 mm 1.27 mm. Modifications

on the ground plane along with systematic placement and

orientation of each antenna on top of the substrate plays a key

role in reducing the mutual coupling which normally degrades

the MIMO array performance. The performance of this

designed MIMO array is assessed through simulations and

measurements of the scattering parameters, radiation patterns,

and correlation coefficients, including an evaluation of the

capacity in indoor and outdoor-to-indoor scenarios with

outstanding results.

Keywords: Antenna array, Capacity, Characteristic Modes,

Correlation, Diversity, Multiple-Input-Multiple-Output

(MIMO) systems, mutual coupling

INTRODUCTION

The operation of MIMO array systems is based on a technique

that allows us to increase the data rate and link reliability when

operating in rich scattering environments. This technique

normally generates low-correlation parallel sub-channels

between elements of the array, thus incrementing the system

capacity by several orders of magnitude [1].

An important factor that has a deleterious effect on the MIMO

system performance is the mutual coupling between antennas.

A high level of mutual coupling can increase the correlation

between the signals received by each antenna, which in turn,

reduces the number of independent parallel sub-channels [2].

There are different techniques to reduce the mutual coupling

between the elements of an array; one way is to increase the

spacing between antenna elements of the array. However, if the

antenna array is built within small mobile phones, expanding

the spacing is not realistic.

Several antenna designs have been proposed to reduce the

mutual coupling of MIMO arrays in small mobile phones.

Corrugated ground plane techniques are presented in [3-4] to

achieve high isolation. A T-shaped ground plane slot is

implemented to reduce the mutual coupling between antennas

[5]. In [6], T-shaped and dual inverted are inserted to reduce

the mutual coupling. Additionally, neutralization techniques

are also used to increase the port-to-port isolation between two

closely placed antennas [7].

In this work, we propose a MIMO array designed to operate at

2.6 GHz (4G-LTE) band. The antenna array consists of four

compact patch antennas on a PCB/dielectric substrate.

Modification of the ground plane (GND) with corrugated slots

in addition to a systematic placement and orientation of each

antenna on top of the PCB plays a key role to increase the

isolation; therefore, reducing the correlation between antennas.

The design of the antenna elements, modifications of the GND,

placement, and orientation of each antenna on the array was

done based on the insights provided by the method of

Characteristic Modes.

The Method of Characteristic Modes (CMs) is a modal analysis

scheme that provides physical insight into the potential

resonant characteristic of a structure by finding and rigorously

examining the natural current modes of the structure [8-9]. We

used the original CMs [9] to design the antenna elements as

well as to study the currents induced on the ground plane of the

MIMO array. Another version of this method is the Network

Characteristic Mode (NCM) technique, in this application; the

network is formed by the 4 input ports of the antenna elements.

DESIGN OF MIMO ARRAY

The goal is to design and build a four-element array within a

compact mobile chassis and with high isolation between the

antennas. The first step was the initial design of a single antenna

that operates at 2.6 GHz. Thanks to miniaturization techniques,

we were able to design an electrically small antenna by properly

inserting slots as depicted in Fig. 1. Note that this antenna has

a radiation efficiency of 63 % after miniaturization.



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Figure 1: Compact patch antenna at 2.6 GHz. (a) Geometry.

(b) Return loss. (c) Radiation Pattern in E-plane. (d) Radiation

pattern in H-plane. The maximum measured gain is 2.79 dBi.

The next step is to place four antennas equal to the one in Fig.

1(a) on the substrate and try to achieve very low mutual

coupling between them.

A. Ground Plane Modifications

We start with a baseline structure; namely, with four equally

spaced antennas and with the same orientation as shown in Fig.

2(a). The initial GND is a solid metal sheet. The first step to

reduce mutual coupling between the array elements is to

introduce slots in the GND. These slots can reduce surface

waves effects in the substrate. The careful design of the slots

allows us to modify the GND taking into considering the

position of each slot as well as the spacing between antennas.

The total current distribution on the original GND (baseline

structure) at 2.6 GHz can be seen in Fig 2(b). Initially, straight

slots were introduced on the GND resulting in a reduction of

the mutual coupling. Further reduction was obtained by

modifying the edges of the slots so they are corrugated as

depicted in Fig. 2(c). The total current of the corrugated GND

is shown in Fig. 2(d). It can be observed how the total current

between antennas decreases substantially when the corrugated

slots are introduced. This means that there is less interaction

between the antennas and therefore a reduction in mutual

coupling between them.

