A Pattern Diversity Antenna with Parasitic SwitchingElements for Wireless LAN Communications

A.Khaleghi, J.C.Bolomey, A.AzoulayElectromagnetics Department, SUPELEC

Gif sur Yvette, FranceKhaleghi(lss.supelec.fr, Bolomey@ supelec.fr, Alain.azoulayW supelec.fr,

Abstract- In this paper, a pattern diversity antenna withswitched parasitic elements is presented for wireless LANapplication. A wire meander-line antenna on top of a circularmetallic ground plane is designed and used as the main radiator.Some conductive rods are installed on the ground plane, aroundthe antenna. Connecting and disconnecting of the rods to theground plane stirrers the antenna farfield radiation pattern.

Two optimized switching categories are used in rods selection. Agood impedance matching and low correlated radiation patternsare generated which results to an efficient diversity antennasystem.

The diversity signals correlation, in a uniform field scatteringenvironment and Rayleigh multipath fading channel, iscomputed from the complex 3D radiation patterns. It is verifiedwith the measurements accomplished inside a mode stirrerreverberation chamber (MSRC). The received signals correlationcoefficient and the diversity gain for selection combining areprovided.

Keywords-switched parasitic antennas; pattern diversity;wireless communication; mode stirred reverberation chamber

I. INTRODUCTIONIn wireless communications, the radio channel is impaired

by the multipath propagation. The effect of multipath at thereceiver end implies large variations of the RF signaltemporarily with deep fading or even sometimes a completeloss of the signal. The short-term variations of the RF signalcan be reduced by using antenna diversity techniques at thereceiver. Antenna diversity can be implemented using multipleantennas with sufficient space separation, differentpolarizations or different directed patterns. To benefit from theadvantages of a diversity antenna the signals combining isnecessary, from which a diversity gain can then be derived.Switch combining is a simple and commonly used method forbest signal selection in diversity configurations. This requires alow loss switches and possibly duplication of some front-endcomponents.

In this paper we present a pattern diversity system with asingle port antenna, so no switch is required in the direct RFbranch. A dual radiation patterns are generated with parasiticelements switching that are installed near to the antenna.Active controlling of the switches and connecting ordisconnecting of these parasites to the ground plane, generates

two low overlapped radiation patterns. The received signalsthrough these patterns are uncorrelated and results to highperformance diversity system with a good diversity gain.

II. PARASITIC ELEMENT CONCEPTThe antenna pattern diversity method considered in our

study is based on the idea of the switch parasitic elements asproposed by Vaughan [1] and Scott [2]. The radiation patternof an antenna is modified by placing a second antenna close toit [1-3]. It was shown that, it is possible to change the pattern ofan antenna by terminating the parasitic antenna to the groundplane or by leaving it open [1]. The radiation pattern changecan be erupted either in pattern amplitude or phase [3]. Then,the diversity gain is obtained by selecting the best uncorrelatedpatterns.

Placing a parasitic element close to the antenna, produces acoupled current on the parasite and the altered currents in turninteract with the active antenna element, modeled by themutual impedance. The mutual impedance is a function of thephysical structure and also the parasite element loadimpedance. The action of the changes in load impedance causeto changes in mutual impedance and their combined effect onthe active element cause the changes in current distribution,consequently the radiation pattern changes.

Using the array form of the parasitic elements andoptimizing the load impedances together with the physicaldimensions, offering directional patterns. Directional patternswith switching facilities provide more received powers at themultipath environment compared to omnidirectional antennas.The pattern switching also suppresses the fast fading effectsand increases the radio link quality.

III. ANTENNA DESIGNFig.1 illustrates the antenna consisting of a wire monopole,

dual meander-line antenna with equally distributed vertical andhorizontal parts, located on a small circularly shaped groundplane. Fourteen conductive rods are installed around themonopole antenna. Connecting or disconnecting of the roads tothe ground plane cause to the different radiation patterns. Thegrounded rods work as reflective elements and thus theradiation pattern is directed away from these rods. In contrast,leaving the rods on air cause the radiation to pass withoutsignificant interference [4]. Different combinations of the

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Figure 1. Meander line antenna with parasitic switched elements; A) Antenna prototype B) Meander-Line antenna on ground plane C) Switch parasiticelements positions (S Short circuited elements 0 Open circuited elements)

parasitic elements allow the formation of different radiationpatterns.

