Research ArticleDesign of Compact 4 times 4 UWB-MIMO Antenna withWLAN Band Rejection

Nguyen Khac Kiem1 Huynh Nguyen Bao Phuong1 and Dao Ngoc Chien2

1 Hanoi University of Science and Technology Hanoi 10000 Vietnam2Ministry of Science and Technology Hanoi 10000 Vietnam

Correspondence should be addressed to Nguyen Khac Kiem kiemnguyenkhachusteduvn

Received 21 January 2014 Revised 9 June 2014 Accepted 15 June 2014 Published 8 July 2014

Academic Editor Joonhyuk Kang

Copyright copy 2014 Nguyen Khac Kiem et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

A compact 4 times 4 UWB-MIMO antenna with rejectedWLAN band employing an electromagnetic bandgap (EBG) structure is pre-sented in this paperTheMIMOantenna is electrically small (60mmtimes 60mm) printed on a FR4 epoxy substrate with the dielectricconstant of 44 and a thickness of 16mm A mushroom-like EBG structure is used to reject the WLAN frequency at 55 GHz Inorder to reduce the mutual coupling of the antenna elements a stub structure acting as a bandstop filter is inserted to suppress theeffect of the surface current between elements of the proposed antenna The final design of the MIMO antenna satisfies the returnloss requirement of less than minus10 dB in a bandwidth ranging from 273GHz to 1068GHz which entirely covers UWB frequencyband which is allocated from 31 to 106GHzThe antenna also exhibits aWLAN band-notched performance at the frequency bandof 536ndash634GHz while the values of all isolation coefficients are below minus15 dB and the correlation coefficient of MIMO antenna isless than minus28 dB over the UWB range A good agreement between simulation and measurement is shown in this context

1 Introduction

Recently a multi-input multi-output (MIMO) system hasbeen proposed [1 2] This system increases channel capacityallowing several users to access to various services at the sametime Since the approval by the Federal CommunicationsCommission (FCC) [3] that the ultrawideband (UWB) tech-nology operates from 31 to 106GHz the technology findsitself in a great number of wireless applications Furthermorethe UWB technology in combinationwithMIMO techniqueshas proven to be a solution for the limitation of short-range communications which requires devices to transmitextremely low power [4] In such a system its antennas aredesigned to ensure that the isolation among their elementsshould be less than minus15 dB One solution to this problem isthat antenna elements are placed apart from the others byat least half a wavelength of the lowest operating frequencyhowever this leads to an increase in dimensions of MIMOantennas Therefore antenna dimensions and its isolationcoefficients are two major criteria to consider in the MIMOantenna design

In order to reduce mutual coupling between MIMOantennas while maintaining their small electrical lengthdifferent approaches have been introduced in numerousdesign [4ndash8] In [4] an inverted Y-shaped stub is insertedinto the ground plane between two elements of a UWB-MIMO antenna Two novel bent slits are etched into theground plane in [7] At lower frequencies the bent slitsreduce the mutual coupling and have a slight effect on thereflection coefficient At higher frequencies the slits work asslit antennas widening the impedance bandwidth becausethe two slits are coupled fed by two 50Ohmmicrostrip linesrespectively Two triangles are cut from the ground planeIn this way the reflection coefficient and isolation of thetwo slit antennas can be improved Moreover in order toenhance isolation and increase impedance bandwidth twolong protruding ground stubs are added to the ground planeon the other side and a short ground strip is used to connectthe ground planes of the two planar monopoles together toform a common ground in [8]

The majority of those antennas are 2 times 2 MIMO-UWBantenna [4 7 8] which has not enhanced so much the quality

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2014 Article ID 539094 11 pageshttpdxdoiorg1011552014539094

2 International Journal of Antennas and Propagation

X

Y

Z

hsub

Rpatch

Lsu

b

Lgn

d

Wsub

TopSubstrate

Bottom

Lf

Wf

(a)

TopSubstrate

BottomVia

webg

g2r

(b)

CC

CC

C

C

L

L

+ +

minus

minus

(c)

Figure 1 Proposed UWB antenna (a) without EBG structures and (b) with EBG structures and (c) equivalent circuit of WLAN notchedbased on EBG structures

(a)

lm

wm

lc

ls

wt

wt2

(b)

Figure 2 Proposed MIMO antenna (a) initial MIMO antenna and (b) final MIMO antenna with MMR stub structure Detailed dimensionsin mm 119897

119898= 12 119908

119898= 05 119897

119888= 65 119897

119904= 55 and 119908

119905= 10

of the communication channels In addition some designswere not able to operate in the entire UWB band allocated bythe FCC [9ndash12] On the other hand a four-element MIMOantenna with better isolation by introducing discontinuitiesbetween elements and the systemgroundplanewas presentedin [5] The obtained results show isolation lower than 20 dBHowever the system only works over the frequency range of20ndash60GHz

In this paper a compact 4 times 4MIMO-UWB antenna withWLAN-notched characteristic is presented The proposedantenna shows isolation less than minus15 dB over its ultrawideoperating frequency band ranging from 273 to 1068GHzand a rejection at the WLAN band of 536ndash604GHz Theunique feature of this design is that the mutual coupling canbe reduced by adjusting the length ofMMR stub correspond-ing to a quarter wavelength of frequency which providesthe lowest coupling coefficient Moreover the notched bandbased onEBG structures helps tomaintain the same radiationpatterns at higher frequency rather than using slots cut on thepatches [13]

The rest part of this paper is organized as follows InSection 2 detailed designs of the UWB antennas without and

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

55mm65mm

75mm85mm

0

S 11

(dB)

Figure 3 Simulated 11987811

of the single UWB antenna without EBGstructures for different parameters of 119877patch

International Journal of Antennas and Propagation 3

Table 1 Design specifications for UWB antennas

Antenna type Design parameters (mm)UWB antenna without EBGs 119871 sub = 30119882sub = 30119882

119891= 3 119877patch = 8 119871

119891= 13 119871gnd = 15

UWB antenna with EBGs 119871 sub = 30119882sub = 30119882119891= 3 119877patch = 8 119871

119891= 13 119871gnd = 15 119908ebg = 62 119892 = 07 119903 = 05

per m]Jsurf [A

30000e + 00127911e + 00125822e + 00123734e + 00121645e + 00119556e + 00117467e + 00115379e + 00113290e + 00111201e + 00191124e + 00070237e + 00049349e + 00028461e + 00075738e minus 001

(a)

Jsurf [A per m]

27911e + 00125822e + 00123734e + 00121645e + 00119556e + 00117467e + 00115379e + 00113290e + 00111201e + 00191124e + 00070237e + 00049349e + 00028461e + 000

30000e + 001

75738e minus 001

(b)

Figure 4 Current distribution on single UWB antenna without EBGs at (a) 4GHz and (b) 9GHz

with EBGs are presented The proposed MIMO antenna isthen introduced in both cases of initial and final design Thesimulated andmeasured results are shown in Section 3 whilesome conclusions are provided in Section 4

2 Design of 4 times 4 MIMO-UWB Antenna

In this work the design of the antenna is divided into twoparts In the first part an antenna is designed for UWBfrequencies ranging from 31 GHz to 106GHz Afterwardsthis antenna is integrated with the mushroom-like EBGstructure to provide WLAN band-notched characteristic Inthe second part the four notched single antennas are utilizedas elements to form a 4 times 4 MIMO antenna Finally the stubstructure is implemented to diminish the mutual coupling ofthe antennas