B. The Placement and Orientation of the Antennas

Additional modifications are necessary to obtain a better

performance, i.e. greater reduction in the correlation. The first

modification is to change the location of each antenna on top of

the substrate. Initially, each antenna is centered in its

corresponding ground plane bounded by slots and all are

oriented in the same direction (see fig. 2(d)), then; further

modifications related to the antenna rotations are performed.

We observed that rotating each antenna helps to improve the

polarization diversity so that the correlation is reduced.

One way to achieve the optimum orientation of each antenna in

the array is by using the physical insight provided by the

method of NCM. The NCM eigenvalue spectrum for the four

antennas of the array with corrugated slots in the GND and each

antenna located in the center of the corresponding ground plane

and with the same orientation can be seen in Fig. 3(a). It can be

seen that the behavior of the modes corresponding to antennas

#1 and #2 show similar behavior and resonate near 2.6 GHz;

however, the resonance frequencies of the dominant modes for

the other two antennas are shifted. We see the magnitude of S11

and S22 resonate near 2.6 GHz, while S33 and S44 are shifted.

After rotating the antennas, the results of these modifications

can be seen in Fig. 3(b) and Fig. 3(d). The latter shows that all

antennas resonate near 2.6 GHz due to the weaker mutual

coupling between them. Note that a more detailed CM analysis

was performed to confirm these results; however, we can’t

present these additional details due to space constraints.

Fig. 4 shows the final arrangement of the four antennas on top

of the dielectric and the geometry of the ground plane.

RESULTS AND DISCUSSION

The results from measurements and simulations of the

proposed array are presented for S-parameters, mutual

coupling, enveloped correlation coefficients, radiation patterns,

and the simulated MIMO ch annel capacity at a frequency of

2.6 GHz.

Figure 2: Design Process. (a) Baseline structure with 4-

element antennas. (b) Total current distribution at 2.6 GHz of

baseline ground plane (mA/m). (c) Modified GND with

corrugated edge slots (5 mm x 2.5 mm). (d) Total current

distribution at 2.6 GHz of modified GND (mA/m).



3245

Figure 3: NCM eigenvalue spectrum of the dominant mode

and Return Loss (RL) for each antenna at 2.6 GHz. (a)

Eigenvalue before rotation. (b) Eigenvalue after rotation. (c)

Simulated RL of each antenna before rotation. (d) After

rotations (see Fig. 4(b)).

Figure 4. The designed MIMO array with total dimensions of

125 mm 62.5 mm 1.27 mm. (a) Four small patch antennas

on top of substrate (r=4.5, tan=0.002) (b) Modified Ground

Plane (GND), with corrugated slots.

A. S-Parameters and Mutual Coupling

Fig. 5 shows the results of the simulated and measured S-

parameters (respect to 50-ohms reference) for all antennas on

the substrate. The return loss values (Sii) show an acceptable

agreement between simulated and measured results. The

measured mutual coupling (Sij, ij) between antennas remains

below -28.42 dB at 2.6 GHz as shown in Fig. 5(c). We show an

excellent agreement with the simulated results where the

mutual coupling is found to be below -25.4 dB at 2.6 GHz.

B. Enveloped Correlation Coefficients

An important parameter that provides very significant

information about the performance of the array for MIMO

systems is the correlation. This correlation between signals

received by the antennas of the array can be computed through

the S-parameters for a lossless MIMO antenna [10-12]. The

results of the coefficients calculated through the measured S-

parameters are plotted in Fig. 5(d).

The results are less than 0.000027 at 2.6 GHz; these very low

values indicate that more isolated parallel sub-channels can

exist with the proposed MIMO array. Fig. 5(b) shows the

coefficients calculated through the simulated S-parameters. It

can be noticed that all the values are still very small. The results

for the coefficients of the baseline array are shown in fig. 5(f).

It shows high correlation with regard to the results of the

proposed array (see Fig. 5d), implying that this baseline array

has poor MIMO performance.

Figure 5: S-parameters and the correlation coefficients of the

MIMO array, and baseline array. (a) Simulated S-parameters.

(b) Correlation using simulated S-parameters. (c) Measured S-

parameters. (d) Correlation using measured S-parameters. (e)

Simulated S-Parameters of the baseline array, the isolation of

this array is 13.79 dB, and all antennas are not well matched at

2.6 GHz (f) Correlation of the baseline array. The S-parameters

are measured while all other antennas are terminated in 50-ohm

loads.