In our application for the wireless LANs communication, inindoor multipath scattering field environment, we will try tocover all of the space angles with two sufficiently directiveradiation patterns. The operating frequency is at 2.45GHz and abandwidth of 80MHz is required.

To form the most directive beam and sufficient coverage,

half of the parasitic rods are shorted to the ground plane andthe other half are left open on air. The generated pattern canthen be stirred in space by alternating the open and short rods.

The main radiator antenna is a dual meander-line wiremonopole prototype with five vertically oriented sections (seeFig.1.B). The antenna is fed through a 50Q SMA typeconnector, penetrating the circular ground plane andconnecting to the meander antenna. The meander-linetechnology allows to design antennas with a small size andprovides wideband performance [5]. One side of the designeddual meander-line antenna is left open circuit. This improvesthe input impedance and the resonance bandwidth of theantenna (Fig.1-B). The current flow on the wire meander-lineantenna is most in the z-axis direction and the x-axis directedcurrents are suppressed by the adjacent opposite currents. Sothe resonance length of the antenna is defined by the verticalheight ofthe antenna on the ground plane.

At the first try of the antenna design, all dimensionsincluding: meander antenna height on the ground plane,circular ground plane radius and the rods lengths and distancesfrom the meander antenna are selected 0.25k (k is thewavelength). This is a more or less good approximation usedfor the Yagi-Uda antenna elements design which selects about0.25k for the reflector length and distance from the mainradiators and directors [6].

Considering the above initial dimensions, the antenna issimulated using 3D time-domain transmission line matrix(TLM) method code which is commercially available withFlomericso/Micro-Strips.

First, the wire meander-line antenna is simulated on thecircular ground plane. The wire radius is considered to beconstant with 0.3mm radius. The meander antenna dimensionsare optimized to generate a good impedance matching at the2.45GHz band. The equally lengths conductive rods withconstant radius of Imm (0.008X) are located environ the wireantenna on the ground circle edges. The rods are equallydistanced with 0=22.5° angular separation (Fig.l-C) and thereare no rods at the x-axis direction. This generates a radiationpattern with more spatial pattern coverage and so lower patterngain. Half of the parasites are connected to the ground planeand the other half are floating on air. The meander antennaimpedance matching at 2.45GHz is highly affected by thepresence of the grounded parasite rods. This is explained by themutual interaction of the induced current on the groundedparasites and their effects on the main antenna currentdistribution. To suppress or compensate this effect we use oneon the next half side parasites for impedance tuning objective(as indicated in Fig.1-C). Connecting the indicated element tothe ground plane recovers the previous optimal impedancematching. This also changes the radiation pattern and generatesa more directive pattern with lower side lobes. A symmetricalconnection is also considered for the second state of theantenna. Using the above assumptions, actually, 6 top or 6bottom rods are used for pattern switching.

The aim of the optimization process is to generate lowoverlapped radiation patterns and good impedance matchingperformance. The antenna optimization factors are: the groundplane radius, the rods length and distance from the antenna, and

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Figure 3. Measured and simulated return loss (dB) versus frequency(GHz)

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(B)Figure 4. Simulated 3D farfield power pattems (dBi) versus azimuth and elevation angles A) +Y axis side rods are switched to the ground plane B) -Y axis

side rods are switched to the ground plane

the meander antenna height on the ground plane. Theoptimization process shows that, there are a wide range of theresults which compromise our criteria. One of the optimizeddimensions is detailed in Fig.l. The ground plane radius is0.2k, the rods length areO.281 and the meander height is 0.23X.

IV. ANTENNA SIMULATION AND MEASUREMENTThe prototype antenna with the optimized dimensions

shown in Fig.l is modeled using the TLM method code. Theantenna is constructed. A photograph of the antenna is shownin Fig.2. The meander antenna is developed from a solid thinwire and then installed on top of the circular metallic groundplane. The antenna is fed through an SMA type connector onthe ground plane. The rods are constructed from long screwsand are installed inside holes that were created around a plasticcylindrical material (see Fig.2). This cylinder is fixed to themetallic ground plane. The rods screwing make the facility ofmanually connecting or disconnecting to the ground base. For aswitch off condition an air distance of 2mm is consideredbetween the rod and the ground base; for the switch on state thescrews are directly connected to the ground. The antenna doesnot have switching ability and it emulates the ideal switchablearrays.

Fig.3 shows the simulated and the measured return lossversus frequency, for one state of the parasitic elements (seeFig.l-C). For the second state, as the structure is symmetric,the same results are obtained. The antenna operates between2280-2488MHz with a return loss less than -10dB. Themeasurements and the simulations are in good accordance.

