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Research ArticleMicrostrip Antenna Design for Femtocell Coverage Optimization

Afaz Uddin Ahmed,1 M. T. Islam,1 Rezaul Azim,1

Mahamod Ismail,2 and Mohd Fais Mansor2

Institute of Space Science (ANGKASA), Universiti Kebangsaan Malaysia, UKM, Bangi, Selangor, Malaysia Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, UKM, Bangi,

Selangor, Malaysia

Correspondence should be addressed to M. . Islam; [email protected]

Received November ; Accepted December ; Published April

Academic Editor: J.S. Mandeep

Copyright Aaz Uddin Ahmed et al. Tis 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 mircostrip antenna is designed ormultielementantennacoverage optimizationin emtocellnetwork.Intererenceis the oremostconcern or the cellular operator in vast commercial deployments o emtocell. Many techniques in physical, data linkand network-layer are analysed and developed to settle down the intererence issues. A multielement technique with sel-conguration eaturesis analyzed here or coverage optimization o emtocell. It also ocuses on the execution o microstrip antenna or multielementconguration. Te antenna is designed or LE Band by using standard FRdielectric substrate.Te perormanceo theproposedantenna in the emtocell application is discussed along with results.

1. Introduction

Femtocell, also known as emtocell access point (FAP), is ashort-ranged, low-powered, and low-cost base station thathas been shrunk down to the size o a paperback book.Femtocell is similar to a wireless internet router and easy toinstallin offices and residences. It is a mini base station or theindoor coverage purpose and an extension o outdoor net-work. It provides high quality indoor coverage and increases

thecapacity o the network by diverting a portiono the trafficthrough wired-backhaul connection []. Cellular operatorsthroughout the world are acing challenges in increasing sys-tem capacity, coverage, and residential connectivity in subur-ban and urban environments due to the huge investment thatollows. Femtocell offers an economically appealing way toimprove the quality, coverage, and the service o the existingnetwork. However, dense deployment o emtocell inducesintererence concern, which remains a strong challenge soar [, ]. Moreover, the indoor wireless environment andshort distances among the cells have made the situationmore complex. Since the cellular operators preer cochanneldeployment or better spectral efficiency, emtocell increases

the capacity without considering the airness o per emtocelluser capacity.

Network planner has no control over emtocells deploy-ment. Femtocell has extensive autoconguration capabilityto ensure plug and play deployment []. For successulresidential deployment, several technical challenges need tobe overcome. Large-scale deployment o emtocell in densearea increasesthe mobility eventsand overshoots the networksubscribers in an unwanted level. Femtocell is usually placed

in the cornero any residential placesor offices where the wireconnection is easier. Tereore, instead o omnidirectionalantenna, multielement antenna is better to optimize thecoverage area. Such a switched based multielement antennaconguration is proposed in []. Switching between theantennas makes it easy to optimize the coverage by control-ling a simplecircuit.Femtocell switches off the antenna at thatdirection where there is no user, thus lowering the chances ointercell intererences. Another coverage optimization tacticusing multielement antenna with tunable attenuator is shownin []. unable attenuator is a reliable option or coverageoptimization. It tunes up the radiation power to conrmthe required radiation pattern. Since a power amplier or

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2014, Article ID 480140, 8 pageshttp://dx.doi.org/10.1155/2014/480140
http://dx.doi.org/10.1155/2014/480140http://dx.doi.org/10.1155/2014/480140


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International Journal o Antennas and Propagation

Digitalattenuator

Digitalattenuator

Digital

attenuator

Digitalattenuator

Femtocell

Diplexer

Poweramplier

Switch

Antenna

Antenna

F : Multielement antenna conguration in emtocell (microstrip antenna).

each antenna will be costly or commercial use, a commonpower amplier with digital actuator or each branch isconventional.

Multielement antenna in emtocell application divides the

coverage area into multisections and each o the antennasoccupies same portion o scanning-area. As such, our direc-tional antennas have the angular coverage o or each othem.