C. Radiation Patterns and Pattern Diversity

The simulated and measured radiation patterns at 2.6 GHz of

each antenna of the proposed array are plotted in the Fig. 6. We

can see that each antenna shows a different radiation pattern

(pattern diversity); this favorable property helps reduce the

correlation because the received multipath distribution in each

antenna will be different. Furthermore, the simulated and

measured radiation patterns show good agreement with a

maximum achieved gain of 3.15 dBi.

(a) (b)

(c) (d)

2.55 2.6 2.650

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3246

D. MIMO Channel Capacity Evaluation

The MIMO channel capacity is an important performance

metric for a wireless communication system. The capacity can

be calculated through the matrix of the MIMO channel (H) [2].

Here, the H matrix for the propagation environment depicted in

fig. 7 was obtained through simulations based a 3D GO/UTD

ray-tracing model [13]. Additionally, to obtain an accurate

interpretation of the capacity, the path loss and receive SNR

effects should be carefully removed [14].

Figure 6. Simulated ( ), and measured ( ) 2-D radiation

patterns. (a) Antenna #1. (b) Antenna #2. (c) Antenna #3. (d)

Antenna #4. See Fig. 4(b). The measurements was performed

by exciting one antenna while the others antennas were

terminated by 50-ohm loads.

Simulated capacity for a 4×4 MIMO system in a specific

propagation environment at a frequency of 2.6 GHz are

presented for two locations of the receiving and transmitting

array as shown in fig 7. We present results for three receiving

arrays: 1) designed MIMO array, 2) baseline array, and 3) four

parallel dipole antenna array with spacing of /2 between them

and without mutual coupling. This latter array together with the

transmitting array form a MIMO system and are used as a

reference for evaluating the MIMO capacity, since these two

arrays do not take into account mutual coupling effects.

Fig. 8(a) shows the capacity calculated for an indoor scenario

for two cases: line of sight, and not line of sight. As expected,

we show that none of the evaluated antenna arrays (proposed

MIMO and baseline arrays) can achieve the performance of the

reference MIMO system due to the presence of mutual

coupling. However, the designed MIMO array presents better

performance than the baseline array due to the high isolation

between its elements.

Figure 7: Layout of propagation environment of the floor #1.

The positions of the transmitting, and receiving arrays are

shown above. The Tx. array consists of four vertical λ/2-

dipoles with a spacing of /2 between them.

In Figs. 8(b-c) the designed MIMO array performance presents

two different behaviors that can be attributed to the multipath

angular distribution arriving at the receiver array. Fig. 8(b)

shows its capacity curve is the lowest when the receiver is

located in a narrow hallway where the various multipath

components arriving at the receiver are concentrated in certain

directions, thus limiting the performance the MIMO system.

Note that the hallway behaves as a waveguide for multipath

propagation [15]. Fig. 8(c) shows its capacity curve is almost

equal to that of the dipole array which has not mutual coupling.

In this case, the receiver array is located in a large room that

allows multipath components to arrive in many directions (high

angular dispersion). Large angular dispersion is one of the

fundamental conditions for the best performance of a MIMO

array [12].

The outstanding results shown here validate the fact that

reducing the mutual coupling greatly increases the performance

of a MIMO system.

(a)

(b)

(d)

(c)



3247

CONCLUSION

In this work, a four-element antenna array with compact

miniaturized patch antennas for MIMO-LTE systems at 2.6

GHz is presented. With the modification of the ground plane

with corrugated slots along with an optimized location and

orientation of the each antenna elements based on the insight

provided by the theory of Characteristic Modes, it was possible

to design and fabricate a MIMO array with very low mutual

coupling and extremely low correlation coefficients.

Furthermore, the radiation characteristics observed in the

proposed array have the ability to overcome propagation

problems of the channel such as multipath fading, line of light

path, and other issues due to polarization and pattern diversity.

Figure 8: Capacity against the SNR. (a) Transmitter at position

#2, and Receiver array at position #1 (LOS), and position #2

(NLOS). (b) Receiver array at position #1 and Tx. at position

#1. (c) Receiver array at position #2 and Tx. at position #1.

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antenna array with low mutual coupling for mimo-lte ...international journal of applied engineering...

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