Direction Finding and "Smart Antennas" Using Software

8/14/2019 Direction Finding and "Smart Antennas" Using Software

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Direction Finding andSmart Antennas"UsingSoftware Radio ArchitecturesSoftware radio architectures,originally develope d for military applications,are now becom ing economically viable in commercial products becauseof therapid advance of DSP technologyJoseph Kennedy and M ar k C. Sullivan

pcrat ional tcs ts and demonstra-tion5 o f 5ystems based on softwarerad io s a r e cu r r en t ly b e in g p c r -fo rm cd i n t h c 800 M H z m o b i l ecel lu lar rad io band . lhesc f ie ldtrials arc formobile unit geo loca-tion systems and adaptive phased array "smartantenna" appliques. The geolocation system trialsar e in response t o the dem and fo r igh confidencegcolocation of mobile un i ts fo r enhanced emer-gcncy91 I scnricc and for usc n thcUS. epartmentof Transportation's Intelligent VehicleHighwaySystem (IVIIS) initiative. The smart antennaarrayappliques address the ccllul;lr service providers'nccd for murcU S C I - channel capacity and/or geo-graphic covcragc from cxisting hasc station instal-lations. Software radio architectu reswere selectedbccausc of thcir ability to provide superior per-f o r m a n c e ; I t low l i fe cycle cost . These systemsuse 4 to 8 widehand coheren t channels andullych a rac t c r i zc t h c a r r iv in gR F en e rg y to e i t h e rgeolocale the emitter or lo maximize the carrier-to-intcrfercncc (CiI) ratio.Software Radio BackgroundT . .he applicationof software radio techniques tomdltary communication?was conccived in the1970s. T h e in i t i a l g o a l w as t o p ro v id e h ig h e rfidelity in the HF band, where over-the-horizonco m m u n ica t io n s v ia i o n o sp h er i c p ro p ag a t io nwere important. ' I 'heHF band is ideally suited fortcsting softwa rc radio principles for wo reason s.First. advanced signal processing s uecessary i nth e IFband because of the highly dynamic natureof the channel due tn ionospher ic p ropagat ionan d a l so because of the h igh d ensity of signals.S cco n d , t h cHF b an d sp an s o n lya f ew t en s o fmcgaherlL, and generally only about 10 to 2C1 per-cent of the band will propagate ovcr the horizon.Softwarc radios capable of processing ev en hun -d r e d s o f k i lo h e r t z of b an d w id th m ay th u s b ceffective i n operating overRF bands of potentialinterest.Sof tware rad io arch i tcctu ral concep ts werereduced LO practice i n about 1Y80 in the first oper-

ationalmilitary digital HFcommunicationssy stem.The hardware required o implement the systemfilled many racks and consumcd a great deal ofpower. Theystem provided simu ltaneoua coverageof only 750 kHzof the HF band with a dynamicrang c of lcss than 60 dB. Still, the ability to employsophisticated signal processing algorithms, evenin th is modest hardware conf igurat ion , p rovedthe worthof software radio techniques.Exponential increases inDSP technology pcr-formancecoupledwith reductionsinsizeandpower(Moore's Law') over the last 20years have openedup applicationsof software radios to ine-of-sightR F b an d s . T h ro u g h o u t t h e 80s and '90s. the sesystems have been upgradcd throughive genera -tions.'The first few generation sof software radi oswe rcdeveloped to provide enhancedidelity and flexi-bility unavailable from analog receivers. Chang-ing needs could be met uickly and inexpensivelywith software upgrades. This abilityo ad ap t t od i f f i c u l t p r o b l e m s m a k e s t h e s o f t w a r e r a d i oa t t r ac tiv e i n co m m erc i a l t e l eco m m u n ica t io n sap p l i ca t io n s , w h ere i n f r a s t ru c tu re eq u ip m en tmight otherwise quickly become ohsolete due tocumulative do uble digit growth in mark ets, rapidlyevolving air standa rds, and rc quirem ents forewservices. Th e action of Moore's Law has driven thecost of the software radio down at a rate greaterthan othcr competitive technologies, providingthe ultimate m olivation for their se in the com-mercial telecommunications industryApplication to CellularIPCSInfrastructureT e current generation f commercial wirelessproducts employs readily availableSP tech-n o lo g y to d ig i t i ze t h e F sp ec t ru m w i th g rea tfidelity and then performall of the base stationfu n c t io n s u s in g so f tw are r ad io a r ch i t ec tu res .Most ol these functions are performedy execut-ing softwarc on a general-purposeDSP processor.but the first stage01 requcncy conversion a nd fil-tering is perfor med by a spccialized decimating