The farfield patterns for 0 and p, spherical coordinatevector components, for each antenna state are calculated inamplitude and phase. Fig.4 shows the farfield power patternsversus space angles. A directive gain (5.5dBi) is indicated at750 azimuth and 300 elevation for a case with upper sideswitched rods to the ground plane. If the lower side rods switchto the ground plane, the maximum gain will move to the -750in azimuth angle. These two states with two different radiationpatterns are selected for our pattern diversity application. Thetwo patterns cover all the azimuth angles and the elevationsgreater than -40° with an antenna gain of more than OdBi. Theantenna gain for lower elevations is reduced by the groundplane effects. Using a cylindrical shaped ground plane withsufficient height increases the lower elevation coverage [6].

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Figure 5. Measurement setup inside MSRC; A transmitter horn antenna,and the two receivers: diversity prototype and a dipole antenna

V. SPATIAL CORRELATION COEFFICIENT

The signals correlation between two antenna configurationsis a performance factor that shows the diversity antenna gainimprovement. It depends on the antenna complex radiationpatterns, incident field angle of arrival (AOA) and crosspolarization ratio (X) [7].

Usually, A uniform scattered field AOA is used for indoorradio channel modeling [8]. To provide a good accordance withthe measurements (section VI) the correlation coefficient iscomputed inside a Rayleigh fading channel with uniform fieldscattering distribution and unpolarized case (X=l). Using theseassumptions the complex correlation in [7] is simplified to:

P0C =If(E6OE;6 + ElnE;,,dQ4, n (1)

Elo, Elq, E20 and E2, are the complex envelopes of the 0 and(p components of the field patterns of the antenna states I and 2respectively. By applying the simulated complex patterns to (1)the correlation coefficient is calculated. Using the previousassumptions the envelope correlation (Pe) is generallyconsidered to be approximately equal to the square of thecomplex correlation. Table I shows the correlation values andthe theoretical diversity gains for selection combining (for anequivalent correlation coefficient [9]) at 0.1 and 0.01cumulative probabilities.

VI. DIVERSITY ANTENNA MEASUREMENT

The diversity antenna is tested inside a large cubic modestirred reverberation chamber (MSRC). A metallic tilted cross

panel stirrer is used inside the chamber which changes thecavity modes of the chamber by turning and provides astatistically uniform field scattering environment. The crosspolarization ratio of the inside field is statistically distributedabout OdB (X=I), so the MSRC models an indoor, uniform andunpolarized, high multipath environment [10-11].

TABLE 1. MEASURED AND SIMULATED CORRELATION COEFFICIENTAND DIVERSITY GAIN FOR SELECTION COMBINING

Diversity parameters Simmiated Measured

Complex correlation (pc) 0.0513 - 0.0046j 0.02+ Oj

Envelop correlation (pt) 0.0026 0.0004

Diversity gain (dB) at 0.1 Theory: 5.6 5.8

Diversity gain (dB) at 0.01 Theory: 10.2 9.7

The measurement setup is shown in Fig.5. A singlefrequency CW signal is injected inside the chamber using ahorn antenna. The antenna is directed toward a metallic edge ofthe chamber to provide a non line of sight (NLOS) propagationchannel. The possibly back lobe radiation pattern of the hornantenna is also reduced by positioning a grounded metal screenat the antenna backward direction. The generation of a timevariable multipath propagation inside the chamber is made bythe rotating stirrer, the rotation being controlled with acomputer code. The statistical properties of the measuredsignals amplitude using any arbitrary antenna inside thechamber show a quasi Rayleigh type of propagation [10]. Themeasurement inside the chamber is statistically repeatable withperiodical rotation of the stirrer.

An E8358A Agilent synchronized vector network analyzeris used in three ports measurement mode for data acquisition(see Fig.5). It is set to work in single trace, single frequency(2.45GHz) and zero span mode. One port is used as transmitterand is connected to the horn antenna and the two others areused as receivers and are connected to the diversity antennaand an arbitrary dipole antenna. The S-parameters between thetransmitter port and the diversity receiving antenna togetherwith the dipole receiving antenna are simultaneously measured.1610 samples of the received signals at each antenna port arerecorded in 100 seconds per turn of the stirrer. The complexsignals at the antenna ports are acquired and are used foroffline processing.

