Multi element antenna systems for diversity and MIMO terminal devices
M. Karaboikis, C. Soras, G. Tsachtsiris, V. Papamichael and V. Makios
Laboratory of Electromagnetics, Department of Electrical and Computer Engineering,
University of Patras, Patras, Greece
Abstract – Compact multi element antenna systems for diversity and MIMO terminal devices operating at
the 5.2 GHz ISM band under combining schemes are presented. The diversity performance is evaluated by
means of the calculated envelope correlation coefficient, the mean effective gain and the effective
diversity gain of the antennas. It is shown that by increasing the number of antenna elements the diversity
gain increases in an asymptotic manner, which is attributed to the reduction of the antennas’ efficiency.
1.
Introduction
Antenna diversity and Multiple-Input Multiple-Output (MIMO) systems have come to counterpoise
the limitations introduced by multipath propagation improving the reliability and increasing the capacity
of the wireless channel [1]. In such wireless terminals, multi element antennas (MEA) are envisaged and
the restricted space available for the antennas is an open issue [2].
In this work MEA systems incorporating up to six elements for wireless handheld devices are
addressed. The antenna elements employed are the inverted F and the Minkowski monopole antenna [3,4],
which have been miniaturized using the device’s ground plane, fractal concepts and short circuit
techniques. The diversity performance evaluation of the proposed MEA systems is conducted by means of
calculating the envelope correlation coefficient [1], the relative mean power levels between the signals
delivered from each antenna branch through the concept of mean effective gain MEG [5] and the effective
diversity gain [6].
2.
Multi element antenna diversity systems
The geometry and dimensions of the investigated configurations are depicted in Figure 1. They have
the dimensions of a PC card while the dimensions of the ground plane are 46 by 89 mm. Each of the
proposed systems consists of two 35 µm thick copper layers with the antennas placed at the upper one and
the ground plane at the bottom. The antennas are placed on an 8 mils-thick substrate, with εr=3.38 and
tanδ=0.0027. In every case the monopoles are placed on the edge of the device’s ground plane and are
terminated to 50-Ω ports.
2
1
2
3
~
~
~
~
1
89mm~
~
~
~
Ground Plane
46 mm
3
1
4
3
4
2
~
~
2
1
5
~
~
~
~
~
~
3.2 mm
5
6
4
3
5.5 mm
2
1
11.7mm
0.5mm
~
~
~
~
1.5
1.8mm
feed
short
Figure 1. The layouts of the five diversity configurations.
The configurations depicted in Figure 1 were simulated using IE3D, a commercial Method of
Moments based electromagnetic field solver [7]. In each case the antennas exhibit large bandwidth and are
well tuned for the 5.2 GHz ISM band (5.15 – 5.35 GHz) as shown in Figure 2, where the Sii parameters of
the six-element case are illustrated. The values of the envelope correlation coefficient ρe shown in Figure
3a indicate that all the antenna branches receive uncorrelated signals since ρe is far below 0.5. This is
attributed to the mutual coupling between the closely spaced antenna elements, which plays a key role to
the decorrelation mechanism as reported in [1]. The MEG values of each antenna element do not exhibit
large differences (2 dB maximum), as depicted in Figure 3b, indicating that the mean power delivered
from each antenna branch is nearly the same. Additionally, it can be noticed that the MEG of the elements
decrease as the number of antennas increases, which is the major drawback of MEA systems.
0
Sii parameters (dB)
-5
-1 0
-1 5
-2 0
S
S
S
S
S
S
-2 5
-3 0
-3 5
1
2
3
4
5
6
1
2
3
4
5
6
-4 0
4 . 5 4 .6 4 . 7 4 .8 4 . 9 5 .0 5 . 1 5 .2 5 . 3 5 .4 5 . 5 5 .6 5 . 7 5 .8 5 . 9 6 .0
F re q u e n cy (G H z)
0.30
0
-1
0.25
-2
-3
0.20
MEG (dBi)
Envelope Correlation Coefficient (-)
Figure 2. Simulated Sii parameters of the six-element diversity system.
0.15
0.10
-4
-5
-6
-7
0.05
-8
0.00
-10
-9
1
2
3
4
5
Number of elements
6
7
1
2
3
4
5
6
7
Number of elements
(a)
(b)
Figure 3. Simulated envelope correlation coefficients and mean effective gain values of the systems (some values are overlapping)
The fulfillment of the diversity criteria (ρe < 0.5 and MEG ratio < 3 dB) ensure that diversity gain is
achieved and Figure 4a depicts the effective diversity gain at 1 % outage probability under maximum ratio
combining with the six-element case illustrating the highest value (15.1 dB). It is obvious that as the
number of antennas increases, the effective diversity gain also increases in an asymptotic manner. The
saturation behavior of the diversity gain for the investigated configurations becomes evident after the 5element case, where the increase in gain becomes negligible (0.1 dB). This asymptotic behavior can be
attributed to the decrease of the antennas’ efficiency illustrated in Figure 4b, where values far below 60%
16
100
15
90
14
80
13
Efficiency (%)
Effective diversity gain (dB)
(six element case) are observed.
12
11
10
9
70
60
50
40
8
30
2
3
4
5
Number of elements
6
1
2
3
4
5
6
7
Number of elements
(a)
(b)
Figure 4. (a) Simulated effective diversity gain at 1% outage probability under maximum ratio combining and (b) efficiency of the
antenna elements for the different diversity configurations.
3.
Conclusion
Multi element antenna systems operating in the 5.2 GHz ISM band using the Inverted F and the
Minkowski monopole antennas were presented. The diversity criteria are easily fulfilled and the maximum
effective diversity gain achieved is 15.1 dB. The observed asymptotic behavior of the diversity gain is
attributed to the reduction of the antennas’ efficiency, which occurs when more elements are employed in
the same physical space.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
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Zeland Software Inc., IE3D, http://www.zeland.com/