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H Figure 1. Tjpical . p e m ayout.

downc onver ter ch ip p roduced y Harr is Semi-conductor and others. This ircuit implements acascaded in teg rato r-co mb f i l ter fo l low ed y aconventional transversal filter to provide compu-tationally efficient sample rate conversion fromthe tens of megahertz at theAID converter to thetens of kilohertz occupied by the signal [ 2 ] .Using software radio techniques,a commonhardware platform may provide all of the currentb ase s t a t i o n fu n c t io n s an d y e t r e t a in en o u g hflexibility to meet future requirements throughalternative software application programs. Thep ro d u c t s a r e co n f ig u red f ro mse t of hardwarean d so f tw are m o d u les . i n t e r co n n ec t ed u s in gin d u s t ry s t an d a rd m ech an ica l , e l ec t r i ca l an dsoftware interfaces. These modules ay be com-b ined to p rov ide user-conf igurab le equ ipmentsuites including completeCellular!PCS macro-and microcell base stations and Smart Ante nna appliquks. Software selectable options includedomestic and international air standards. bascs t a t i o n co n t ro l l e r - to - sw i t ch in t e r f aces , f i x edomnilsector or smart antenn as, and ew servicefeatures such as fraud detection and mobile ter-minal gcolocation.

CAPITAL Direction FindingOperational Demonstration0 e example of commercial equipment usingApplied to IVHSTrackingg i L ocation (CAPITAL)sof tware rad io arch i tectu res s the Cel lu larproject, currently undergoing field trials in theWashington, D.C. suburbs. The concept behindCAPITALis that automobile trafficcongestionaybe monitored by tracking the progressof vehiclesequippedwithceIlularphones.Thiscomprehensivep ro g ram d em o n s t r a t e s a l lf thc capabilitiesofsuch a system, beginn ing with th e collection andprocessing of cellular data and proceeding to thedissemination of t raf f ic in fo rmat ion to remoteusersandin-vehicleequipment.Theoperationa1testmakes extensive use of the in-place cellular infra-structure for both wide area automobile trafficco n g es t io n m o n i to r in g an d co m m u n ica t io n s .G eo lo ca t io n eq u ip m en t h as b een co lo ca t ed a tselected towers to collect cellular phone usagestatistics and to geolocate phones n designatedroadways.