The diversity antenna working state is manually changed tothe next one. The measurement inside the chamber is repeated,and the received signals with the diversity and dipole antennasare acquired. Periodically rotation of the stirrer inside MSRCprovides the same scattering field environment. This results tothe same antenna signals if no variations are made at theantenna location. The dipole antenna signals are used as thereference at two measurement procedures. The received signalsby the diversity antenna at the two working states are timesynchronized using the dipole signals information.

The Correlation coefficient between the diversity signalsis calculated from the two sets of the recorded signals (Table 1).The cumulative distribution function (CDF) plot of the

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Figure 6 Measured CDF plot of the signal powers(dB),normalized to mean, by the pattern diversity antenna with andwithout applying the parasitic elements switching. Measurements

are performed inside a MSRC

recorded signal powers is computed for each state and afterapplying selection combining (SC) (Fig.6). The diversity gainsfor the cumulative probabilities of both 0.1 and 0.01 areextracted from the CDF plots and are tabulated in Table I.

Small correlation coefficient and good diversity gain areobtained from the measurement and the simulation. Theseresults make the antenna appropriate for Wireless LANcommunication with pattem diversity ability.

VII. CONCLUSION

A patter diversity antenna with a single RF port andswitching parasitic elements is presented for wireless LANcommunication. Two sets of uncorrelated patterns aregenerated with the parasitic elements switching to the groundplane. The antenna impedance matching is established byfixing a rode element to the ground base. Good impedancebandwidth of 200M1-z at 2.45GHz is observed for a return lossless than -10OdB.

The signals correlation coefficient is first calculated fromthe modeled complex patterns and then measured inside a

unifonn field scattering environment of MSRC. A specialmeasurement setup is proposed which avoids the use of theactive switches in the test procedure. Envelop correlation of0.0004 is measured. The diversity gains of 5.8dB and 9.7dB at0.1 and 0.01 cumulative probabilities are obtained withselection combining. The gain factor of the pattem diversityantenna is close to the theoretical diversity gain of spacediversity antennas with equivalent correlation values.

REFERENCES[1] R. G. Vaughan, "Switched parasitic elements for antenna diversity,"

IEEE Trans. Antennas & Propagation., vol. 47, pp. 399 405, Feb. 1999.[2] N. L. Scott, M. 0. Leonard-Taylor, and R. G. Vaughan, "Diversity gain

from a single-port adaptive antenna using switched parasitic elementsillustrated with a wire and monopole prototype," IEEE Trans. Antennas&Propagaion., vol. 47, pp. 1066 1070, June 1999

[3] A.khaleghi, A.Azoulay, J.C.Bolomey "Diversity techniques with dipoleantennas in indoor multipath propagation" PIMRC 2005

[4] D.V.Thiel, S.Smith, " Switched parasitic antennas for cellularcommunications" Boston, MA: Artech House, 2001

[5] M. Ali, S.S. Stuchly, and K. Caputa, "A wide-band dual meander-sleeveantenna," Journal of Electromagnetic Waves and Applications, Vol. 10,No. 9, pp: 1223-1236, 1996

[6] Schlub, R.; Thiel, D.V.; "Switched parasitic antenna on a finite groundplane with conductive sleeve" IEEE Transactions on Antennas andPropagation, Vol.: 52, Iss.: 5, pp. :1343 - 1347, May 2004

[7] J.D.Parson, "The mobile radio propagation channel ", Second edition2000, John Willey and sons, pp:330-335

[8] Alayon Glazunov, A." Theoretical analysis of mean effective gain ofmobile terminal antennas in ricean channels" 56th IEEE Conference onVTC, Vol.3, pp: 1796 - 1800, Fall 2002

[9] Mattheijssen, P.; Herben, M.H.A.J.; Dolmans, G.; Leyten, L.; "Antenna-pattern diversity versus space diversity for use at handhelds", IEEETransactions on Vehicular Technology, Vol.53, Iss.4, pp: 1035 - 1042,July 2004

[10] Khaleghi. A; Azoulay. A; Bolomey .J.C "Diversity antennacharacteristics evaluation in narrow band rician fading channel usingrandom phase generation process" 61th IEEE Vehicular TechnologyConference, Spring 2005, Stockholm, Sweden

[ 11] N. K. Kouveliotis, P. T. Trakadas, and C. N. Capsalis , "FDTDmodelling of a vibrating intrinsic reverberation chamber "Journal ofElectromagnetic Waves and Applications, Vol.17, Pages: 849-850, 2003

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