-
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1,
JANUARY 2014 237
Dual-Band Dual-Polarized Compact Bowtie AntennaArray for
Anti-Interference MIMO WLANWen Chao Zheng, Long Zhang, Qing Xia Li,
Member, IEEE, and Yi Leng, Member, IEEE
AbstractSmart antennas have received increasing interestfor
mitigating interference in the multiple-inputmultiple-output(MIMO)
wireless local area network (WLAN). In this paper, adual-band
dual-polarized compact bowtie dipole antenna array isproposed to
support anti-interferenceMIMOWLAN applications.In the antenna
array, there are 12 antennas, six for horizontalpolarization and
six for vertical polarization. In order to achievedual linear
polarizations and beam switching, six horizontalantennas are placed
in a sequential, rotating arrangement on ahorizontal substrate
panel with an equal inclination angle of 60to form a symmetrical
structure, while the other six antennasfor vertical polarization
are inserted through slots made on thehorizontal substrate panel.
Furthermore, six pairs of meanderedslits are introduced to reduce
the mutual coupling between hori-zontal antennas in the lower band.
A prototype of the array with adimension of 150 150 60 mm is
manufactured and exhibitsthe characteristics of high isolation,
good front-to-back ratio, andaverage gains of 4.5 and 5 dBi over
the 2.4- and 5-GHz band,respectively. The MIMO performance of the
array is analyzedand evaluated by mutual coupling, the total active
reflectioncoefficient (TARC) and the envelope correlation
coefficient. Theanti-interference capability of the array is also
investigated by theexperiment.
Index TermsAntenna array, anti-interference, bowtie, com-pact,
dual-band, dual-polarization, MIMO, WLAN.
I. INTRODUCTION
H IGH data rate and anti-interference capability are
thenecessary characteristics for the wireless local areanetworks
(WLANs). Multiple-inputmultiple-output (MIMO)antenna systems have
attracted considerable interests as aneffective way of improving
the data rate and increasing thechannel capacity in WLANs [1].
However, MIMO WLANsystems suffer severe interference problems when
more andmore wireless access points (APs) have been deployed orthe
number of WLAN users are abruptly increasing [2], [3].Smart antenna
technology is an effective way to overcome thisflaw [4], [5].
Switched beam and adaptive beamforming arrays
Manuscript received February 04, 2013; revised September 03,
2013; ac-cepted October 11, 2013. Date of publication October 24,
2013; date of currentversion December 31, 2013. This work was
supported in part by the NationalNatural Science Foundation of
China under Grants 41176156, 41275032, and61201123.W. C. Zheng, L.
Zhang, and Q. X. Li are with the Science and Technology
on Multi-Spectral Information Processing Laboratory, Huazhong
University ofScience and Technology, Wuhan 430074, China (e-mail:
[email protected]; [email protected];
[email protected]).Y. Leng is with the Department of
Information Counter, Air Force Early
Warning Academy, Wuhan 430019, China (e-mail:
[email protected]).Color versions of one or more of the
figures in this paper are available online
at http://ieeexplore.ieee.org.Digital Object Identifier
10.1109/TAP.2013.2287287
are two main smart antenna technologies. Switched beamarrays
with directional antennas [6], [7] have the advantage ofsimplicity
since several fixed beams could be chosen to reducethe interference
by controlling the state of a number of RFswitches. Compared to the
adaptive beamforming arrays, thesimplicity of the switched beam
arrays makes it suitable forlow-cost, low-power applications in
anti-interference MIMOWLANs.MIMOWLAN antenna arrays with
directional antennas have
been developed [8][11]. In [8] and [9], two different kinds
ofhigh-gain, dual-loop antennas were applied to
three-antennasystems for MIMO AP applications. A printed
YagiUdaantenna with integrated balun was reported, and an MIMOarray
was obtained in a triangular configuration at 5.2-GHzband [10].
Moreover, a two-port dual-band dual-polarizationarray [11] was
proposed for MIMO WLAN. However, theseantenna arrays do not support
beam switching for anti-interfer-ence applications.This paper
presents a dual-band dual-polarized compact
bowtie dipole antenna array for MIMOWLAN, which supportsbeam
switching. In the array, there are 12 antennas, six forhorizontal
polarization and six for vertical polarization. Sixantennas for
horizontal polarization are placed in a sequential,rotating
arrangement on a horizontal substrate panel with anequal
inclination angle of 60 to form a symmetrical struc-ture, while the
other six antennas for vertical polarization areinserted through
slots made on the horizontal substrate panel.Furthermore, six pairs
of meandered slits are introduced toreduce the mutual coupling
between horizontal antennas in thelower band. Each of the 12
antennas comprises two bowties,a director, a microstrip line to
feed the antenna, a widebandtransition from the microstrip line to
a parallel stripline (PSL),and a ground plane.Compared to the
existing MIMO WLAN antenna arrays, the
proposed array has the advantage of compact structure,
duallinear polarizations, high isolation, and anti-interference
capa-bility. The compact structure of the arraymainly results from
thefeeding structure and the dual linear polarizations. In the
array,the high isolation is obtained by two kinds of decoupling
strate-gies, which are meandered silts on the ground and
dual-polar-ization arrangement. In addition, the anti-interference
character-istic of the array is mainly due to the radiation pattern
featuresof the 12 sector antennas.A prototype of the array with a
dimension of
150 150 60 mm is manufactured to support threedata streams
system with beam switching over the 2.4-GHzband and the 5-GHz band.