Cellular-basedwide areasurveillance techniquesarc bcing evaluated for se in automob ile trafficmonitoringapplicationsbecauseoftheircomparativeadvan tage over magnet ic sensors bur ied in theroadway. video cam eras, and other cc~n ventionaltechniques. These advantages nclude:Area coverage based on square miles, not vehiclecount or road miles.No disruption of road scrvicc for installation orrepair.Ordersofmagnitude lower incost than oop-basedapproaches.V ery h ig h r e l i ab i li t y w i th o w m ain t en an cecosts.* O t h e r u s e s i n c l u d e f l e e t m a n a g e m e n t a n demergency assistance.G eo lo ca t io n eq u ip m en t co u ld b e o w n ed an doperated by cellular se nic e providers.CAPITAL geolocatesmobile users of cellularrad ios th rough the recep t ion and p rocess ingfR F signals emit ted by o rd inary mobi le cel lu lartelephones. In this application the contentof th csignal is o f no interest and the privacy of the useris protected by not demod ulating th e aud io po r-tion of the signal and by replacing the electronicseri-a l number wi th a randomly ch osen ide n t i f ier .Geoloc ation is achieved u sing either the ReverseVoice Channel (RVC) or the Reverse Con tro lC h a n n e l ( R E C C ) t r a n s m i t t e d y t h e m o b il epro be behicle. The system ge olocates the emit-ter towithin -100 meters using a combinationo ltechniques. The ine of bearing to the emitter sestima ted at each eceiving site using the Alter-n a t in g P ro j cc t io n (A P ) m ax im u m l ik e l ih o o ddirection-finding algorithm [4 , 71. Time of arrivalfo r each signal is also measured with respec t toa GPS t ime reference so that t ime-difference ofarrival (TDO A) tech niqu es ay also be employedto fix the location. If the RV C is being processed,then thc requency of the Supervisory Audio Tone(SAT) is used to determine f the receiwdsignalsare from a common mobile nit, or from interfer-ing mobile transmi tten.RF signals are receive d at multiple eceivings i t e s , c h a r a c t e r i z e dby ang le of a r r i v a l , t i m estamped, and hen passed o a common site wherethe geolocation is d e te rm in ed .A typical layoutis shown in Fig. 1.Geolocation equipment basedonsoftwareadiosis located at selected towers and interconnected

-The actionof MooresLaw hasdriven thecost of thesofhvareradio downat a rategreater thanothercompetitivetechnologies.

-IEEE Communicationsagazine May 1995 63


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Antenna RF a -array RF recewer(8 channel)Baseband 8 %

Digitalnarrowband(8 channel)

Frequency referenceFigure 2. Hardware block diagram.

through exist ing back haul l inks to a control sub-system at the Mobile Teleph one Switching Office(MTSO). In Fig. 1,a transit bus s shown commu-nicating through a base tation via a cellular tele-phone. Geolocation equipment located at one ortwo other si tes are receiving the mobile cellulartransmission and estimating a osition. Th e algo-ri thms employed by the system to comba t multi-

Mobile at R e s t6031 Hz SAT837.06MHz

Ii

l Figure 3. PDAgram showing mulilputh.

1

Figure 4. PDAgram showing muitipnth and interference.

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path and interference would be ll hut impossibleto implement wi thout the use f sof tware rad iotechnology. Geolocation accuracy is qnhanced bythe following features:GPS-der ived c locks provide less than100 nsdifferential timing error from site to site.GPS-based tower siting removesny ding-basedbias errors.Coherent multichannel t imedelay allowsDSP-b a s e d s p a t i a l f i l t e r i n g t o b e u s e d t o c o m b a tmultipa th and interferenc e ffects.Phase i s ca l ib ra ted to hundredthsf a degreethrough DSP.A block diagram of the CAPITAL hardwaresshown inFig. 2. Signals from the eight-elementantenn a array are lock downconverted to an Fbefore the entire bandf interest i s digitized. Eightdigital tuner ASICs coherently downconvert eachnar rowband channel be ing moni torgd to a zeroIF complex baseband format.The resuping narrow-band da ta s then p rocessed by Intel is60 chips inthe Signal Processing Unit.S o f t w a r e i m p l e m e n t e d i n t h e C anguagee n u m e r a t e s a n d i s o l a t e s t h e c o m p o n e n t sf thereceived signal using he AP algorithm and spa-tial filtering.T h e SAT frequency of each compo-n e n t i s t h e n m e a s u r e d t o d i s t i n g u i s h b e t w e e n