In this paper, a microstrip antenna is designed ormultielement antenna conguration or emtocell coverageoptimization (Figure ). Te perormance o the antenna issimulatedin a -element emtocell like assembly. Te antennadesign and substrate choice result in a exible beam ormingin emtocell application. Te rest o the paper is arrangedas ollows: antenna design in Section , results inSection ,array conguration in emtocell in Section , discussion inSection , and conclusion inSection .

2. Antenna Design

Inside the emtocell, the available space is limited or antennapatterns. Only ew antenna types are possible or emto-cell operation, or example, planner inverted F antenna,microstrip antenna, and wire antenna []. Microstripantenna is a smart solution or small, efficient, and econom-ical wireless communication system. Features like small size,light weight, low prole, low assembly cost, ease o mountingon the surace, and integration with printed circuits board(PCB) enable its use in a wide range o wireless appliances[,]. However, compared with the nonprinted antennas,

it has ew disadvantages, or example, poor radiation patterndue to the excitation o surace wave, narrow impedancebandwidth, and low gain. Since the emtocell coverage regionis small (around m), microstrip antenna is a suitable choiceor a multielement emtocell conguration. It is capableo miniaturing design and cost effective solution [].However, to improve the bandwidth, radiation-efficiency, anddirectivity o the patch antenna, there are methods that usethick substrate, low dielectric substrate, multiresonator, stackcongurations, and various impedance matching and eedingtechniques. Te use o substrates with low thickness andpermittivity helps to reduce the dimension o the antennas[]. One o the methods o widening the bandwidth is using

slot on the patch o different shapes such as I, H, M, and U[,].

Te patch antenna in this paper is designed or LEBand . It is widely used in European, South American,

Australian, Asian, and Arican regions [, ]. It has oneo the highest bandwidth among the LE bands and it issuitable or emtocell operation. LE Band has uplink rom MHz to MHz and downlink rom MHz to MHz. Tereore, the coverage bandwidth o the antennais rom . GHz to . GHz. Substrate FR is used to designthe antenna. FR is a low cost and available substrate andpopular or commercial microstrip antenna production.

Te length and width calculation o the antenna isdetermined by using the ollowing ormulas.

Te width o the patch is given by Bhartia et al. [] as

=

2+ 1 /2,

()

where and are resonant requency and relative dielectricconstant o the substrate, respectively.

Now, the effective dielectric constant is expressed by Gilband Balanis [] as

=+ 12 + 1

2 1 + 12

1/2

, ()whereis the height o dielectric substrate.

Te actual length o the patch is given by [] as

= 2 2,

()

where is the extension o the patch length on both ends othe patch that is given by Hammerstad [] as

= 0.412+ 0.3 ((/)+ 0.264) 0.258 ((/)+ 0.8) . ()Te ground-plane length and width can be calculated as

[]

= 6 + ,= 6 + .

()


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Wg

W

L

Lg

b

c

f

d

e

g

Copper

Noncopper

a

F : Design layout o the proposed antenna.

: Design specication parameters.

Parameters Unit (mm) . . . . .

Te beamwidth o each microscript antenna and theresultant o any two particular antenna elements is a keyact in antenna designing. For design exibility, two resonantrequencies were considered to saeguard the whole bandcoverage. Tough the band is not that wide, two resonantpoints are taken so that it cutbacks the narrow beamwidthcharacteristics o the patch antenna. Te resonantrequenciesare about the middle points o the uplink and downlinkbands: . GHz and . GHz. Te theoretically calculatedpatch length and width or the two requencies are . mmand . mm and mm and mm,respectively. Te param-eter or these two requencies is then optimized with arectangular slot in the lower middle portion o the patch.Te dimension and position o the slot are also optimized

according to the ull coverage o the bandwidth []. Teopposite copper layer o the substrate is considered as theground plane. Te antenna and array structure are modelledthrough a commercially available nite element package CSStudio Suite. Te main patch is set on the top with . mmFR dielectric substrate with relative permittivity o . andtangent loss o .. Figure shows the dimension o theantenna andable contains the overall design specicationparameters.