21 Design ofUWBAntenna Thetwo configurations ofUWBantennawithout andwith EBG elements are shown in Figures1(a) and 1(b) respectively The antenna consists of a circularradiating patch excited by a 50Ω microstrip feed line (119882

119891=

3mm) On the other side of the substrate the ground planewith a length of 119871gnd covers the section of the microstrip fedlineThe antennas are printed on a FR4 epoxy substrate withthe dielectric constant of 44 and a thickness of 16mm

The antenna integrated with EBG structures is shown inFigure 1(b) Here the microstrip feed line is placed betweentwo pairs of EBG cells which are designed to act as stopbandfiltersWith the intent of prohibitingWLAN frequency banda pair of EBG cells is inserted rather than only using one cellThe equivalent-circuit model of WLAN notched based onEBG structures also is shown in Figure 1(c) The capacitance119862119862denotes the coupling between EBG and microstrip line

The capacitance 119862 is due to the voltage gradient between the

patch and ground plane while the inductance 119871 is generatedby the current flowing through the shorting via Thereforethe center resonant frequency is 119891

119862= 12120587radic119871(119862 + 119862

119862) and

the width of the stopband increases with the rise of 119862119862or

the decrease of the distance between feed line and EBG cellsThe dimensions of the EBG cells the gap between EBG cellsand the microstrip line are optimized to have band rejectionfrom5 to 6GHz corresponding toWLAN frequencies All thedimensions of these antennas are summarized in Table 1

22 Design of 4 times 4 MIMO-UWB Antenna In this designthe four UWB antennas with EBG structures are rotatedclockwise and placed orthogonally to each other to form the4 times 4 MIMO antenna which has an overall size of 60mmtimes 60mm The layout of the MIMO antenna is shown inFigure 2(a)

With the aim of reducing the mutual coupling in theinitial MIMO antenna a stub structure is adopted to formthe final antenna depicted in Figure 2(b) The design ofthis structure was based on the principle of the microstripmultimode resonator (MMR) [14 15] These stubs are placedbetween the antennas and connected to each other by asquare placed at the center of theMIMOantennaThemutualcoupling between the antennaswill be investigated for findingthe range of frequency in which the isolation coefficients arenot lower than minus15 dB The length 119897

119898of the MMR stub is

then set at a value that is approximately equal to a quarterwavelength at the defined frequency

3 Results and Discussions

This part will discuss the performance of the proposedantennas through both simulation and measurement results

4 International Journal of Antennas and Propagation

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 5 Simulated radiation patterns of the UWB antenna without EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 120

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

Figure 6 Simulated peak gain of the single UWB antenna withoutEBGs

31 UWB Antenna The simulated 11987811

results of the UWBantenna without EBG structure with different values of 119877patchare shown in Figure 3 It can be seen that the bandwidth ofthe antenna defined by the 119878

11less thanminus10 dB entirely covers

the UWB frequency range The decrease of 119877patch causes theincrease of the impedance mismatching at lower frequencyrange The optimized bandwidth is obtained when the 119877patchis chosen as 8mm

Current distributions of the antenna at certain frequen-cies are exhibited in Figure 4 As observed in Figure 4(a) thecurrent distribution at 4GHz proves that the antenna oper-ates at its fundamental mode while the current distribution

Jsurf [A per m]

83217e + 00177275e + 00171333e + 00165390e + 00159448e + 00153505e + 00147563e + 00141621e + 00135678e + 00129736e + 00123793e + 00117851e + 00111909e + 00159663e + 00023907e minus 002

Figure 7 Surface current distribution on UWB antenna with EBGstructures at 55 GHz

at 9GHz in Figure 4(b) indicates the higher resonant modewhich is corresponding to the third order harmonic Hencethe antenna can work over the broad range of UWB

Typical radiation patterns of the antenna at 4GHz and9GHz in 119909119911-plane are plotted in Figures 5(a) and 5(b)respectively The solid lines display the 119864

120593 and the dotted

lines represent 119864120579 From Figure 5(a) the antenna possesses a

dipole-like radiation pattern confirming its operation in thefundamental resonantmodeThe pattern at higher frequencyfrom Figure 5(b) corresponds to the harmonics of the fun-damental resonant mode which are closely spaced [16] Thesimulated peak gain of the antenna is depicted in Figure 6in which the higher frequencies provide larger antenna gainThis is a good agreement with the theory

International Journal of Antennas and Propagation 5

2 4 6 8 10 12Frequency (GHz)

minus30

minus20

minus10

05mm06mm

07mm08mm

0S 1

1(d

B)

(a)

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

0

5mm6mm

7mm8mm

S 11

(dB)

(b)

Figure 8 Simulated 11987811of the single UWB antenna integrated with EBG structures for different parameters of (a) 119892 and (b) 119908ebg

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 9 Simulated radiation patterns of the UWB antenna integrated with EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

The effect of EBG structures onUWBantenna is shown inFigure 7 At the center frequency of WLAN band (55 GHz)the current distribution on antenna mainly focuses on theEBG structures and therefore the patch antenna will notradiate resulting in a notched frequency band

In order to obtain the desired notched band parametricstudies on the dimension of EBG cell 119908ebg and the distance 119892between EBG cells and the feed line are investigated Figure 8shows the simulated 119878

11of antenna for different values of the

gap 119892 In Figure 8(a) the width of the notched band reducedwhen the value of gap 119892 increases while the simulated 119878

11

of antenna for different values of EBG size 119908ebg is alsodepicted in Figure 8(b) It should be noticed in Figure 8(b)that the frequency of the notched band also reduces with theincreasing value of 119908ebg Therefore the notched-band tuningrequires a combination of both values

Figures 9(a) and 9(b) show the radiation pattern of theproposed UWB antenna at 4GHz and 9GHz respectively

6 International Journal of Antennas and Propagation

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

minus2

minus4

UWB range (31ndash106GHz)

Figure 10 Simulated peak gain of the single UWB antenna inte-grated EBGs

Figure 11 Fabricated UWB integrated EBGs antenna and finalMIMO antenna

0WLAN

minus30

minus25

minus20

minus15

minus10

minus5UWB range (31ndash106GHz)

2 4 6 8 10 12Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 12 Simulated and measured results of 11987811

of the UWBintegrated EBGs with WLAN band notched antenna

minus30

minus20

minus10

S11S22

S33S44

2 4 6 8 10 12

0

Retu

rn lo

ss (d

B)

Frequency (GHz)

(a)

minus50

minus40

minus30

minus20

minus10

0Is

olat

ion

(dB)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 13 Simulated results of the initial MIMO antenna (a) returnloss and (b) isolation coefficient

The radiation patterns at high frequency of the antenna withand without EBGs are of small variation due to the lack ofetching or cutting on the surface of the patch antenna

The simulated peak gain versus frequency is shown inFigure 10 It reveals a significant drop in the peak gain withinthe rejected bands when the antenna employs the EBGcells A relatively stable peak gain remains over the UWBbandwidth except a 5 dB gain drop in the notched band