com pone nts of the desired signal,wbich will alle x h i b i t t h e s a m e f r e q u e n c y , a n d t h o s ef co-channel in te r fe rence , whichw i ll g e n e r a l l y b eassigned a differe nt frequency. The isolated com -m o n S A T s i g n a l s a r e t h e n c r o s s c o r r e l a t e d t oidentify the component f the desired signal thata r r ives ear l ies t . The components a r r iv ing la te rare deem ed to ave resulted from multipath prop-aga t ion and a re d i scarded . The l ine f b e a r i n gand t ime f arrivalof the irect-raycomponent aresen t to a cen t ra l s i te over a 1 link to comple tethe geolocation of the emitter.T h e m a j o r i m p e d i m e n t t o r e l i a b l e g e o l o c a -t ion of mobi le un i t s in the ce l lu la r te lephoneb a n d i s s e v e r e m u l t i p a t h p r o p a g a t i o n . T h eeffects of this phenom ena m ust be m itigated inord er to achieve accurate po sition estimates withlow variance. Typical examples of the multipathencountered in the suburban Washiqgton , .C.o p e r a t i n g e n v i r o n m e n t w e r e c o l l e c t e ds ingins t rumcnta t ion in the geo loca t ion equipm ent .Specular multipath can be characterizedn t e rmsof power, delay, and angle-of-arrival relative tothe direct ray.

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.i i i i . t -

f N.

a n d b u t t er PDigitalbeamformer

Isolatedsignal 1

SATs

Find f i r s t rece lp tl a n de n d to GCS

iAngle and TO A

Figure 5. PDAgram iignu! isolutionprocess.

Th e ideal tool to measure these parameters isthe software radio,with its inhercnt fidclity anddigital storage capabil i ty. The signal parametersmay b e displayed n a PDAgram (Power-Delay-A n g l e d i a g r a m ) s u c h a s t h e e x a m p l e s h o w nnFig. 3. This PD Agram was gcncra ted f rom da tacollected by the CAPITAL system and shows threesignal comp onents, the dircct ay shown on the 0delay ax is and two mul t ipa th components wi thdiffcrcntial path length delaysof about 80meters.Figure 4 shows another PD Agram ohrained froma s i g n a l d a t a r e c o r d e x h i b i t i n g m u l t i p a t h a n dinterf eren ce. On e ignal exhibits a SAT frequencyof 6000 Hz a n d t h e o t h e r . i n t e r f e r i n g , s i g n a loriginates from a cell using a 6030Hz S A T f r e -quency assignment. Note that the interlerer alsohas a multipath component present.This record was processed to determine direc-tion of arrival for thc desired ignal. Th e desiredmobile signal multipath components, he interfer-ing mobile (shown n a different color), and inter-f e r i n g m o b i l e m u l t i p a t h c o m p o n e n t h a d t o h eremoved to determine the correct angle andim e ofarrival for the correct direct ray. nce thc ignalc o m p o n e n t s h a v e b e e n i s o l a t e d , t h e s o f t w a r eexamines the SATo detcrminc which compo nentscame from interfering cellular users or from mul-tipath reflcctions. The process used to isolate thesignals is shown in Fig. 5.This process s employedin thc DF equipment to raise the confidence nthe geo locationestimates y being sure the rightsignal is bcing geoloc ated. The result s accurategeolocation needed for Emergency91 1 service.