In Section II, the geometry anddesign consideration of the array
are described. In Section III,advantages of the array are
demonstrated by the measured and
0018-926X 2013 IEEE
-
238 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO.
1, JANUARY 2014
Fig. 1. Coupling between the vertical antennas.
computed parameters, including -parameters, total
activereflection coefficient (TARC), envelop correlation
coefficients,radiation pattern, and signal-to-interference ratio
(SIR). Finally,our conclusion is presented in Section IV.
II. DUAL-BAND DUAL-POLARIZED COMPACT BOWTIEANTENNA ARRAY
The proposed dual-band dual-polarized antenna array con-sists of
12 compact antennas. The dual-band compact bowtieantenna is first
introduced. Then, a whole dual-polarized an-tenna array, which
consists of 12 of the compact antennas, ispresented.
A. Compact Directional AntennaThe single antenna is a
double-bowtie dipole structure. Fig. 1
shows the geometry of the proposed antenna. The antenna
con-sists of two bowties, a director, a microstrip line to feed
theantenna, a transition from the microstrip line to a PSL, and
aground plane. Each bowtie has two printed arms, one on the topand
the other on the bottom of the substrate. The radii (R2 andR3) and
the flare angles of the arms control the resonance fre-quencies.
The benefit of the bowtie shape is to reduce the sizeof the antenna
compared to a normal dipole antenna.The single antenna has a simple
feeding structure. In the
feeding part of the antenna, the top and bottom arms of
bowtiesare connected to the PSL, while the PSL is linked to
themicrostrip line through a wideband transition. This
transitiontapers from the ground plane to the width of the PSL with
amanner of quarter-circles, which has a simple geometry. Thebest
matching can be achieved just by tuning the parameter R1.With this
tapered wideband transition, the proposed antennahas wider
bandwidth and smaller size than other designs withbalun in [10],
[12] because the balun in these designs is alwaysbased on a
half-wavelength delay line, which is designed at thecenter
frequency.Moreover, the directional radiation pattern of the single
an-
tenna is owing to its double quasi-Yagi structures. The
smallerbowtie, which has a quarter-wavelength (at the center
frequencyof the higher band) radius, is considered to be a dipole
workingat higher band. Since the bigger bowtie can act as a
reflector, aquasi-Yagi antenna structure for the higher band is
obtained by
Fig. 2. Antenna prototype.
Fig. 3. Measured reflection coefficients for the single
antenna.
the combination of the smaller bowtie, the bigger bowtie, andthe
director. For the lower band, the quasi-Yagi structure con-sists of
the ground plane, the bigger bowtie with a quarter-wave-length (at
the center frequency of the lower band) radius, and thesmaller
bowtie, which act as the reflector, the driven element,and the
director, respectively. Thus, with the double quasi-Yagistructures,
the antenna has directional pattern in dual bands.A prototype is
manufactured on a 1-mm-thick FR-4 substrate
for 2.4-GHz (24002484-MHz)/5-GHz (51505850-MHz)WLAN. This
low-cost antenna is much smaller than the similardesign in [13].
The dimension of the antenna is 26 60 mmas shown in Fig. 2. W1, W2,
W3, R1, R2, R3, L1, L2, L3,L4, and L5 are 2.50, 2.46, 12.5, 6,
17.2, 6.5, 5, 2, 7, 3, and3 mm, respectively. Fig. 3 shows the
measured reflectioncoefficients of the antenna, which fully covers
the 2.4-GHzband and the 5-GHz band. The bandwidth of the antenna
overthe 2.4-GHz band and 5-GHz band is 5.76% (140 MHz) and17.27%
(960 MHz), respectively.