Finally the fabrication of the proposed antenna is pre-sented in Figure 11The simulated andmeasured results of 119878

11

of the UWB antenna with EBG cells are shown in Figure 12From this figure it is observed that the antenna can operate

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

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VLSI Design

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Shock and Vibration

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DistributedSensor Networks

International Journal of

2 International Journal of Antennas and Propagation

X

Y

Z

hsub

Rpatch

Lsu

b

Lgn

d

Wsub

TopSubstrate

Bottom

Lf

Wf

(a)

TopSubstrate

BottomVia

webg

g2r

(b)

CC

CC

C

C

L

L

+ +

minus

minus

(c)

Figure 1 Proposed UWB antenna (a) without EBG structures and (b) with EBG structures and (c) equivalent circuit of WLAN notchedbased on EBG structures

(a)

lm

wm

lc

ls

wt

wt2

(b)

Figure 2 Proposed MIMO antenna (a) initial MIMO antenna and (b) final MIMO antenna with MMR stub structure Detailed dimensionsin mm 119897

119898= 12 119908

119898= 05 119897

119888= 65 119897

119904= 55 and 119908

119905= 10

of the communication channels In addition some designswere not able to operate in the entire UWB band allocated bythe FCC [9ndash12] On the other hand a four-element MIMOantenna with better isolation by introducing discontinuitiesbetween elements and the systemgroundplanewas presentedin [5] The obtained results show isolation lower than 20 dBHowever the system only works over the frequency range of20ndash60GHz

In this paper a compact 4 times 4MIMO-UWB antenna withWLAN-notched characteristic is presented The proposedantenna shows isolation less than minus15 dB over its ultrawideoperating frequency band ranging from 273 to 1068GHzand a rejection at the WLAN band of 536ndash604GHz Theunique feature of this design is that the mutual coupling canbe reduced by adjusting the length ofMMR stub correspond-ing to a quarter wavelength of frequency which providesthe lowest coupling coefficient Moreover the notched bandbased onEBG structures helps tomaintain the same radiationpatterns at higher frequency rather than using slots cut on thepatches [13]

The rest part of this paper is organized as follows InSection 2 detailed designs of the UWB antennas without and

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

55mm65mm

75mm85mm

0

S 11

(dB)

Figure 3 Simulated 11987811

of the single UWB antenna without EBGstructures for different parameters of 119877patch

International Journal of Antennas and Propagation 3

Table 1 Design specifications for UWB antennas

Antenna type Design parameters (mm)UWB antenna without EBGs 119871 sub = 30119882sub = 30119882

119891= 3 119877patch = 8 119871

119891= 13 119871gnd = 15

UWB antenna with EBGs 119871 sub = 30119882sub = 30119882119891= 3 119877patch = 8 119871

119891= 13 119871gnd = 15 119908ebg = 62 119892 = 07 119903 = 05

per m]Jsurf [A

30000e + 00127911e + 00125822e + 00123734e + 00121645e + 00119556e + 00117467e + 00115379e + 00113290e + 00111201e + 00191124e + 00070237e + 00049349e + 00028461e + 00075738e minus 001

(a)

Jsurf [A per m]

27911e + 00125822e + 00123734e + 00121645e + 00119556e + 00117467e + 00115379e + 00113290e + 00111201e + 00191124e + 00070237e + 00049349e + 00028461e + 000

30000e + 001

75738e minus 001

(b)

Figure 4 Current distribution on single UWB antenna without EBGs at (a) 4GHz and (b) 9GHz

with EBGs are presented The proposed MIMO antenna isthen introduced in both cases of initial and final design Thesimulated andmeasured results are shown in Section 3 whilesome conclusions are provided in Section 4

2 Design of 4 times 4 MIMO-UWB Antenna

In this work the design of the antenna is divided into twoparts In the first part an antenna is designed for UWBfrequencies ranging from 31 GHz to 106GHz Afterwardsthis antenna is integrated with the mushroom-like EBGstructure to provide WLAN band-notched characteristic Inthe second part the four notched single antennas are utilizedas elements to form a 4 times 4 MIMO antenna Finally the stubstructure is implemented to diminish the mutual coupling ofthe antennas

21 Design ofUWBAntenna Thetwo configurations ofUWBantennawithout andwith EBG elements are shown in Figures1(a) and 1(b) respectively The antenna consists of a circularradiating patch excited by a 50Ω microstrip feed line (119882

119891=

3mm) On the other side of the substrate the ground planewith a length of 119871gnd covers the section of the microstrip fedlineThe antennas are printed on a FR4 epoxy substrate withthe dielectric constant of 44 and a thickness of 16mm

The antenna integrated with EBG structures is shown inFigure 1(b) Here the microstrip feed line is placed betweentwo pairs of EBG cells which are designed to act as stopbandfiltersWith the intent of prohibitingWLAN frequency banda pair of EBG cells is inserted rather than only using one cellThe equivalent-circuit model of WLAN notched based onEBG structures also is shown in Figure 1(c) The capacitance119862119862denotes the coupling between EBG and microstrip line

The capacitance 119862 is due to the voltage gradient between the

patch and ground plane while the inductance 119871 is generatedby the current flowing through the shorting via Thereforethe center resonant frequency is 119891

119862= 12120587radic119871(119862 + 119862

119862) and

the width of the stopband increases with the rise of 119862119862or

the decrease of the distance between feed line and EBG cellsThe dimensions of the EBG cells the gap between EBG cellsand the microstrip line are optimized to have band rejectionfrom5 to 6GHz corresponding toWLAN frequencies All thedimensions of these antennas are summarized in Table 1

22 Design of 4 times 4 MIMO-UWB Antenna In this designthe four UWB antennas with EBG structures are rotatedclockwise and placed orthogonally to each other to form the4 times 4 MIMO antenna which has an overall size of 60mmtimes 60mm The layout of the MIMO antenna is shown inFigure 2(a)

With the aim of reducing the mutual coupling in theinitial MIMO antenna a stub structure is adopted to formthe final antenna depicted in Figure 2(b) The design ofthis structure was based on the principle of the microstripmultimode resonator (MMR) [14 15] These stubs are placedbetween the antennas and connected to each other by asquare placed at the center of theMIMOantennaThemutualcoupling between the antennaswill be investigated for findingthe range of frequency in which the isolation coefficients arenot lower than minus15 dB The length 119897

119898of the MMR stub is

then set at a value that is approximately equal to a quarterwavelength at the defined frequency

3 Results and Discussions

This part will discuss the performance of the proposedantennas through both simulation and measurement results

4 International Journal of Antennas and Propagation

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 5 Simulated radiation patterns of the UWB antenna without EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 120

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

Figure 6 Simulated peak gain of the single UWB antenna withoutEBGs

31 UWB Antenna The simulated 11987811

results of the UWBantenna without EBG structure with different values of 119877patchare shown in Figure 3 It can be seen that the bandwidth ofthe antenna defined by the 119878

11less thanminus10 dB entirely covers

the UWB frequency range The decrease of 119877patch causes theincrease of the impedance mismatching at lower frequencyrange The optimized bandwidth is obtained when the 119877patchis chosen as 8mm

Current distributions of the antenna at certain frequen-cies are exhibited in Figure 4 As observed in Figure 4(a) thecurrent distribution at 4GHz proves that the antenna oper-ates at its fundamental mode while the current distribution