Smart Antenna ArraysA other comme rc ia l appl ica t ion of sof twarer a d i o t e c h n o l o g y i s t h e S m a r t A n t e n n a appliquC for exixing base stations.A field trial ata service providers network this Springil l beused to evaluate thebility of the Sm art Ant enna t o i n c r e a s eCi I for a n A d v a n c e d M o b i l e P h o n eS e r v i c e ( A M P S ) s y s t e m i n a n a r e a w i t hveryhigh densityof users.Incrcascd uscr capac i ty and expanded geo-graphic coverage from exist ing installat ions arethe two s t rong economic mot iva t ions vo icedyservice providersas the impetu s for these de vel-opments. Th e need for increased areaoverage isof primary concern in rural environments and forp r o s p e c t i v e PC S p r o v i d e r s . I n c r e a s e s i n u s e rcapacity are desir ed by existing service providersfor high-use areas where they are faced withossof poten t ia l revenue , cus tomer d issa t i s fac t ion ,a n d r e d u c t i o n n spec t rum ava i lab le for AMPScreated by the introductionof digi tal voicc scrviccs.Focus ing o n the capacity challenge, the realneed is to make more oice channels available ina g iven geographic a rea in the f in i te spec t rumallocated. Wh en translatin g this operatio nal needto a echnical requirement for an antenna system,the need is not for a steerable higher gain array,but instead an arraywhich can increase the carrierto interference ratio CII) at both thebase stationand the mobileterminal.f this can be achieved, thenmore RF channels can e added to the base s ta -tions in the area withou t riving the CII helow an


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-Preliminaryanalysesindicate thatsoftware-radio-basedSmartAntenna appliqukscan enhancethe CjI inboth theforward andreverne linkto allowf i e 0reuse of3 to 1.

acceptable limit. Suppose that the channels n aconven tional system without an array havc bccnallocated to maximke capacity subject to a minimumacceptable CII. If the smart array is employ ed, theC/I will increase, allowing the ad ditionof morcchannels until the/l is again brought to a minimumaccep tab le l ev e l . By in c reas in g th eC/I o f thedesired signal forhoth theorward and reverselinks,the smart array provides more channel capacity.There are th ree approachcshich can be usedwhen design ing a smarl ar ray to max imize/I.Th e first and most obvious s to creategain on th earray in the intended directionsing antenna aper-ture . This s do ne by using physically directive ele-m e n t s , o r b y c o m b i n i n g t h e o u t p u t sof morcelemen ts to create aper tu re , and thus a in . It isimportant to notehat it is not the ain that is ofvalue,

bu t the d i rect iv i ty which haa value in t ry ing tomaximize C,a.The lack of gain in the directionofinterference, ideallya null, may be mor e effectivethan gain in the directionof the desired signal.Thc secon d appro ach s the mitigationof mul-tipath fading. C/I budge ts are ge nerally d esignedwith a significant margin or slow and fast fading.For conventionalmobilc radio systcms, fast fad-ing inducedby multipalh propagation requires anadditionallink margin of 8 dB [3] . f the smart arraycan minimize the effectsof destructive specularmultipath combining, most of this margin mayberecovered. The multipath components presentnthe PDAg rams shown in Figs.3 and 4 are typicalo f thosefound inalong- termexper imen tperfo rmedin the Washington,D. C. a rea t o ch a rac t e r i~ e t h emobile radio propagation environment. Preventing

~i1

II

27 T

PDAgram an d array response or single signal withno muitlpath.

1 PDAgram andrrayesponseoruitlpath

PDAgram and a rray resp ons e for combined multipath and tnterferen ce.Figure 6 . PDAgrams and away response.y o f f h e SrnaflAntenna.