B. Dual-Band Dual-Polarized Antenna ArrayA compact dual-band
dual-polarized antenna array for anti-
interference MIMO WLAN applications, which supports threedata
streams, is presented. The array comprises six antennas
forhorizontal polarization and six antennas for vertical
polariza-tion as shown in Fig. 4. In order to implement MIMO and
beamswitching flexibly, six antennas (H1H6) are placed in a
sequen-tial, rotating arrangement on the horizontal substrate panel
with
-
ZHENG et al.: DUAL-BAND DUAL-POLARIZED COMPACT BOWTIE ANTENNA
ARRAY FOR ANTI-INTERFERENCE MIMO WLAN 239
Fig. 4. Antenna array geometry.
an equal inclination angle of 60 to form a symmetrical
struc-ture. The other six antennas (V1V6) are inserted through
slotsmade on the horizontal substrate panel. The central space of
thehorizontal substrate panel could be used to construct an RF
cir-cuit. This configuration not only keeps the full broadside
360coverage on the azimuth plane for the WLAN band, but alsooffers
a dual-linear polarization to decrease the correlation be-tween
different streams in MIMO.Since the proposed antenna involves so
many antennas
in such a limited space, it is inevitable to induce the
severemutual coupling problems. Though the dual-linear
polarizationarrangement is an efficient method to decrease the
couplingbetween antennas with different polarization, the
couplingbetween horizontal antennas (H1H6) still needs to be
reduced.The horizontal antennas (H1H6) share the same ground
plane;a certain portion of excited currents will flow to other
antennas,which raises the mutual coupling. Moreover, the distance
be-tween two adjacent horizontal antennas is only approximately
at lower band ( is the wavelength at the centerfrequency of the
lower band in the substrate), which causes thenear-field radiation
coupling. Thus, the isolation between thehorizontal antennas will
be worse if no strategies are appliedhere. The proposed isolation
structure for horizontal antennasat lower band consists of six
pairs of meandered slits. Unlikeother decoupling strategies such as
adding a strip resonatorbetween the antennas [14] or incorporating
a neutralizationline in between the antennas [15], the proposed
slits are easyto achieve high isolation in our array and occupy
little space.Each meandered slit has a configuration as shown in
Fig. 5. Themeandered slits work as radiating slots. It is effective
to makethe surface current at lower band converge around the slits
andthus mitigate the coupling.The specialty of the proposed array
is that it could support the
beam switching technique so as to mitigate interferences in
anMIMO scenario. In the 3 3 (three receive and three
transmitantennas) anti-interference MIMO system we proposed,
thereare numerous possible antenna combinations for MIMO andbeam
switching. For instance, the three groups (H1, H4, V2,V5), (H2, H5,
V3, V6), and (H3, H6, V1, V4) are one kind oftypical selection for
three data streams. In this case, there arefour antennas in one
data stream, two for horizontal polariza-tion and two for vertical
polarization. With the RF switch cir-cuits, proper antennas are
selected so as to direct the beam to
Fig. 5. Meandered slits on ground plane.
Fig. 6. Prototype of the antenna array.
the desired signal and direct sidelobes or nulls to
interferencesto mitigate interferences. A prototype of the array
with a dimen-sion of 150 150 60 mm is shown in Fig. 6. Some
designparameters related to the meandered slits are also described
byFigs. 4 and 5. Detailed discussions of the array with measuredand
simulated results are illustrated in Section III.
III. RESULTS AND DISCUSSION
In this section, the merits of the array are analyzed by
themeasured and computed results. The measured -parameters ofthe
array are firstly presented. Then, the envelope
correlationcoefficient and TARC are calculated to investigate the
potentialMIMO performance of the array. Finally, the radiation
patternand anti-interference performance of the array are
described.
A. Reflection Coefficient
The -parameters of the proposed array are measured by mi-crowave
vector network analyzer E5071C, employing a coaxialcable at the
desired antenna port and connecting the othersto 50- loads. Fig. 7
indicates that the horizontal antennasoperate from 2.32 to 2.54 GHz
in the lower band (220 MHz,9.05% bandwidth) and from 5.00 to 5.85
GHz in the upperband (850 MHz, 15.67% bandwidth). Also, the
bandwidthsof the vertical antennas are 6.21% (150 MHz) from 2.34
to2.49 GHz and 16.07% (900 MHz) from 5.15 to 6.05 GHz, asshown in
Fig. 8. The bandwidth of the array ensures that thesystem could
cover the 2.4-GHz (24002484-MHz) and 5-GHz(51505850-MHz) bands for
WLAN.
-
240 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO.
1, JANUARY 2014
Fig. 7. Reflection coefficients of the horizontal antennas.
Fig. 8. Reflection coefficients of the vertical antennas.
Fig. 9. Coupling between the horizontal antennas.
Fig. 10. Coupling between the vertical antennas.
B. Isolation
Isolations between antennas are important factors for
anti-in-terference MIMOWLAN. Due to the symmetry of the array,
theisolation between any two of the 12 antennas is sufficiently
indi-cated by , , , , , ,
, , and . The measured -parameters inFigs. 911 indicate that all
the isolations between the antennasare above 20 dB.