Rectangular shape microstrip antenna is a basic shapein antenna engineering eld. However, the intention odesigning is to keep it as simple as possible. Te antenna isexcited with general microstrip line-eeding technique. Te

2.4 2.5 2.6 2.7 2.8

0

5

10

15

20

Frequency (GHz)

S11

parameter

(dB)

F :S-parameter o the designed antenna.

advantage o this method is that the connection o the patchand the eed is direct and all on the same substrate to provideplanar structure [,].

3. Results Analysis

Te impedance bandwidth o the antenna is given in Figure .Te return loss (S) o the antenna is


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120

150

180

150

120

90

60

30

40 15 10

Fareld (f = 2.53) [1]Fareld (f = 2.66) [1]

Frequency= 2.53

Main lobe magnitude= 6.1 dB

Main lobe direction = 4.0 deg

Angular width (3 dB) = 91.9 deg

Side lobe level= 10.3 dB

Frequency= 2.66

Main lobe magnitude= 6.5 dB

Main lobe direction = 5.0 deg

Angular width (3 dB) = 82.1 deg

/deg versus dB

Fareld gain abs ( = 0)

F : Radiation pattern at .GHz and .GHz.

(A

/m)

57.1

46.7

36.3

26

15.6

0

15.6

26

36.3

46.7

57.1

Surface current (f = 2.53) [1] (peak)

3D maximum:56.62

Frequency:2.53

1

(a)

(A

/m)

1

Surface current (f = 2.66) [1] (peak)

3D maximum:42.23

Frequency:2.66

42.8

35

27.2

19.5

11.7

0

11.7

19.5

27.2

35

42.8

(b)

F : Surace current on the patch at (a) . GHz and (b) . GHz.

Figure illustrates the radiation characteristics o theantenna at . GHz and . GHz. In both requencies, itshows broad beam and maximum radiation at the ront side.Te direction o the main lobes is almost at . Besides, thepeak-gains are . dB and . dB in . GHz and . GHz,respectively. However, the backward radiation in the mea-surement cannot be ignored. Since the antennas are placedin the outer surace o the house that contains the circuitryboard o the emtocell, a shielding plane or cavity can reduce

the backward radiation level. However, a shielded plane canreduce the antenna efficiency, while cavity can narrow downthe beamwidth [].

Figure shows the current density on the antenna patch.At . GHz, maximum surace current density occurs on themiddle o the patch and above the slot. However, at . GHz,the maximum surace current shifs to lower portion o theslot. For uplink and downlink bands, different side o the sloton the patch gets higher surace current density.


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1

2

3

4

F : Multielement antenna conguration.

4. Array Configuration inFemtocell Application

Single antenna emtocell with coverage optimization ea-ture can help very little in intererence reduction. Dueto omnidirectional radiation pattern, it cannot optimizeits radiation pattern according to the shape o the houseand users position. A switch based multielement antennacoverage with sel-optimization technique can select oneor a combination o two or three antennas according tothe users position and the shape o the house. Althoughsingle antenna emtocell ensures a aster convergence timecomparing with the multielement antenna, it also increasesthe mobility events that result in unnecessary core networksignalling. However, multielement antenna decreases themobility events and increases the indoor coverage moreeffectively without overshooting the outdoor users.

o analyse the perormance o the antenna in emtocellapplication, our microscript antennas are placed outside oa rectangular box on the same plane. Te our antennas areaced in our directions, mounted on each surace o the box,and separated by uniorm angle o as given inFigure .