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the significant losses inC induced by multipathpropagation is one very effective usc ofa smar tantenna array.The final approach uscd o maximize CiI is theidentif ication and null ing of in te r fe re rs . Whend e s i g n i n g a n a n t e n n a e l e m e n t c o m b i n e r , t i sgenerally much easiero formsharp nulls than beamsfor a given numb er of elements in the array. I t sgenerallyeasiertoplace a lOdBull on aninterfe rerthan to c rea te beam wi th 10dB of gain in thedirection of thc desired signal. The operation ofthe Smar t Antenna s i llus t ra ted in F ig . 6 . I nthc f i r s t pa i rof graphs , the PDAgram showss ingle component and the cor rcsponding a r rayresponse shows a single beam. In the second pairo f g r ap h s , t h e P D A g r a m s h o w s w o m u l t i p a t hcompo nents . The a r ray resp onse ow inc ludesg a i n w i t h p h a s c s h i f t s t o c o h e r e n t l y c o m b i n ethese com ponents with the direct ray. In thc thirdpairofgraphsasourceofinterferenceispresentwithmult ipa th and the a r ray response shows cor re -sponding nulls.Other important factors o consider in designingSma rt A ntenna ppliques are theability to formb c a m s o n t h e r e v e r s e c o n t r o l c h a n n e l a c c e s sreque sts which arrive at unpredictable directionsand t imes, the abil i ty to interfaceo an existingbasestationwithoutviolatingproprietarydatainter-face agreen~ents and disruptingand-off manage-ment, and thc abil i ty to maintain the necessarylinearity in the equipment with increased dynamicrange (> 80 dB) imposed by higher antenna gainsand morechannels per base station.Software radio architectures support the imple-menta t ion of a l l th ree approache s to ax imizeCII an d also address the factors mentioned above.The architccture for the N-channel beamformeris shown in Fig. 7 . Each an tenna c lcmcnt s fo l -lowed by a wideband RF to baseban d conve rter ,where the entire operatingRF band is moved tobaseband at vely high dynamic range. The digitalhardware is a combination of custom boards andcommercial off-the-shelf hoards.Digital tuncrASICs coherently downconvert each narrowbandchannel to zero IF complex bascband format .T h e d i g i t a l b e a m f o r m e r i s i m p l e m e n t e d i nassembly language software cxccuted by a Tex asI n s t r u m e n t s TMS320C40 DSP chip. Assemblylanguage was chosen after C anguage imple-mentation lai led to execute n real timc. Each ofthc result ing narrowband signals s then upcon-verted digital ly and summ ed to produc e singlewideband slgnal thats then converted back to ananalog format using a high-speed D/A converter.An RF block upconverter com pletes the processby translating thc wideband IF back to the originalcellular R F band.The s igna l p rocess ing a lgor i thms providefully adaptive receive b eam an d switchcd trans-m i t b e a m w i t h f o u r a z i m u t h a l b e a m ser 1 2 0degree sec tor . The ne t apcr ture ga ins in excessof 20 dBi. The adaptive receive beams re createdby a specialircd variant f the constant modulusalgorithm (CMA) [I ,6, 71which execntes in soft-ware and maximizes CII through a combinationof spatial beam steering, interfcrcr null steering.and maximal ratio combining l multipath.Th e beamformcr so ftware stores the incom ingsignals in random access mem ory (RAM) whilethe bcam steering coefficients are being calculated.

$7ormervulttplex

converter

1 Basebandto RFconverstonIt

To Input port of exlsting base station-~ ~~-~Figure 7.Wideband digital beamfonner architecture.

Oncetheyarecalculated,thedataisrcadoutofRAhlthrough the beamformer. Signalswith unpredictablearrival timcsand directionsof arrival may thus beaccommodated ua ing the RAM delay buf fe r togive the beamformingalgorithmenoughime toni-t ia l ize the s teer ing vec tor . The a lgor i thm a lsoseparates and identifies signalsy SAT frequencyso tha t the beamformcr response has ga inn th edirections of desired signal components and nullsin thc dircctions of interfering components.This architecture also uses newly developedt e c h n i q u e s t o i m p l e m e n t s o f t w a r e t r a n s m i s -sion. Once the spatial filtering has been accom-plished, the signal is reconsti tuted digital lyndconverted with high fideli ty to R F so t h a t t h ebeamformed signals can be receiv ed by a stan-d a r d base s t a t i o n t h r o u g h t h e o m n ir s e c t o rantenna port .