Fig. 11. Coupling between the horizontal and vertical
antennas.
Fig. 12. Current distribution of the reference array (H1 is
excited).
Fig. 13. Current distribution of the proposed array (H1 is
excited).
Traditionally, the coupling always derives from the
near-fieldradiation and current flow on the ground plane. In an
arraywithout proposed slits, the coupling mainly stems from a
cer-tain portion of the excited surface currents on the ground
plane,especially at 2.4-GHz band for horizontal antennas. When H1is
excited, the behavior of the surface currents on the groundplane in
a reference array (without slits) could be illustrated bythe
current distribution as shown in Fig. 12. From Fig. 12, it
isnoticed that a certain portion of the excited surface current
flowsto other antennas. The coupling caused by this behavior
couldbe effectively reduced by incorporating themeandered slits.
Theproposed meandered slit as shown in Fig. 6 has a total length
of21 mm ( at 2.44 GHz) and a slit width of 0.5 mm. This21-mm-length
ground slit acts as a resonator that is equivalentto a series
resonator. At resonant frequency, the resonatorextracts the
substrate ground current related to mutual coupling.The surface
current distribution on the ground plane of the an-tenna array with
slits is shown in Fig. 13. It is noticed that thecurrent that flows
to other antennas is trapped around the slitswhen H1 is excited.
Therefore, the isolation between two portsis enhanced. In Fig. 14,
it describes results of the coupling be-tween antennaH1 andH2 in an
arraywith andwithout themean-
-
ZHENG et al.: DUAL-BAND DUAL-POLARIZED COMPACT BOWTIE ANTENNA
ARRAY FOR ANTI-INTERFERENCE MIMO WLAN 241
Fig. 14. Coupling between the H1 and H2.
Fig. 15. Coupling between the H1 and H3.
dered slits. It is noticed that the of the array can be as
highas about 14 dB over the 2.4-GHz band in an array withoutslits.
Fig. 15 also describes a comparison of the coupling be-tween the H1
and H3. According to the contrast of the couplingsas shown in Figs.
14 and 15, the highest in-band coupling (at2.4-GHz band) is
approximately 30 dB. Thus, approximately15 dB port isolation is
improved by meandered slits at 2.4 GHzbetween the horizontal
antennas.
C. TARC
The scattering matrix does not accurately characterize
theradiating efficiency and bandwidth of a multiport antennaarray
[16]. TARC must also be considered [17][19]. TARCcan be considered
as a measure of the MIMO array radiationefficiency for a multiport
antenna and accounts for both cou-pling and random signals
combining [16]. We use TARC ratherthan the traditional scattering
matrix to evaluate the radiatingefficiency and bandwidth of an
antenna array. TARC is definedas the ratio of the square root of
total reflected power divided
Fig. 16. Calculated TARC of three antennas with and without
proposed slits.
by the square root of total incident power [16]. The TARC atthe
-port antenna array can be described as
(1)
In the proposed 3 3 anti-interference MIMO array, thereare
numerous combinations for data transmission. Here, threegroups (H1,
H4, V2, V5), (H2, H5, V3, V6), and (H3, H6, V1,V4) are considered
as an example to compute the TARC. ATARC for three antennas (H1, H2
and H3) is computed by (2)at the bottom of the page, where is the
random phase angleof port excitation.The calculated TARC of the
array with and without slits is
shown in Fig. 16. It is noticed that the calculated TARCwith
slitsis lower than 10 dB at 2.4-GHz band and 5-GHz band. How-ever,
the TARC without slits is higher than 10 dB in 2.4-GHzband and in
most of the 5-GHz band. Thus, the computed TARCresults show that
our array has a good radiating efficiency andlow mutual coupling so
as to improve the MIMO performance.
D. Envelope Correlation CoefficientThe correlation coefficient
is an important MIMO perfor-
mance metric, as it quantifies the capability of the MIMOchannel
to provide parallel subchannels, which facilitates goodcapacity
performance. It is associated with the loss of spec-tral efficiency
and degradation of performance of an MIMOsystem [20]. The
correlation coefficient is usually computedfrom radiation patterns.
Considering the complex calculationprocedure, recent research
indicates that for uniform signalpropagation environments, the
correlation coefficient and
(2)
-
242 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO.
1, JANUARY 2014
Fig. 17. Correlation coefficients of the (H1, H2) and (H1,
H3).
Fig. 18. Correlation coefficients of the (H1, V1) and (V1,
V2).
envelop correlation coefficient can be easily derived from
the-parameters [20][22] with the following expression:
(3)
where and denote correlation coefficient and envelop
cor-relation coefficient, respectively.Based on the measured
-parameters, the envelope correla-
tion coefficient of two ports of the array is less than 40 dBin
the band of 2.42.5 GHz, and less than 60 dB in the bandof 5.06.0
GHz. The low envelope correlation coefficients inWLAN band indicate
a good support for MIMO performanceand demonstrate that our
decoupling strategies are effective.In addition, the longer the
distance between two ports, the
lower envelope correlation the results indicate. Thus,
envelopecorrelation coefficients in Figs. 17 and 18 are sufficient
for eval-uating all the envelope correlation coefficients of the
array.
