1. International Journal of Electronics and Communication Engineering & Technology (IJECET),
INTERNATIONAL JOURNAL OF ELECTRONICS AND
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Special Issue (November, 2013), pp. 134-139
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IJECET
©IAEME
A Pattern Diversity Compact MIMO Antenna Array Design for WLAN
Application
Prof. S S Khade1, Dr. S L Badjate2
1Dept.
2Dept.
of Electronics and Telecommunication Engg., Y.C.C.E, Nagpur, India
of Electronics and Telecommunication Engg., S.B.J.I.T., Nagpur, India
1sac_mob@rediffmail.com, 2
s_badjate@rediffmail.com
ABSTRACT: In this paper, design of multiple-input multiple-output (MIMO) arrays is presented
for WLAN applications. The planar directive antenna is of interest in the proposed work. To
enhance channel capacity in rich multipath environment low mutual coupling among radiating
elements is necessary. To achieve low mutual coupling among radiating elements, we need
orthogonal patterns. In this paper, printed Yagi-Uda antenna with an integrated balun is
presented and MIMO array is obtained by placing three printed Yagi-Uda antenna in a
triangular configuration to achieve orthogonal patterns. The proposed MIMO array is designed
on FR4 substrate. The proposed array exhibit very low mutual coupling less than -20 dB in the
impedance bandwidth ranging around 4.8 GHz to 6.2 GHz.
KEYWORDS: Multiple-input multiple-output system, multipath channels, Printed Yagi antenna,
directive antenna, integrated balun, wireless local area network (WLAN).
I.
INTRODUCTION
MULTIPLE-INPUT–MULTIPLE-OUTPUT (MIMO) wireless systems, characterized by multiple
antenna elements at the transmitter and receiver, have demonstrated the potential for
increased capacity in rich multipath environments. Such systems operate by exploiting the
spatial properties of the multipath channel, thereby offering a new dimension which can be
used to enable enhanced communication performance. The technology figures prominently on
the list of recent technical advances with a chance of resolving the bottleneck of traffic capacity
in future Internet-intensive wireless networks. Perhaps even more surprising is that just a few
years after its invention the technology seems poised to penetrate large-scale standards-driven
commercial wireless products and networks such as broadband wireless access systems,
wireless local area networks (WLAN), third generation networks (3G) and beyond.
In wireless environment, signals are scattered from various structures and reach the receiving
terminal from any unpredicted direction. An omnidirectional antenna not only receives signal
from all directions but also receives noise from all directions. A directional antenna receives
noise only from a particular direction, resulting in better communication. Also, for WLAN APs,
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
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2. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
directional and end fire antennas are promisingly suitable. Also for outdoor-indoor scenarios
directive antennas are used to enhance MIMO capacity [1].
The performance of the MIMO antenna is studied by considering some parameters. Mutual
coupling is one of the important parameter because higher mutual coupling means lower
antenna efficiency, correlation coefficient and total active reflection coefficient are another
parameters [2].To achieve low mutual coupling among radiating elements in a MIMO array,
diversity techniques are used. There are essentially three antenna diversity techniques
commonly employed in MIMO array designs which are space, polarization, and pattern
diversity [3-6]. In typical MIMO systems, size and cost constraints often prevent the antennas
from being placed far apart therefore space-diversity techniques may be insufficient for next
generation wireless handsets. Therefore pattern diversity technique is considered
advantageous over other techniques. To exploit pattern diversity, the antennas are designed to
radiate with orthogonal radiation patterns as a means to create uncorrelated channels across
different array elements. There are many designed to achieve pattern orthogonality for
example by exciting orthogonal modes within the same geometrical structure in co-located
patch antennas [7] or in spirals [8], pattern synthesis based on properly tailored current
distribution in a theoretical array [9], or reconfigurable planar arrays made of combined
Landstorfer and Yagi-Uda antennas [10].
In this paper, to exploit pattern diversity or to achieve orthogonal patterns, a very simple and
easy to implement design has been developed. In particular, the printed Yagi-Uda antenna with
an integrated balun has been designed and MIMO array has been obtained in a triangular
configuration. The proposed array has been designed for WLAN application.
II.
SIMULATED ANTENNA STRUCTURE
A. Single Printed Yagi-Uda Antenna
The printed Yagi Uda antenna with an integrated balun was designed on both sides of 27.6 mm
x 24 mm FR4 with a dielectric constant 4.4 and thickness of 1.6mm as shown in fig. 1. FR4 is
more available and much cheaper than any other PCB material and it has good parameters for
most applications, mechanical, electrical & climatic stabilities and even economical aspects too.
The antenna comprises of an integrated balun feeding, two printed dipoles, a parasitic strip
and a ground plane. The two printed dipoles, the larger dipole and the smaller dipole, act as a
reflector and driver respectively. The parasitic strip in the proposed structure acts as a
director.