Jsurf [A per m]

83217e + 00177275e + 00171333e + 00165390e + 00159448e + 00153505e + 00147563e + 00141621e + 00135678e + 00129736e + 00123793e + 00117851e + 00111909e + 00159663e + 00023907e minus 002

Figure 7 Surface current distribution on UWB antenna with EBGstructures at 55 GHz

at 9GHz in Figure 4(b) indicates the higher resonant modewhich is corresponding to the third order harmonic Hencethe antenna can work over the broad range of UWB

Typical radiation patterns of the antenna at 4GHz and9GHz in 119909119911-plane are plotted in Figures 5(a) and 5(b)respectively The solid lines display the 119864

120593 and the dotted

lines represent 119864120579 From Figure 5(a) the antenna possesses a

dipole-like radiation pattern confirming its operation in thefundamental resonantmodeThe pattern at higher frequencyfrom Figure 5(b) corresponds to the harmonics of the fun-damental resonant mode which are closely spaced [16] Thesimulated peak gain of the antenna is depicted in Figure 6in which the higher frequencies provide larger antenna gainThis is a good agreement with the theory

International Journal of Antennas and Propagation 5

2 4 6 8 10 12Frequency (GHz)

minus30

minus20

minus10

05mm06mm

07mm08mm

0S 1

1(d

B)

(a)

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

0

5mm6mm

7mm8mm

S 11

(dB)

(b)

Figure 8 Simulated 11987811of the single UWB antenna integrated with EBG structures for different parameters of (a) 119892 and (b) 119908ebg

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 9 Simulated radiation patterns of the UWB antenna integrated with EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

The effect of EBG structures onUWBantenna is shown inFigure 7 At the center frequency of WLAN band (55 GHz)the current distribution on antenna mainly focuses on theEBG structures and therefore the patch antenna will notradiate resulting in a notched frequency band

In order to obtain the desired notched band parametricstudies on the dimension of EBG cell 119908ebg and the distance 119892between EBG cells and the feed line are investigated Figure 8shows the simulated 119878

11of antenna for different values of the

gap 119892 In Figure 8(a) the width of the notched band reducedwhen the value of gap 119892 increases while the simulated 119878

11

of antenna for different values of EBG size 119908ebg is alsodepicted in Figure 8(b) It should be noticed in Figure 8(b)that the frequency of the notched band also reduces with theincreasing value of 119908ebg Therefore the notched-band tuningrequires a combination of both values

Figures 9(a) and 9(b) show the radiation pattern of theproposed UWB antenna at 4GHz and 9GHz respectively

6 International Journal of Antennas and Propagation

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

minus2

minus4

UWB range (31ndash106GHz)

Figure 10 Simulated peak gain of the single UWB antenna inte-grated EBGs

Figure 11 Fabricated UWB integrated EBGs antenna and finalMIMO antenna

0WLAN

minus30

minus25

minus20

minus15

minus10

minus5UWB range (31ndash106GHz)

2 4 6 8 10 12Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 12 Simulated and measured results of 11987811

of the UWBintegrated EBGs with WLAN band notched antenna

minus30

minus20

minus10

S11S22

S33S44

2 4 6 8 10 12

0

Retu

rn lo

ss (d

B)

Frequency (GHz)

(a)

minus50

minus40

minus30

minus20

minus10

0Is

olat

ion

(dB)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 13 Simulated results of the initial MIMO antenna (a) returnloss and (b) isolation coefficient

The radiation patterns at high frequency of the antenna withand without EBGs are of small variation due to the lack ofetching or cutting on the surface of the patch antenna

The simulated peak gain versus frequency is shown inFigure 10 It reveals a significant drop in the peak gain withinthe rejected bands when the antenna employs the EBGcells A relatively stable peak gain remains over the UWBbandwidth except a 5 dB gain drop in the notched band

Finally the fabrication of the proposed antenna is pre-sented in Figure 11The simulated andmeasured results of 119878

11

of the UWB antenna with EBG cells are shown in Figure 12From this figure it is observed that the antenna can operate

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

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Active and Passive Electronic Components

Control Scienceand Engineering

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 3

Table 1 Design specifications for UWB antennas

Antenna type Design parameters (mm)UWB antenna without EBGs 119871 sub = 30119882sub = 30119882

119891= 3 119877patch = 8 119871

119891= 13 119871gnd = 15

UWB antenna with EBGs 119871 sub = 30119882sub = 30119882119891= 3 119877patch = 8 119871

119891= 13 119871gnd = 15 119908ebg = 62 119892 = 07 119903 = 05

per m]Jsurf [A

30000e + 00127911e + 00125822e + 00123734e + 00121645e + 00119556e + 00117467e + 00115379e + 00113290e + 00111201e + 00191124e + 00070237e + 00049349e + 00028461e + 00075738e minus 001

(a)

Jsurf [A per m]

27911e + 00125822e + 00123734e + 00121645e + 00119556e + 00117467e + 00115379e + 00113290e + 00111201e + 00191124e + 00070237e + 00049349e + 00028461e + 000

30000e + 001

75738e minus 001

(b)

Figure 4 Current distribution on single UWB antenna without EBGs at (a) 4GHz and (b) 9GHz

with EBGs are presented The proposed MIMO antenna isthen introduced in both cases of initial and final design Thesimulated andmeasured results are shown in Section 3 whilesome conclusions are provided in Section 4

2 Design of 4 times 4 MIMO-UWB Antenna

In this work the design of the antenna is divided into twoparts In the first part an antenna is designed for UWBfrequencies ranging from 31 GHz to 106GHz Afterwardsthis antenna is integrated with the mushroom-like EBGstructure to provide WLAN band-notched characteristic Inthe second part the four notched single antennas are utilizedas elements to form a 4 times 4 MIMO antenna Finally the stubstructure is implemented to diminish the mutual coupling ofthe antennas

21 Design ofUWBAntenna Thetwo configurations ofUWBantennawithout andwith EBG elements are shown in Figures1(a) and 1(b) respectively The antenna consists of a circularradiating patch excited by a 50Ω microstrip feed line (119882

119891=

3mm) On the other side of the substrate the ground planewith a length of 119871gnd covers the section of the microstrip fedlineThe antennas are printed on a FR4 epoxy substrate withthe dielectric constant of 44 and a thickness of 16mm

The antenna integrated with EBG structures is shown inFigure 1(b) Here the microstrip feed line is placed betweentwo pairs of EBG cells which are designed to act as stopbandfiltersWith the intent of prohibitingWLAN frequency banda pair of EBG cells is inserted rather than only using one cellThe equivalent-circuit model of WLAN notched based onEBG structures also is shown in Figure 1(c) The capacitance119862119862denotes the coupling between EBG and microstrip line

The capacitance 119862 is due to the voltage gradient between the

patch and ground plane while the inductance 119871 is generatedby the current flowing through the shorting via Thereforethe center resonant frequency is 119891

119862= 12120587radic119871(119862 + 119862

119862) and

the width of the stopband increases with the rise of 119862119862or

the decrease of the distance between feed line and EBG cellsThe dimensions of the EBG cells the gap between EBG cellsand the microstrip line are optimized to have band rejectionfrom5 to 6GHz corresponding toWLAN frequencies All thedimensions of these antennas are summarized in Table 1