E. Radiation PerformanceThe radiation characteristics of the
array are obtained by the
two-axis dual-polarization pattern measurement system. In
thissection, only the results of one horizontal antenna (H1) and
onevertical antenna (V1) are reported since the array has a
symmet-rical arrangement. The measured and simulated radiation
pat-terns at the operating band center frequency, 2.44 and 5.5
GHz,are shown in Figs. 1926. A good agreement is noticed,
whichfurther verifies the simulation results by HFSS. The
measured3-D radiation patterns are presented in Figs. 2831.
Fig. 19. Measured and simulated radiation pattern at 2.44 GHz in
the E-plane(H1).
Fig. 20. Measured and simulated radiation pattern at 2.44 GHz in
the H-plane(H1).
Fig. 21. Measured and simulated radiation pattern at 5.5 GHz in
the E-plane(H1).
According to the measured results, the front-to-back ratioof the
horizontally and vertically polarized arrays is approxi-mately 12
dB at 2.44 GHz and approximately 15 dB at 5.5 GHz,respectively. The
vertical and horizontal antennas exhibit lowcross-polarization
levels in both 2.44 and 5.5 GHz. The mea-sured results indicate a
good directivity of antennas. Becauseof this directional
characteristic of the array, it is facilitative to
-
ZHENG et al.: DUAL-BAND DUAL-POLARIZED COMPACT BOWTIE ANTENNA
ARRAY FOR ANTI-INTERFERENCE MIMO WLAN 243
Fig. 22. Measured and simulated radiation pattern at 5.5 GHz in
the H-plane(H1).
Fig. 23. Measured and simulated radiation pattern at 2.44 GHz in
the E-plane(V1).
Fig. 24. Measured and simulated radiation pattern at 2.44 GHz in
the H-plane(V1).
make the null or the sidelobe of the radiation pattern direct
tointerferences.The single horizontal or vertical antenna presents
a fixed end-
fired main beam in the -plane. The E-plane beamwidth ofthe
single horizontal or vertical antenna is about 60 , while
theH-plane beamwidth is about 120 . The E-plane beamwidth of
Fig. 25. Measured and simulated radiation pattern at 5.5 GHz in
the E-plane(V1).
Fig. 26. Measured and simulated radiation pattern at 5.5 GHz in
the H-plane(V1).
Fig. 27. Azimuth plane coverage with H1, H4, V2, and V5 at 2.44
GHz.
one of these antennas is narrower than the H-plane. The
nar-rower beamwidth in E-plane can promote distinguishing the
de-sired signal from interferences, while the wider beamwidth
inH-plane could improve the coverage area in elevation plane.The
E-plane radiation patterns of the horizontal antennas andthe
H-plane radiation patterns of the vertical antennas ensurethe
coverage of the whole azimuth plane. For instance, Fig. 27describes
an example of azimuth plane coverage with H1, H4,V2, and V5 at 2.44
GHz. The four antennas cover the whole
-
244 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO.
1, JANUARY 2014
Fig. 28. Measured 3-D radiation patterns at 2.44 GHz for
horizontal antennas.
Fig. 29. Measured 3-D radiation patterns at 5.5 GHz for
horizontal antennas.
Fig. 30. Measured 3-D radiation patterns at 2.44 GHz for
vertical antennas.
Fig. 31. Measured 3-D radiation patterns at 5.5 GHz for vertical
antennas.
-plane with a crossover depth of 3 dB. With the RF
switchcircuits, proper antennas could be selected so as to direct
thebeam to the client to avoid interferences. If the scenario is
in
Fig. 32. Layout of the measurement site.
a building, the proposed array not only covers the floor in
az-imuth plane, but also covers the clients in other adjacent
floorsdue to the wide beamwidth in H-plane of antennas. In
addition,the measured average gains of the array are 4.5 dBi over
the2.4-GHz band and 5 dBi over the 5-GHz band, respectively.