Figure (a) illustrates the radiation pattern o the twoantennas and their resultant beam that is directed almost at angle with respect to their individual beams.Figure (b)shows the resultant beams o antennas and with vari-able eeding power through the digital attenuator or eachantenna. Te higher the amplitude o the eed-signal withrespect to one another, the more the resultant beam deectsto that direction. Tus, by changing the power level in eachport, the direction and coverage o the resultant beam can bechanged. Tis is an advantage that emtocell can exercise tonarrow down the beamwidth to serve a particular subscriberwithout interering with other subscribers. Figure (b) showsthe radiation pattern o our antennas and the resultant

combinations. One mentionable effect that might have anarray coupling is the spacing between the elements. However,in this case, the directions o the antennas are separated by, so it is not a bigger concern. However, the antennas arepositioned keeping the same distance among each other alikethe commercially available emtocell.

Finally, Figure shows the resultant path-loss o theantenna combinations. As all the antennas are directed

apart rom one another, the side lopes do not have anymentionable effect on their radiations.

5. Discussion

Te Femtocell operates on the same requency band alongwith the existing neighbouring emtocell andoutdoor macro-cell. Tereore, signicant impetus towards the deploymento emtocell in the dense coverage area is essential. Te user-equipment measurement report (UMR), inorms the mea-surement results on the uplink regarding the coexisting FAPsand macrocell service level. An effective intererence detec-tion algorithm senses the UMR and emtocell cooperation

message (FCM). Using multielement antenna congurationwith adjacent power controller (attenuator), emtocell canchoose proper radiation pattern to serve particular homeusers without overshooting the neighbouring emtocell useror outdoor macrocell user. Assigning the required SINRthreshold level will make it more convenient to emtocell toadjust the radiation pattern or each user.

6. Conclusion

Te design o a microstrip antenna or multielement antennaconguration or emtocell device and analyses o its peror-mance or emtocell operation are presented. Te proposed


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1050510

/deg versus dB

Fareld (f = 2.53) [1]

Fareld (f = 2.53) [1] + [2]Fareld (f = 2.53) [2]


(a)

/deg versus dB

Fareld (f = 2.53) [1[0.5] + 2[1.0]]

Fareld (f = 2.53) [1[0.5] + 2[1.5]]Fareld (f = 2.53) [1[1.0] + 2[0.5]]

Fareld (f = 2.53) [1[1.0] + 2[1.0]]

Fareld (f = 2.53) [1[1.5] + 2[0.5]]

0

30

60

90

120

150

180

150

120

90

60

30

1050510


(b)

/deg versus dB

0

30

60

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120

150

180

150

120

90

60

30

1050510

Fareld (f = 2.53) [1[1.0] + 2[1.0]]

Fareld (f = 2.53) [1[1.0] + 4[1.0]]

Fareld (f = 2.53) [1[1.0]]

Fareld (f = 2.53) [2[1.0] + 3[1.0]]Fareld (f = 2.53) [2[1.0]]

Fareld (f = 2.53) [3[1.0] + 4[1.0]]Fareld (f = 2.53) [3[1.0]]

Fareld (f = 2.53) [4[1.0]]


(c)

F : Radiation patters (a) two antennas and their resultant beam, (b) resultant beam o antennas and with different power eeding,and (c) resultant beams o possible combinations.


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2.4 2.5 2.6 2.7 2.8

Axis title

Chart title

S(1,2)

S(1,3)

S(1,4)

S(2,1)S(2,3)

S(2,4)

S(3,1)

S(3,2)

100

S1

1

parameter

F : S- parameters or different combination o antennas.

conguration allows each o the our antennas to adjust thepilot power to optimize the antenna coverage. Along with theswitching techniques, the microstrip antennas shape up theradiation pattern according to the house model and usersposition. Te antenna characteristics and the substrate choiceindicates that it is suitable or commercial deployment inemtocell devices. It will also reduce the cotier and crosstierintererence in dense emtocell network.

Conflict of Interests

Te authors declare that there is no conict o interestsregarding the publication o this paper.

Acknowledgment

Te authors sincerelyacknowledgethe nancial and technicalsupport rom Institute o Space Science (ANGKASA), Uni-

versiti Kebangsaan Malaysia.

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