22 Design of 4 times 4 MIMO-UWB Antenna In this designthe four UWB antennas with EBG structures are rotatedclockwise and placed orthogonally to each other to form the4 times 4 MIMO antenna which has an overall size of 60mmtimes 60mm The layout of the MIMO antenna is shown inFigure 2(a)

With the aim of reducing the mutual coupling in theinitial MIMO antenna a stub structure is adopted to formthe final antenna depicted in Figure 2(b) The design ofthis structure was based on the principle of the microstripmultimode resonator (MMR) [14 15] These stubs are placedbetween the antennas and connected to each other by asquare placed at the center of theMIMOantennaThemutualcoupling between the antennaswill be investigated for findingthe range of frequency in which the isolation coefficients arenot lower than minus15 dB The length 119897

119898of the MMR stub is

then set at a value that is approximately equal to a quarterwavelength at the defined frequency

3 Results and Discussions

This part will discuss the performance of the proposedantennas through both simulation and measurement results

4 International Journal of Antennas and Propagation

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 5 Simulated radiation patterns of the UWB antenna without EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 120

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

Figure 6 Simulated peak gain of the single UWB antenna withoutEBGs

31 UWB Antenna The simulated 11987811

results of the UWBantenna without EBG structure with different values of 119877patchare shown in Figure 3 It can be seen that the bandwidth ofthe antenna defined by the 119878

11less thanminus10 dB entirely covers

the UWB frequency range The decrease of 119877patch causes theincrease of the impedance mismatching at lower frequencyrange The optimized bandwidth is obtained when the 119877patchis chosen as 8mm

Current distributions of the antenna at certain frequen-cies are exhibited in Figure 4 As observed in Figure 4(a) thecurrent distribution at 4GHz proves that the antenna oper-ates at its fundamental mode while the current distribution

Jsurf [A per m]

83217e + 00177275e + 00171333e + 00165390e + 00159448e + 00153505e + 00147563e + 00141621e + 00135678e + 00129736e + 00123793e + 00117851e + 00111909e + 00159663e + 00023907e minus 002

Figure 7 Surface current distribution on UWB antenna with EBGstructures at 55 GHz

at 9GHz in Figure 4(b) indicates the higher resonant modewhich is corresponding to the third order harmonic Hencethe antenna can work over the broad range of UWB

Typical radiation patterns of the antenna at 4GHz and9GHz in 119909119911-plane are plotted in Figures 5(a) and 5(b)respectively The solid lines display the 119864

120593 and the dotted

lines represent 119864120579 From Figure 5(a) the antenna possesses a

dipole-like radiation pattern confirming its operation in thefundamental resonantmodeThe pattern at higher frequencyfrom Figure 5(b) corresponds to the harmonics of the fun-damental resonant mode which are closely spaced [16] Thesimulated peak gain of the antenna is depicted in Figure 6in which the higher frequencies provide larger antenna gainThis is a good agreement with the theory

International Journal of Antennas and Propagation 5

2 4 6 8 10 12Frequency (GHz)

minus30

minus20

minus10

05mm06mm

07mm08mm

0S 1

1(d

B)

(a)

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

0

5mm6mm

7mm8mm

S 11

(dB)

(b)

Figure 8 Simulated 11987811of the single UWB antenna integrated with EBG structures for different parameters of (a) 119892 and (b) 119908ebg

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 9 Simulated radiation patterns of the UWB antenna integrated with EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

The effect of EBG structures onUWBantenna is shown inFigure 7 At the center frequency of WLAN band (55 GHz)the current distribution on antenna mainly focuses on theEBG structures and therefore the patch antenna will notradiate resulting in a notched frequency band

In order to obtain the desired notched band parametricstudies on the dimension of EBG cell 119908ebg and the distance 119892between EBG cells and the feed line are investigated Figure 8shows the simulated 119878

11of antenna for different values of the

gap 119892 In Figure 8(a) the width of the notched band reducedwhen the value of gap 119892 increases while the simulated 119878

11

of antenna for different values of EBG size 119908ebg is alsodepicted in Figure 8(b) It should be noticed in Figure 8(b)that the frequency of the notched band also reduces with theincreasing value of 119908ebg Therefore the notched-band tuningrequires a combination of both values

Figures 9(a) and 9(b) show the radiation pattern of theproposed UWB antenna at 4GHz and 9GHz respectively

6 International Journal of Antennas and Propagation

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

minus2

minus4

UWB range (31ndash106GHz)

Figure 10 Simulated peak gain of the single UWB antenna inte-grated EBGs

Figure 11 Fabricated UWB integrated EBGs antenna and finalMIMO antenna

0WLAN

minus30

minus25

minus20

minus15

minus10

minus5UWB range (31ndash106GHz)

2 4 6 8 10 12Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 12 Simulated and measured results of 11987811

of the UWBintegrated EBGs with WLAN band notched antenna

minus30

minus20

minus10

S11S22

S33S44

2 4 6 8 10 12

0

Retu

rn lo

ss (d

B)

Frequency (GHz)

(a)

minus50

minus40

minus30

minus20

minus10

0Is

olat

ion

(dB)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 13 Simulated results of the initial MIMO antenna (a) returnloss and (b) isolation coefficient

The radiation patterns at high frequency of the antenna withand without EBGs are of small variation due to the lack ofetching or cutting on the surface of the patch antenna

The simulated peak gain versus frequency is shown inFigure 10 It reveals a significant drop in the peak gain withinthe rejected bands when the antenna employs the EBGcells A relatively stable peak gain remains over the UWBbandwidth except a 5 dB gain drop in the notched band

Finally the fabrication of the proposed antenna is pre-sented in Figure 11The simulated andmeasured results of 119878

11

of the UWB antenna with EBG cells are shown in Figure 12From this figure it is observed that the antenna can operate

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

4 International Journal of Antennas and Propagation

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 5 Simulated radiation patterns of the UWB antenna without EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 120

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

Figure 6 Simulated peak gain of the single UWB antenna withoutEBGs

31 UWB Antenna The simulated 11987811

results of the UWBantenna without EBG structure with different values of 119877patchare shown in Figure 3 It can be seen that the bandwidth ofthe antenna defined by the 119878

11less thanminus10 dB entirely covers

the UWB frequency range The decrease of 119877patch causes theincrease of the impedance mismatching at lower frequencyrange The optimized bandwidth is obtained when the 119877patchis chosen as 8mm

Current distributions of the antenna at certain frequen-cies are exhibited in Figure 4 As observed in Figure 4(a) thecurrent distribution at 4GHz proves that the antenna oper-ates at its fundamental mode while the current distribution

Jsurf [A per m]

83217e + 00177275e + 00171333e + 00165390e + 00159448e + 00153505e + 00147563e + 00141621e + 00135678e + 00129736e + 00123793e + 00117851e + 00111909e + 00159663e + 00023907e minus 002

Figure 7 Surface current distribution on UWB antenna with EBGstructures at 55 GHz

at 9GHz in Figure 4(b) indicates the higher resonant modewhich is corresponding to the third order harmonic Hencethe antenna can work over the broad range of UWB

Typical radiation patterns of the antenna at 4GHz and9GHz in 119909119911-plane are plotted in Figures 5(a) and 5(b)respectively The solid lines display the 119864