In cur ren t ce l lu la r sys tems wi th omnid i rec-t ional or sectored antennas, interfcrcncc l imitsthe ch oice of frequ encies in each cell o one outof every seven. P reliminary analyscs indicate hatsoftware-radio-based Smart Antenna appliquescan enhance thcC,fl in both the forward andeverselink to allow Irequency reuse of 3 to 1.Use of this

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-In the filture,softwareradios willbe thetechnologyofchoice in a nexpandingaway ofmobile radioapplications.

type of a smart array represents a greater thant o 1 increase in capac i ty and suppor t s a 3 to 1increase in call arrival ratc for the same probabil-ity of blocked calls.SummaryS f tware radio architectures, originally devel-oped for military applicalions, areow becom-ing economically viable in commercial productsbecause of the rapid advance of DSP technology.The advantages of d ig i ta l t echniques over con-ventional analog approachcs arcmuch the sameas those of CD audio over the now-obsoletevinylLP technology: higher dynamic range and betterfidelity.As t h e demand for radio spectrum grows,the ab i l i tyof sophis t ica ted DSP a lgor i thms tocombat mul t ipa th fad ing ando rcduce interfer-ence will become increasingly valuable s a meansof adding capacity o cellular and PC S systems.In the fu ture , so f tware rad ios i ll be the tech -nology of choice in an expanding arrayf mobileradio applications.References[lID N Godard."Self-recovenngequalizatlonandcarrlertrackingintwo-dimens~onaldatacommun~cat~onsystems."lEEETrans.onCommun..

[21 B Hoyenauer, "A n economtcal class of digltal fllters for decima-V I . COM-28,o 11 . No v 1980. p 1867-1875tlon and Interpolation," lEEE Trans. on Acoustics, Speech, and SignaI Processmg. "01. ASP-29. no. 2. Aprll 1981, p 155-162.131 W. C. Y. Lee. Mobile Cellular Te lecommunlra t lonr . 2nd Ed., (NewYork McGraw-Hill, 1995)

[dl 5. K. Oh and C. K. Un, "Simple computatlonal methods of the APalqorlthm for maximum lhkelihood locallzatlon of multlole radlatIny sources." IEEE Trans. on Srgnal Process,ng. vol. 40. no 1 1 ,Nov. 1992,pp. 2848-2854[51 0 Smlth and J. S Abel, "Closed-form least-squaressource l o c a ~tlon estlmation from ranoe-dlfference measurements." IEEE Trans.onacoustics. Speech, a n d 5 g n a I Processing. vol. ASP-35 . no. 12,Dec. 1987. p 1661-1669

[61B I Sublett. R P Gooch, and 5. H. Goldberq, Separatlon an dbearing estimation of co-channel signals," Proceedings of the1989 EEE Milttaty Communlcations Conference, Oct. 1989.

[ I : I.Zlsklnd and M. Wax, "Maxlmum Ilkellhood locallzatlon of multbple sources by alternatlng projection." IEEE Trans on Acou5tiqSpeech, and Signal Processing. "01. 36, no. 10, pp 1553-1 60,oct. 1988.

BiographiesJOSPEH KENNEDY5the dmctor of the AdvancedComrnunlcatlonrSystemsgroup at Englneerlng Research Airsocldter, Vlenna. Virglnla, where heIS currently responsible for the develop ment of smart antenna andgeOlocatlon technology for government and commerclal markets. Herecewed an M.E.A. from Vlrgbnla Polytechnlc Instutlte. and M.5 andB . 5 . degrees ~nelectrical engmeerlng from the George Wah ny to nUnlverslty and the Univemlty of Vnrglnla, respectivelyMARKC. SULLIVAN IM 851 is a sclentlst at Englneerlng Research ASSOCI~ates. Vlenna. Vlrglnla. where he ~ surrentlydeveloping adaptivebeamformers for commercial applications. H e received a Ph D tnaEIS.lnsignalprocesslngin1989,fromGeorgeMasonUn~verslty~nFalrinformat~ontechnologyin1994,anM.S.~nstat~st~calrc~ence~n1991,andfax. Virglnia HIS ema ll address IS sullivan8era com.

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68 IEEE Cornn~unicat~onsagarme May 1995

Direction Finding and "Smart Antennas" Using Software

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