F. Anti-Interference Performance
The performance of anti-interference of the array is
simplyevaluated by an experiment similarly to that in [23]. The
anti-in-terference property of the array is investigated by the SIR
forchanging of the interference direction. The SIR is equivalentto
the interference suppression. If the receiving antenna is
om-nidirectional, the SIR of the receive antenna is 0 dB when
thedesired and interfering signals have equal transmitting
power.Since the proposed array is directional, when the main lobe
ofthe antenna directs to the desired signal and sidelobe or nulls
tointerferences, the SIR will be obviously improved, and the
in-terference is effectively suppressed.An indoor laboratory was
chosen as the measurement envi-
ronment, as shown in Fig. 32. The array was installed at
thecenter of the test site (point A) as a receiving antenna. The
outputsignal of the receiving antenna was obtained by the
spectrumanalyzer linked with a low noise amplifier (LNA). The
desiredand interfering signals coming from various directions were
as-sumed by placing the transmitting antenna around the
receivingarray with a distance of 1 m (point A to point B-G). The
trans-mitting antenna was a horn antenna with linear polarization.
Thetransmitting power of 0 dBm was applied for both of the de-sired
and interfering signals. The directions of the interferencewere
given by the directions of 45 (C), 135 (E), 225 (F), and315
(G).Since there were no switches in the prototype of the array,
the
desired signal was assumed to be in 0 from the point B, and
H1was selected to be a receiving antenna in the array. When
thedesired signal transmitted from B with horizontal
polarization,the received power of H1 was 5.83 and 11.67 dBm at
2.44and 5.5 GHz, respectively.Table I shows the measured received
power of H1 when the
interference came from different directions with different
linearpolarizations. From Table I, it is found when the
interferencewith different polarization was close to the desired
signal suchas in location C or G, at least 6 dB SIR discrepancy is
achieved
-
ZHENG et al.: DUAL-BAND DUAL-POLARIZED COMPACT BOWTIE ANTENNA
ARRAY FOR ANTI-INTERFERENCE MIMO WLAN 245
TABLE IMEASURED RECEIVED POWER OF H1 WHEN THE INTERFERENCE CAME
FROM
DIFFERENT DIRECTIONS WITH DIFFERENT LINEAR POLARIZATIONS
TABLE IIMEASURED RECEIVED POWER OF V2 WHEN THE INTERFERENCE CAME
FROM
DIFFERENT DIRECTIONS WITH DIFFERENT LINEAR POLARIZATIONS
at 2.44 GHz. This discrepancy is mainly attributed to the
hor-izontal polarization characteristic of the H1, which
suppressesthe interference with vertical polarization.When the
interferences were in the backlobe or the sidelobe
areas such as locations E and F, the SIRs were at least 16
dB.The directional characteristic of radiation pattern ensured
thatinterferences were effectively suppressed.Moreover, in another
case, the desired signal was assumed
to be in 90 from the point D with vertical polarization, and
V2was selected to be a receiving antenna in the array. The
receivedpower of V2 was 3.50 and 16.67 dBm at 2.44 and 5.5
GHz,respectively. Table II shows the measured received power
ofinterference and SIR. The main difference between these
twoexperiments is that the former presented a higher SIR when
theinterference was close to the desired signal. The possible
reasonfor this is the difference of radiation pattern in H1s
E-planeand V2s H-plane. Therefore, different quantity of power
willbe received with different gains.
IV. CONCLUSION
A dual-band dual-polarized compact bowtie dipole antennaarray is
proposed for anti-interference MIMO WLAN applica-tions. In the
array, there are 12 antennas, six for horizontal polar-ization and
six for vertical polarization. Six antennas are placedin a
sequential, rotating arrangement on a horizontal substratepanel
with an equal inclination angle of 60 to form a symmet-rical
structure for horizontal polarization, while the other six
an-tennas for vertical polarization are inserted through slots
madeon the horizontal substrate panel. It is flexible to support
MIMOand beam switching.