120593 and the dotted

lines represent 119864120579 From Figure 5(a) the antenna possesses a

dipole-like radiation pattern confirming its operation in thefundamental resonantmodeThe pattern at higher frequencyfrom Figure 5(b) corresponds to the harmonics of the fun-damental resonant mode which are closely spaced [16] Thesimulated peak gain of the antenna is depicted in Figure 6in which the higher frequencies provide larger antenna gainThis is a good agreement with the theory

International Journal of Antennas and Propagation 5

2 4 6 8 10 12Frequency (GHz)

minus30

minus20

minus10

05mm06mm

07mm08mm

0S 1

1(d

B)

(a)

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

0

5mm6mm

7mm8mm

S 11

(dB)

(b)

Figure 8 Simulated 11987811of the single UWB antenna integrated with EBG structures for different parameters of (a) 119892 and (b) 119908ebg

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 9 Simulated radiation patterns of the UWB antenna integrated with EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

The effect of EBG structures onUWBantenna is shown inFigure 7 At the center frequency of WLAN band (55 GHz)the current distribution on antenna mainly focuses on theEBG structures and therefore the patch antenna will notradiate resulting in a notched frequency band

In order to obtain the desired notched band parametricstudies on the dimension of EBG cell 119908ebg and the distance 119892between EBG cells and the feed line are investigated Figure 8shows the simulated 119878

11of antenna for different values of the

gap 119892 In Figure 8(a) the width of the notched band reducedwhen the value of gap 119892 increases while the simulated 119878

11

of antenna for different values of EBG size 119908ebg is alsodepicted in Figure 8(b) It should be noticed in Figure 8(b)that the frequency of the notched band also reduces with theincreasing value of 119908ebg Therefore the notched-band tuningrequires a combination of both values

Figures 9(a) and 9(b) show the radiation pattern of theproposed UWB antenna at 4GHz and 9GHz respectively

6 International Journal of Antennas and Propagation

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

minus2

minus4

UWB range (31ndash106GHz)

Figure 10 Simulated peak gain of the single UWB antenna inte-grated EBGs

Figure 11 Fabricated UWB integrated EBGs antenna and finalMIMO antenna

0WLAN

minus30

minus25

minus20

minus15

minus10

minus5UWB range (31ndash106GHz)

2 4 6 8 10 12Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 12 Simulated and measured results of 11987811

of the UWBintegrated EBGs with WLAN band notched antenna

minus30

minus20

minus10

S11S22

S33S44

2 4 6 8 10 12

0

Retu

rn lo

ss (d

B)

Frequency (GHz)

(a)

minus50

minus40

minus30

minus20

minus10

0Is

olat

ion

(dB)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 13 Simulated results of the initial MIMO antenna (a) returnloss and (b) isolation coefficient

The radiation patterns at high frequency of the antenna withand without EBGs are of small variation due to the lack ofetching or cutting on the surface of the patch antenna

The simulated peak gain versus frequency is shown inFigure 10 It reveals a significant drop in the peak gain withinthe rejected bands when the antenna employs the EBGcells A relatively stable peak gain remains over the UWBbandwidth except a 5 dB gain drop in the notched band

Finally the fabrication of the proposed antenna is pre-sented in Figure 11The simulated andmeasured results of 119878

11

of the UWB antenna with EBG cells are shown in Figure 12From this figure it is observed that the antenna can operate

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 5

2 4 6 8 10 12Frequency (GHz)

minus30

minus20

minus10

05mm06mm

07mm08mm

0S 1

1(d

B)

(a)

2 4 6 8 10 12Frequency (GHz)

minus30

minus25

minus20

minus15

minus10

minus5

0

5mm6mm

7mm8mm

S 11

(dB)

(b)

Figure 8 Simulated 11987811of the single UWB antenna integrated with EBG structures for different parameters of (a) 119892 and (b) 119908ebg

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 9 Simulated radiation patterns of the UWB antenna integrated with EBGs in 119909119911-plane at (a) 4GHz and (b) 9GHz

The effect of EBG structures onUWBantenna is shown inFigure 7 At the center frequency of WLAN band (55 GHz)the current distribution on antenna mainly focuses on theEBG structures and therefore the patch antenna will notradiate resulting in a notched frequency band

In order to obtain the desired notched band parametricstudies on the dimension of EBG cell 119908ebg and the distance 119892between EBG cells and the feed line are investigated Figure 8shows the simulated 119878

11of antenna for different values of the

gap 119892 In Figure 8(a) the width of the notched band reducedwhen the value of gap 119892 increases while the simulated 119878

11

of antenna for different values of EBG size 119908ebg is alsodepicted in Figure 8(b) It should be noticed in Figure 8(b)that the frequency of the notched band also reduces with theincreasing value of 119908ebg Therefore the notched-band tuningrequires a combination of both values

Figures 9(a) and 9(b) show the radiation pattern of theproposed UWB antenna at 4GHz and 9GHz respectively

6 International Journal of Antennas and Propagation

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

minus2

minus4

UWB range (31ndash106GHz)

Figure 10 Simulated peak gain of the single UWB antenna inte-grated EBGs

Figure 11 Fabricated UWB integrated EBGs antenna and finalMIMO antenna

0WLAN

minus30

minus25

minus20

minus15

minus10

minus5UWB range (31ndash106GHz)

2 4 6 8 10 12Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 12 Simulated and measured results of 11987811

of the UWBintegrated EBGs with WLAN band notched antenna

minus30

minus20

minus10

S11S22

S33S44

2 4 6 8 10 12

0

Retu

rn lo

ss (d

B)

Frequency (GHz)

(a)

minus50

minus40

minus30

minus20

minus10

0Is

olat

ion

(dB)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 13 Simulated results of the initial MIMO antenna (a) returnloss and (b) isolation coefficient

The radiation patterns at high frequency of the antenna withand without EBGs are of small variation due to the lack ofetching or cutting on the surface of the patch antenna

The simulated peak gain versus frequency is shown inFigure 10 It reveals a significant drop in the peak gain withinthe rejected bands when the antenna employs the EBGcells A relatively stable peak gain remains over the UWBbandwidth except a 5 dB gain drop in the notched band

Finally the fabrication of the proposed antenna is pre-sented in Figure 11The simulated andmeasured results of 119878

11

of the UWB antenna with EBG cells are shown in Figure 12From this figure it is observed that the antenna can operate

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

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Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

6 International Journal of Antennas and Propagation

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

minus2

minus4

UWB range (31ndash106GHz)

Figure 10 Simulated peak gain of the single UWB antenna inte-grated EBGs

Figure 11 Fabricated UWB integrated EBGs antenna and finalMIMO antenna

0WLAN

minus30

minus25

minus20

minus15

minus10

minus5UWB range (31ndash106GHz)

2 4 6 8 10 12Frequency (GHz)

SimulatedMeasured

S 11

(dB)

Figure 12 Simulated and measured results of 11987811

of the UWBintegrated EBGs with WLAN band notched antenna

minus30

minus20

minus10

S11S22

S33S44

2 4 6 8 10 12

0

Retu

rn lo

ss (d

B)

Frequency (GHz)

(a)

minus50

minus40

minus30

minus20

minus10

0Is

olat

ion

(dB)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 13 Simulated results of the initial MIMO antenna (a) returnloss and (b) isolation coefficient

