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provides better call reliability.Normallya user is covered by morethan one satellite.If the user movesinto an area that shadowsor blocksaccess to one satellite, the spacediversitylinkthrougha satellitethatis not blocked maintainsuninterruptedusercommunications.

Gateways are illuminated by anearth coverage beam. The Gatewayconnects the user telephone to theterrestrialnetworkthroughan earthterminal that acts as the gatewaybetween the Globalstat system andthe terrestrialnetwork.

UserTelephoneEquipment

The user telephonescome in severalvarieties - hand held units, mobileunits and fixed station units. Thecandidateuser telephonesare listedin table1.

units typically have a higher gainantenna,a lower noise receiver,anda higher RF power output that isprovidedby the adapterkit.

Fixed. Fixed station terminals areGlobalstar only. The fixed userterminals have multiple antennaspointed at different portions of thesky. Each antenna element hashighergainthanmobileorhandheldunits and one antenna is used at atime. This permits lower satellitepower in the forward direction andlower interference in the reversedirection.

Gutewoy

The Gateways are geographicallydistributedby the serviceprovidersto serve their customer base.Gateways are designed forunmannedoperation. The Gatewayconsists of up to four identical

Table 1 Pre-ProductionUser Telephones

I Fixed Tennina/ I Mobile and Hand Held IIGlobalStarOnly IGlobalstarOnly I

I IDual ModeGlobalstar& GSM I

I Tri Mode Globalstar& TerrestrialCDMA&AMPS

The radiatingelementofthe antenna is positioned above thehead of the user. The antenna ispositioned vertically to effectivelyutilize the symmetrical radiationpatternof the handheld antenna.

parabolic dish antennas, drivemechanisms for positioning theantenna,the Code DivisionMultipleAccess (CDMA) equipment, PSTNinterface equipmentthat interfaceswith the terrestrial telephonenetworkand computerequipmentto

Md2ik memobileunitsconsistof a operate the Gateway and collecthand held unit inserted in an statusandperformancedata.adapter in the vehicle. The mobile

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The Gateway connects theGlobalstar space segment toterrestrialswitchingequipment.TheGateway receives telephone callsfrom the terrestrial switchingequipment and generates CodeDivision Multiple Access (CDMA)carriers to transmit through thesatellite. The satellite then re-transmits the signal to the usertelephones. These user telephonesmay be either hand held, fixed ormobile and located anpvhere withinthe satelliteantennafootprint.

In the return direction,the Gatewayreceivestransmissionsfromanyuser

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telephone and connects the call to “terrestrialswitchingequipmentthatcan then connect to any subscriberusing the standard telephonesystem. Connections can also bemade to terrestrial cellularsubscribers or to other Globalstarusertelephones.

GlobakturSatellite

The Globalstarsatellite is a simplelow cost satellite designed tominimize both satellite costs andlaunch costs. A pictorial of thesatellite and some of the majorcharacteristicsare shownin figure3.

SatelliteDepth=68.6cm(Earthto Anti-EarthPanel)

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A user telephone transmits to thesatellite by L-Band. The signalenters the satellite through the L-Band low noise amplifier. It isamplifiedand then convertedinto aC-Band signal. This is radiated tothe Gateway.The Gatewayreceivesthe signal and downconverts to anintermediate frequency. Thecommunicationstraffic is presentedto the CDMA equipment fordemodulation.

In the transmit direction, theGateway combines the up linkCDMA signals with the signal hornthe command transmitter andradiates it at C-Band up to thesatellite. The satellite thendownconverts the signal andradiatesan S-Banddownlink signalto the usertelephones.

Gmurd o~rutions contro/CefMer/SutdlifeOperationsControlCenfer

Ground Operations Control Centeris responsible for planning andmanagementof the communicationsresourcesof the Globalstarsatelliteconstellation. This is coordinatedwiththe SatelliteOperationsControlCenter that manages the satellites,controls the orbits and providesTelemetry and Command servicesfor the satelliteconstellation.

ChannelPropagationConsiderations

The channelpropagationmodelingisof paramount importance indevelopinga communicationsystem

for both performancepredictionmidsystem design. For the terrestrialcellular telephone system, thepropagationchannel has been wellstudied.However,in the Globalstarenvironment further work isrequiredto developa comprehensivemodel. For the terrestrial system,typically the user equipment doesnot receive a direct line of sightsignalcomponent.Instead,the signalis composedof the reflections bornmany diffuse scatterers. InterrestrialCDMA,we employa Rakereceiver design to combine energiesilom differentpathsto offsetsomeofthe signalfading.For the Globalstarsystem, the typical scenario willenable the user telephone tomaintain a direct line of sightcomponent with the satellites inview. For the Globalstar systemlocatedin rurallocations,mostof theobstructionsof thedirectline of sightcomponentwill be due to vegetativeshadowingthat does not completelyblock the direct line of sightcomponent, but rather simplyattenuates it. Therefore, theGlobalstar channel is significantlybetter than the terrestrial channelhorna signalfadingperspective.

Significantimprovementin channelpropagationcomes from the use ofcontinuous multiple satellitediversityemployed in the design ofthe Globalstar system. The use ofsatellitediversitywill help mitigatesignalfadingand greatlyreduce thepowerrequirementsto originateandmaintaincalls relativeto operationswith a single satellite. Given twosatellitesin view at the same time,the probabilityof signal blockageshadowing to both satellites

oris

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significantly less than theprobability of blockage to a singlesatellite.

Another difference with terrestrialsystem is vastly different forwardand reverse link frequencies. Thecorrelationbetween signal fades onthe forward and reverse linkfrequencies will be used indeveloping strategies for powercontrol. One of the major problemsassociated with the land mobilesatellite channel is the long delaysassociatedwith the transit times toand born the Gatewaysthroughthesatellite.This satellite link delay istensof millisecondslongcomparedtoless than a couple of millisecondsinthe terrestrialcase.