In order to diminish the size of the antenna array, a
simplefeeding structure is applied to reduce the dimension of the
singleantenna. The feeding part of the antenna comprises a PSL,
awideband transition from the microstrip line to a PSL, and aground
plane. Moreover, in the array, because of the dual
linearpolarizations, more antennas could be arranged in a
compactspace, which also contributes greatly on diminishing the
size ofthe array.Furthermore, six pairs of meandered slits are
introduced to
reduce the mutual coupling between the antennas in the
lowerband. The decoupling mechanism is analyzed with the
currentdistribution of the array. Meanwhile, the dual linear
polariza-tions arrangement is beneficial to reduce the coupling
betweenthe antennas.The anti-interference characteristic of the
array is mainly due
to the radiation pattern performance of the 12 sector
antennas,such as directivity, high front-to-back ratio ( ), and
lowcross-polarization level. Each compact antenna in the arrayhas a
directional radiation pattern due to its double
quasi-Yagistructure. The E-plane beamwidth of one of these
antennasis narrower than the H-plane. The narrower beamwidth
inE-plane is useful for mitigating interferences, while the
widerbeamwidth in H-plane could expand the coverage area inanother
dimension. High of the array is valuable formitigating
interferences.The prototype of the compact array has a dimension
of
150 150 60 mm . The measured results indicate that theantenna
array has the characteristics of wide bandwidth, highisolation,
good front-to-back ratios, and average gains of 4.5and 5 dBi over
the 2.4- and 5-GHz band, respectively. The mea-sured good radiation
pattern characteristic ensures the abilityto fulfill the beam
switching strategy to mitigate interferences.The MIMO performance
of the array is analyzed and evalu-
ated by mutual coupling, envelope correlation coefficient,
andTARC. The measured mutual coupling is at least 20 dB be-tween
any two antennas in the array. Approximately 15 dBport isolation is
improved by meandered slits at 2.4 GHz be-tween the horizontal
antennas, which demonstrates the decou-pling strategy we proposed
is effective to obtain high isola-tion. The calculated envelope
correlation coefficient is less than40 dB in all cases, and the
TARC is around 10 dB in the
desired frequency range. The anti-interference capability is
alsoinvestigated by an indoor experiment. The SIR is efficiently
im-proved when using the proposed array. According to the
exper-iments, the maximum interference suppression is about 35
dB.The proposed antenna array is a possible candidate in
anti-in-
terference MIMO WLAN applications, as well as other MIMOsystems
such as Worldwide Interoperability for Microwave Ac-cess (WiMAX),
Long Term Evolution (LTE), other mobile com-munication systems, and
so on.
REFERENCES[1] D. Gesbert, M. Shafi, D. Shiu, P. J. Smith, and A.
Naguib, From theory
to practice: An overview of MIMO space-time coded wireless
sys-tems, IEEE J. Sel. Areas Commun., vol. 21, no. 3, pp. 281302,
Apr.2003.
[2] E. G. Desautel, D. Kim, J. S. Kenney, and D. Kiesling,
Interferencemitigation in WLAN networks using client-based smart
antennas, inProc. IEEE RAWCON, Boston, MA, USA, 2002, pp. 6366.
-
246 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO.
1, JANUARY 2014
[3] J. A. Park, S. K. Park, D. H. Kim, P. D. Cho, and K. R. Cho,
Ex-periments on radio interference between wireless LAN and other
radiodevices on a 2.4 GHz ISM band, in Proc. IEEE VTC
2003-Spring,Jeju, Korea, 2003, pp. 17981801.
[4] C. Hermosilla, R. Feick, R. Valenzuela, and L. Ahumada,
ImprovingMIMO capacity with directive antennas for outdoor-indoor
scenarios,IEEE Trans. Wireless Commun., vol. 8, no. 5, pp.
21772181, May2009.
[5] T. H. Kim, T. Salonidis, and H. Lundgren, MIMO wireless
networkswith directional antennas in indoor environments, in Proc.
IEEE IN-FOCOM, Orlando, FL, USA, 2012, pp. 29412945.
[6] N. Honma, T. Seki, K. Nishikawa, K. Tsunekawa, and K.
Sawaya,Compact six-sector antenna employing three intersecting
dual-beammicrostrip Yagi-Uda arrays with common director, IEEE
Trans. An-tennas Propag., vol. 54, no. 11, pp. 30553062, Nov.
2006.
[7] M. Lai, T. Wu, J. Hsieh, C. Wang, and S. Jeng, Compact
switched-beam antenna employing a four-element slot antenna array
for digitalhome applications, IEEE Trans. Antennas Propag., vol.
56, no. 9, pp.29292936, Sep. 2008.
[8] S. Su, High-gain dual-loop antennas for MIMO access points
in the2.4/5.2/5.8 GHz bands, IEEE Trans. Antennas Propag., vol. 58,
no. 7,pp. 24122419, Jul. 2010.
[9] S. W. Su and C. T. Lee, Low-cost dual-loop-antenna system
for dual-WLAN-band access points, IEEE Trans. Antennas Propag.,
vol. 59,no. 5, pp. 16521659, May 2011.
[10] A. Capobianco, F. M. Pigozzo, A. Assalini, M. Midrio, S.
Boscolo, andF. Sacchetto, A compact MIMO array of planar end-fire
antennas forWLAN applications, IEEE Trans. Antennas Propag., vol.
59, no. 9,pp. 34623465, Sep. 2011.
[11] J. M. Steyn, J. W. Odendaal, and J. Joubert, Dual-band
dual-polarizedarray for WLAN applications, Prog. Electromagn. Res.
C, vol. 10, pp.151161, 2009.
[12] F. C. Costa, G. Fontgalland, A. G. DAssuncao, T. P. Vuong,
and L.M. Mendonca, A new quasi-Yagi bowtie type integrated antenna,
inProc. Int. Telecommun. Symp., 2006, pp. 468471.
[13] J. M. Steyn, J. W. Odendaal, and J. Joubert, Doubel dipole
antennafor dual-band wireless local area networks
applications,Microw. Opt.Technol. Lett., vol. 51, no. 9, pp.