The radiation patterns at high frequency of the antenna withand without EBGs are of small variation due to the lack ofetching or cutting on the surface of the patch antenna

The simulated peak gain versus frequency is shown inFigure 10 It reveals a significant drop in the peak gain withinthe rejected bands when the antenna employs the EBGcells A relatively stable peak gain remains over the UWBbandwidth except a 5 dB gain drop in the notched band

Finally the fabrication of the proposed antenna is pre-sented in Figure 11The simulated andmeasured results of 119878

11

of the UWB antenna with EBG cells are shown in Figure 12From this figure it is observed that the antenna can operate

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 7

per m]Jsurf [A

30000e + 00127859e + 00125718e + 00123577e + 00121435e + 00119294e + 00117153e + 00115012e + 00112871e + 00110730e + 00185885e + 00064474e + 00043062e + 00021651e + 00023907e minus 002

(a)

per m]Jsurf [A

20000e + 00118573e + 00117145e + 00115718e + 00114291e + 00112864e + 00111436e + 00110009e + 00185817e + 00071544e + 00057271e + 00042998e + 00028726e + 00014453e + 00017976e minus 002

(b)

Figure 14 Surface current distribution at 67 GHz on (a) initial MIMO antenna and (b) final MIMO antenna

Port 1 Port 2

(a)

4 5 6 7 8

0

S-pa

ram

eter

s (dB

)

Frequency (GHz)

minus30

minus40

minus20

minus10

S11S21

(b)

Figure 15 (a) Bandstop filter model and (b) simulated 119878-parameters of the filter

over the range spreading from 298GHz to 1116GHz andexhibits a good rejection at frequencies of WLAN from471 GHz to 583GHz

32 4 times 4 MIMO-UWB Antenna The simulated resultsof reflection coefficients of the initial MIMO antenna areshown in Figure 13(a) As can be seen from Figure 13(a)the reflection coefficients of the antenna do not satisfyimpedance matching from 4 to 45GHz On the other handthe isolation coefficients between the elements of the initialantenna (see Figure 13(b)) are very low at 6ndash8GHz (aboutminus10 dB) This fact is clearly demonstrated by the surfacecurrent distribution on the initial antenna in Figure 14(a) Ascan be observed from Figure 14(a) when the first elementis excited the surface current is strongly induced on theopposite element resulting in a rise of the mutual coupling(11987813and 11987824) Actually themutual coupling can be reduced by

increasing the distance between the elements However thiswill lead to the larger size of the proposed MIMO antenna

These drawbacks of the initial MIMO antenna can besolved thanks to the use of stub structure in the final MIMOdesign It can be seen from Figure 13(b) that the mutualcoupling has a high value at about 67GHz Therefore thelength 119897

119898of the stub structure is selected as 12mm approx-

imately a quarter wavelength at 67 GHz The configurationof this stub is designed based on the principle of MMRstructure Actually the stub structure acts as a bandstop filterwhich produced a stopband from635 to 687GHzThereforethe filter will suppress the induced currents caused by thecopolarization elements at the center frequency of 67GHzThe bandstop filter model and simulated 119878-parameters of thefilter are given in Figures 15(a) and 15(b) respectively

In Figure 16(a) the bandwidth of the final antennaentirely covers the UWB operating band whereas the

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

8 International Journal of Antennas and Propagation

minus30

minus20

minus10

S11S22S33

S44

2 4 6 8 10 12

0Re

turn

loss

(dB)

Frequency (GHz)

(1) (2)

(1)(2)

514GHz582GHz

(a)

minus40

minus30

minus35

minus20

minus25

minus10

minus15

Isol

atio

n (d

B)

2 4 6 8 10 12Frequency (GHz)

S12S13

S23S24

S14 S34

(b)

Figure 16 Simulated results of the final MIMO antenna (a) return loss and (b) isolation coefficient

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 17 Simulated radiation patterns of the initial MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

notched band appears in WLAN range (from 514 to582GHz) Moreover in Figure 16(b) the mutual couplingbetween the opposite-side elements decreases to belowminus15 dB The current distribution of the final antenna at67 GHz is focused on the stub structures shown inFigure 14(b) Therefore the effect of the surface current onthe copolarization element is reduced

Figures 17 and 18 show the simulated radiation patternsof initial and final MIMO antenna respectively From thesefigures the radiation patterns of both antennas at lowerfrequency (4GHz) are directional while nearly omnidirec-tional patterns are observed at higher frequency (9GHz)Thesimulated peak gain of final MIMO antenna is presented inFigure 19 The stable gain is maintained over the UWB range

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 9

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(a)

0

100

30

60

90

120

150

180

210

240

270

300

330

0

10

minus40

minus30

minus20

minus10

minus30

minus20

minus10

E120593

E120579

(b)

Figure 18 Simulated radiation patterns of the final MIMO antenna in 119909119911-plane at (a) 4GHz and (b) 9GHz

2 4 6 8 10 12

0

2

4

6

8

Peak

gai

n (d

Bi)

Frequency (GHz)

WLAN

UWB range (31ndash106GHz)minus2

minus4

minus6

Figure 19 Simulated peak gain of the final MIMO antenna

while a gain drop of 45 dB is observed at theWLAN-notchedband

Figure 20 presents the measured return loss of the fab-ricated final MIMO antenna shown in Figure 11 It workswell over a broad range of frequenciesfrom 273 to 1068GHzand possesses a rejected band at the WLAN band from536 to 604GHz Meanwhile Figure 21 shows the results ofmutual coupling of the cross- and copolarization elementsof the final MIMO antenna The measured results show thatthe mutual coupling between cross- (119878

12) and copolarization

elements (11987813) is less than minus17 dB and minus16 dB over UWB

0WLAN

2 4 6 8 10 12Frequency (GHz)

minus10

minus5

minus15

minus20

minus25

minus30

SimulatedMeasured

Retu

rn lo

ss (d

B)

UWB range (31ndash106GHz)

Figure 20 Simulated andmeasured results of return loss of the finalMIMO antenna

range respectively It should be noted that the measuredresults are in a good agreement with the simulated results

33 MIMO Characteristics It is required that MIMO anten-nas must be characterized for their diversity performance Ina diversity system the signals are usually correlated due to thedistance between the antenna elements [4] The parameterused to assess the correlation between radiation patterns isthe envelope correlation coefficient It is required tominimize

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

10 International Journal of Antennas and Propagation

2 4 6 8 10 12Frequency (GHz)

S 12

(dB)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(a)S 1

3(d

B)

2 4 6 8 10 12Frequency (GHz)

minus10

minus15

minus20

minus25

minus30

minus35

minus40

minus45

minus50

SimulatedMeasured

(b)

Figure 21 Simulated and measured results of mutual coupling of the final MIMO antenna (a) cross-polarization elements and (b)copolarization elements

the correlation because the relationship of the correlationwith diversity gain is that the lower is the correlation thehigher will be the diversity gain and vice versa Normallythe value of the envelope correlation at a certain frequencyis small in case that the radiation patterns of one antennaare different from each other Otherwise the same patternsof these antennas exhibit the larger value of the envelopecorrelation