A printed Yagi-Uda antenna has been presented in [11] where a broadband Microstrip-tocoplanar strips (CPS) transition is employed. The ground plane below the transition acts also
as the Yagi-Uda reflector. Here we construct different Microstrip-to-CPS transition with a
shaped ground for optimization of the reflector element. The shaped ground also allows
reduction of the metallization near the feeding point. The entire structure is excited by
proximity coupling, using an open ended microstrip above a rectangular hole in the ground
plane. We used a 50Ω impedance microstrip line on a back side of substrate while the coplanar
stripline was designed on the front side of the substrate with a characteristic impedance of
100Ω. The antenna was simulated using CADFEKO software.
The dimensions of the proposed antenna are as described below. The length of substrate (ls)
and width of substrate (ws) is 27.6 mm and 24 mm respectively. Thus the proposed antenna is
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
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3. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
compact in size. The length of driver (l1) is optimized to 17 mm which is responsible for
resonant frequency and bandwidth response of proposed Yagi – Uda antenna. The length of
director (l3) value is adjusted to 11.5 mm in order to reach high directivity. The length of
reflector (l1) and finite ground plane length (lg) and width (wg) values are 21.5 mm, 6.2 mm
and 9 mm respectively which improves back lobe suppression which results in high directivity.
The ground plane extension (h1) value is 2.6 mm. The width of director (w0), driver (w1) and
reflector (w2) is 2 mm. Separation between reflector and driver (d2) and separation between
director and driver (d3) is adjusted to 4.5 mm and 5.5 mm respectively. Length of rectangular
slot (a1) and its width (b1) values are 4 mm and 1 mm respectively. The distance (d1) of slot
from finite ground plane is 5 mm. The ratio of transition (a2/b2) is maintained to 0.5 mm and
magnitude of cut “c” is 1 mm which is also responsible for bandwidth response of proposed
antenna. The impedance matching was realized by acting again on the ratio of the transition
and on the driven element-to-director distance.
(a)Top View
(b) Bottom View
Fig. 1: Proposed single antenna design
B. Array Configuration
To achieve orthogonal patterns, MIMO array is obtained by placing three printed Yagi-Uda
antenna in a triangular configuration as shown in fig. 2. The ground plane extension of single
printed antenna is delimited by a Y-shaped slot whose branches are spaced by 120 degrees,
which enhance isolation among radiating elements in an array since each antenna in an array
configuration is having separate ground plane.
(a) Top View
(b) Bottom View
Fig. 2: Proposed antenna array design obtained by rotating each printed antenna by 120
degree
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
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4. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
III.
RESULTS AND DISCUSSIONS
For a proposed antenna array we got upper WLAN band operation at 5 GHz ranging from 5.15
GHz – 5.825 GHz. The return loss plot of each antenna in an array configuration is below -10 dB
in the interested frequency band. The value of mutual coupling is low in an interested
frequency range due to separate ground plane of each antenna in an array configuration. It is
less than -20 dB in an impedance bandwidth of proposed array. Thus the proposed antenna
array is designed and simulated for upper band WLAN application.Simulated results are
visualized in the following figures.
Fig. 3: Combined Return loss plot for array
The performance of all three antennas of a MIMO Array is nearly same. Bandwidth range
obtained is from 4.8 GHz to 6.2 GHz which covers upper WLAN band ranging from 5.15 GHz to
5.825 GHz as shown in fig. 3. The operating frequency is 5.69 GHz.
Fig. 4: VSWR graph of a MIMO Array
Fig. 5: Impedance graph of a MIMO Array
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B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
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5. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
VSWR is in between 1 and 2 in the impedance bandwidth and impedance is around 50 Ώ in the
bandwidth obtained for an array, as shown in fig. 4 and 5 respectively.
Fig. 6: Plot representing transmission coefficients for array
The mutual coupling between the radiating elements should be -10 dB below for good
performance of an array. The value of mutual coupling is less than -20 dB in the impedance
bandwidth of an array, as shown in figure 6.
Fig. 7: Gain (3D View) of an array
Fig. 8: Directivity plot (2D View) of a MIMO Array
Gain of a MIMO array is around 2.29 dBi at operating frequency as shown in figure 7, where red
colour indicates maximum value of gain in the particular direction. Directivity of an MIMO
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
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6. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
array is around 2.97 dBi at the operating frequency. Figure 8 represents polar plot of an array,
showing directivity.
IV.
CONCLUSION
In this paper, pattern diversity is exploited to achieve orthogonal patterns. Here, printed YagiUda antenna is of interest since it is directive antenna and has many advantages. Here, we have
designed MIMO array by placing three printed antennas in an equilateral triangular
configuration to achieve isolation among radiating elements. The proposed array is of size 60
mm x 55 mm and thus compact in nature. The proposed array has been designed for WLAN
application and it has low cost. The proposed antenna array is used for indoor applications due
to its compact size. Also the proposed antenna design principle can be extended to arrays
having a large number of antennas due to compact size of antenna.
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International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
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