20342038, Sep. 2009.
[14] T. W. Kang and K. L. Wong, Isolation improvement of
2.4/5.2/5.8GHz WLAN internal laptop computer antennas using
dual-band stripresonator as a wavetrap, Microw. Opt. Technol.
Lett., vol. 52, no. 1,pp. 5864, Jan. 2010.
[15] S. W. Su, C. T. Lee, and F. S. Chang, Printed MIMO-antenna
systemusing neutralization-line technique for wireless USB-dongle
applica-tions, IEEE Trans. Antennas Propag., vol. 60, no. 2, pp.
456463,Feb. 2012.
[16] M. Manteghi and Y. Rahmat-Samii, Multiport characteristics
of awide-band cavity backed annular patch antenna for
multipolariza-tion operations, IEEE Trans. Antennas Propag., vol.
53, no. 1, pp.466474, Jan. 2005.
[17] D. W. Browne, M. Manteghi, M. P. Fitz, and Y. Rahmat-Samii,
Ex-periments with compact antenna arrays for MIMO radio
communica-tions, IEEE Trans. Antennas Propag., vol. 54, no. 11, pp.
32393250,Nov. 2006.
[18] S. H. Chae, S. K. Oh, and S. O. Park, Analysis of mutual
coupling, cor-relations, and TARC in WiBro MIMO array antenna, IEEE
AntennasWireless Propag. Lett., vol. 6, pp. 122125, 2007.
[19] S. Zhang, B. K. Lau, Y. Tan, Z. Ying, and S. He, Mutual
couplingreduction of two PIFAs with a T-shape slot impedance
transformer forMIMO mobile terminals, IEEE Trans. Antennas Propag.,
vol. 60, no.3, pp. 15211531, Mar. 2012.
[20] S. Blanch, J. Romeu, and I. Corbella, Exact representation
of antennasystem diversity performance from input parameter
description, Elec-tron. Lett., vol. 39, no. 9, pp. 705707, May
2003.
[21] P. Hallbjorner, The significance of radiation efficiencies
when using-parameters to calculate the received signal correlation
from two an-tennas, IEEEAntennasWireless Propag. Lett., vol. 4, pp.
9799, 2005.
[22] J. Thaysen and K. B. Jackobsen, Envelope correlation inMIMO
antenna array from scattering parameters, Microw. Opt.Technol.
Lett., vol. 48, no. 5, pp. 832834, May 2006.
[23] P. Ngamjanyaporn, C. Phongcharoenpanich, P. Akkaraekthalin,
andM. Krairiksh, Signal-to-interference ratio improvement by using
aphased array antenna of switched-beam elements, IEEE Trans.
An-tennas Propag., vol. 53, no. 5, pp. 18191828, May 2005.
Wen Chao Zheng received the B.S. degree in elec-trical
engineering from Xian institute of Posts andTelecommunications,
Xian, China, in 2008, and theM.S. degree in electrical engineering
from WuhanResearch Institute of Posts and Telecommunications,Wuhan,
China, in 2011, and is currently pursuing thePh.D. degree at
Huazhong University of Science andTechnology, Wuhan, China.His
research interests include microwave remote
sensing and multiband antenna design.
Long Zhang received the B.S. degree in communica-tion
engineering and M.S. degree in electromagneticfields and microwave
technology from HuazhongUniversity of Science and Technology,
Wuhan,China, in 2009 and 2012, respectively.He is currently a
Research Assistant with the
Science and Technology on Multi-Spectral Informa-tion Processing
Laboratory, Huazhong Universityof Science and Technology. His
research interestsinclude smart antenna, mobile terminal
antennas,and microstrip antenna array.
Qing Xia Li (M08) received the B.S., M.S.,and Ph.D. degrees in
electrical engineering fromHuazhong University of Science and
Technology,Wuhan, China, in 1987, 1990, and 1999, respectively.He
is presently a Professor with the Science
and Technology on Multi-Spectral InformationProcessing
Laboratory, Department of Electronicsand Information Engineering,
Huazhong Universityof Science and Technology. His research
interestsinclude microwave remote sensing and deep
spaceexploration, electromagnetic theory and application,
antenna array, and signal processing.
Yi Leng (M13) received the B.S. degree in elec-tronic
engineering from the National University ofDefense Technology,
Changsha, China, in 1999,and the Ph.D. degree in electronic science
andtechnology from Huazhong University of Scienceand Technology,
Wuhan, China, in 2008.He is currently a Research Associate and
Vice
Director of the Electromagnetic Engineering Re-search Center,
Air Force Early Warning Academy,Wuhan, China. His current research
interests includemicrowave antennas, wireless communication,
and
electromagnetic countermeasure.