The correlation coefficient can be calculated from radi-ation patterns or scattering parameters Assuming uniformmultipath environment the envelope correlation (120588

119890) simple

square of the correlation coefficient (120588) can be calculatedconveniently and quickly from 119878-parameters [17] as follows

120588119890(119894 119895119873) =

10038161003816100381610038161003816sum119873

119899=1119878lowast

119894119899119878119899119895

10038161003816100381610038161003816

2

prod119896=(119894119895)[1 minus sum

119873

119899=1119878lowast

119894119899119878119899119896]

(1)

Equation (1) depicts the envelope correlation between theantennas 119894 and 119895 in the (119873119873) MIMO system In case of 119894 =1 119895 = 2 and 119873 = 4 the envelope correlation of proposedMIMO antenna can be defined as follows

120588119890(1 2 4)

=1003816100381610038161003816119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842

1003816100381610038161003816

2

times ([1 minus (119878lowast

1111987811+ 119878lowast

1211987821+ 119878lowast

1311987831+ 119878lowast

1411987841)]

times [1 minus (119878lowast

1111987812+ 119878lowast

1211987822+ 119878lowast

1311987832+ 119878lowast

1411987842)])minus1

(2)

The simulated envelope correlation coefficient is shownin Figure 22 From this figure the proposed MIMO antennahas a minimum correlation coefficient of minus56 dB and less

2 4 6 8 10 12

0

Frequency (GHz)

minus30

minus40

minus50

minus60

minus20

minus10

UWB range (31ndash106GHz)

Cor

relat

ion

coeffi

cien

t (dB

)

Figure 22 Simulated proposed MIMO antennarsquos envelope correla-tion coefficient

than minus28 dB over the UWB frequency range It should benoticed that a very low value of correlation coefficient resultsin ensured high diversity gain Therefore the presentedcorrelation coefficient is suitable for mobile communicationwith aminimum acceptable correlation coefficient of 05 [18]

The simulated result of group delay of the proposedMIMO antenna is shown in Figure 23 It can be seen thata distortion (ge1 ns) occurred at 55 GHz while the otherpart of operating band keeps relatively flat The variationof group delay is found to be less than 1 ns showing goodphase linearity and thus it fulfills the requirement for UWBoperations

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of Antennas and Propagation 11

3 4 5 6 7 8 9 10 11

0

1

2

3

4

5

Gro

up d

elay

(ns)

Frequency (GHz)

minus3

minus4

minus5

minus2

minus1

Figure 23 Simulated group delay of proposed MIMO antenna

4 Conclusions

Thecompact 4times 4MIMO-UWB antenna is designed to oper-ate in ultrawide frequency rangewith rejected band atWLANfrequency based on EBG structures The stub structuresacting as bandstop filter are inserted to suppress the effect ofsurface current on the elements of the proposed antenna forreducing themutual couplingThe fabricatedMIMOantennashows isolation less than minus15 dB over its ultrawide operatingfrequency band spreading from273ndash1068GHz and rejectionat the WLAN band of 536ndash604GHz The proposed MIMOantenna has also aminimum correlation coefficient ofminus56 dBand less than minus28 dB over the UWB frequency range makingit a good candidate for UWB-MIMO applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] G J Foschini and M J Gans ldquoOn limits of wireless communi-cations in a fading environment when usingmultiple antennasrdquoWireless Personal Communications vol 6 no 3 pp 311ndash3351998

[2] L Zheng and D N C Tse ldquoDiversity and multiplexinga fundamental tradeoff in multiple-antenna channelsrdquo IEEETransactions on Information Theory vol 49 no 5 pp 1073ndash1096 2003

[3] F C C (FCC) ldquoRevision of part 15 of the commissions rulesregarding ultra-wideband transmission systemsrdquo First Reportand Order ET Docket 98-153 FCC 02-48 2002

[4] A Najam Y Duroc and S Tedjni ldquoUWB-MIMO antenna withnovel stub structurerdquo Progress In Electromagnetics Research Cvol 19 pp 245ndash257 2011

[5] S-Y Lin and H-R Huang ldquoUltra-wideband MIMO antennawith enhanced isolationrdquo Microwave and Optical TechnologyLetters vol 51 no 2 pp 570ndash573 2009

[6] S Zhang Z Ying J Xiong and S He ldquoUltrawidebandMIMOdiversity antennas with a tree-like structure to enhancewideband isolationrdquo IEEE Antennas and Wireless PropagationLetters vol 8 pp 1279ndash1282 2009

[7] J Li Q Chu and T Huang ldquoA compact wideband MIMOantenna with two novel bent slitsrdquo IEEE Transactions onAntennas and Propagation vol 60 no 2 pp 482ndash489 2012

[8] L Liu S W Cheung and T I Yuk ldquoCompact MIMO antennafor portable devices in UWB applicationsrdquo IEEE TransactionsonAntennas and Propagation vol 61 no 8 pp 4257ndash4264 2013

[9] KWong S Su and Y Kuo ldquoA printed ultra-wideband diversitymonopole antennardquoMicrowave and Optical Technology Lettersvol 38 no 4 pp 257ndash259 2003

[10] T S P See and Z N Chen ldquoAn ultrawideband diversityantennardquo IEEE Transactions on Antennas and Propagation vol57 no 6 pp 1597ndash1605 2009

[11] A Rajagopalan G Gupta A S Konanur B Hughes and GLazzi ldquoIncreasing channel capacity of an ultrawidebandMIMOsystem using vector antennasrdquo IEEE Transactions on Antennasand Propagation vol 55 no 10 pp 2880ndash2887 2007

[12] S Zhang B K Lau A Sunesson and S He ldquoClosely-packedUWB MIMOdiversity antenna with different patterns andpolarizations for USB dongle applicationsrdquo IEEE Transactionson Antennas and Propagation vol 60 no 9 pp 4372ndash43802012

[13] X L Liu Z DWang Y Z Yin and J HWang ldquoClosely spaceddual band-notched UWB antenna for MIMO applicationsrdquoProgress in Electromagnetic Research C vol 46 pp 109ndash1162014

[14] J W Baik S M Han C Jeong J Jeong and Y S KimldquoCompact ultra-wideband bandpass filter with EBG structurerdquoIEEE Microwave Wireless Components Letter vol 51 pp 364ndash370 2008

[15] K Ma K Liang R M Jayasuriya and K S Yeo ldquoA widebandand high rejectionmultimode bandpass filter using stub pertur-bationrdquo IEEE Microwave and Wireless Components Letters vol19 no 1 pp 24ndash26 2009

[16] K Bahadori and Y Rahmat-Samii ldquoAminiaturized elliptic-cardUWB antenna withWLAN band rejection for wireless commu-nicationsrdquo IEEE Transactions on Antennas and Propagation vol55 no 11 pp 3326ndash3332 2007

[17] J Thaysen and K B Jakobsen ldquoEnvelope correlation in (N N)mimo antenna array from scattering parametersrdquo Microwaveand Optical Technology Letters vol 48 no 5 pp 832ndash834 2006

[18] M P Karaboikis V C Papamichael G F Tsachtsiris C FSoras and V T Makios ldquoIntegrating compact printed antennasonto small diversityMIMO terminalsrdquo IEEE Transactions onAntennas and Propagation vol 56 no 7 pp 2067ndash2078 2008

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of