Signal Components

The signal received at a user phoneis composed of three components:direct line-of-sight, specularreflectionand difise reflection.Thediffise componentis composed of asum of a large numberof individualterrain scatterers horn outside thefirstFresnelzoneof thevehicle.Thisdiffise component is characterizedby phase incoherentmultipathwitha uniform phase distributionand aRayleigh amplitude distribution(hence known as Rayleigh fading).Thesignalfadingassociatedwiththedifise component combining withthe direct component produces thefast-fading characteristics of thepropagationchannel.

The direct signal component issubjectto shadowingby obstaclesinthe local environmentsuch as treesand buildings. This form of fading

has fade rates that are significantlyless than the Rayleigh componentandis primarilydue to the motionofthe mobile unit with respect to thegeometry of the object causing theshadowing. In the presence ofvegetative shadowing,a direct line-of-sight signal is subject toattenuation by the surroundingvegetation. The direct line-of-sightsignal has an amplitudecharacteristic that can best bedescribed as being log-normallydistributed [2]. This direct line-of-sight signal combined with theRayleigh distributed multipathsignal from the diffime scatterersprovides a composite Riciandistribution with a line of sightsignal amplitudevariability definedby the log normaldistribution.

In the case of travel through a city,the most significantcharacterizationis the percentageof time the directsignal path is blocked due to thegeometryof the local buildingswithrespect to the signal line-of-sight.Ithas been found that the descriptionof the distribution fhnction duringtimes of direct line-of-sight signalblockage is best described by aRayleigh distribution whose meanreceivedpoweris describedby a log-normal distributed variable. Ininstances where there is no signalblockage, the distribution is bestdescribedby a Riciandistribution.

ChannelModeling

Giventhe limitedsatelliteresources,estimationof systemperfoxmmnceiscriticallydependentupon the signalpropagation losses. The purpose ofthe channel modeling activity is to

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accurately characterize the signalpropagationfrom (or to) the satellitewhen communicating with a GStelephone user. Signal propagationcharacteristics are stronglydependent on the environmentsurroundingthe usertelephonesuchas the height and density ofvegetation,the locationof manmadestructures, nearby hills, etc.Therefore, the sign alcharacterization must account forthe type of terrain one expects toencounterandthe locationof objectsin the environmentthat impedethedirect line of sight between thesatelliteandusertelephone.

At different locations and terraintypes, we are concerned withdetermining the following signalcharacterizations for each of thestatesidentifiedabove:

1.

2.

3.

4.

5.

6.

7.

The signalfadepowerprobabilitydistributioniimctions,

Characterizationof the modelingparameters of the magnitudefunction for the direct line ofsight componentand the difisecomponent

Temporalcharacterizationof thediifbsesignalcomponent

Average fade and non-fadedurations

Probability distributionsof fade(and non fade) durations below(above) a series of signalthresholds

Timedelayspread

Cross correlation of signalmagnitudefunctionsbetween L-bandandS-bandsignals

This signal characterizationat eachlocation is dependent upon thegeometry between the satellite andthe user telephone (primarilysatelliteelevationangle and the lineof sight vector between the satelliteand the user telephone), motion ofthe user telephone, and the usertelephoneantennacharacteristics.

Characterization of theenvironments to be encountered isalso a big factor to be considered.This characterization involvesdeterminingthe percentage of timeone would anticipate encounteringthe various terrain types in a givenlocale.

S yDesignConsiderations

The sections below provide anoverviewof someof the designissuesin the Globalstar system withchannelpropagationconsiderations.

Contfo/

To achievethe high capacity,qualityand other benefits inherent in theCDMA waveform design, theGlobalstarsystemwill employpowercontrol on both the forward link(Gateway to user telephone) andreverse link (user telephone toGateway) communications. On thereverse link, the objective of thepower control process is to producethe same nominal received signalpower from each transmitting usertelephone. On the forward link, theobjectiveof the powercontrolprocessis to provide a minimum power at

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each user telephone adequate to

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achievethe desiredqualityof servicefor eachusertelephone.

The principal problem facing thedesigner of the Globalstar powercontrol is the large round trippropagation delay that reduces thetracking capability of closed looppower control. Fortunately, thesatellitechannelis inherentlyRiciandue to the strong line-of-sightcomponent, as opposed to theRayleigh channel prevalent interrestrialcellularsystems.Thishasthe implication of reducing thetracking requirement for powercontrol, which now has to contendonly with the effects of shadowingand/or blocking by trees andbuildings. These effects havedifferent dynamics than diffusemultipath fading effects. TheGlobalstar system design alsoprovides for multiple satellitediversityfor mostusers.Thishastheeffectof reducingthe fluctuationsinsignal level. Due to the large roundtrip delay, the bandwidth of thefading processes that can becompensatedfor with power controlis limitedand must be compensatedusing a combinationof interleaving,codingandopenlooppowercontrol.

To compensatefor the sluggishnessin the closed loop power control,investigations of open loop powercontrol strategies are beingconsidered. In the open loop powercontrol, the user telephone (orGateway) measures the receivedforward (or reverse) link signal andadjusts the power of his transmitreverse(or forward)link signal.The -open loop power control can beoperated at higher operational

bandwidths and can thereforereactmore quickly to its environment.Essential to the development ofclosed loop power control strategiesis a betterunderstandingof the crosscorrelation between L-band and S-band signaling.

SystemCapacity

Computation of the capacity of theGlobalstar system is a complexproblem. However, one of theoverridingfactorsin the computationof the channel capacity is theinclusion of channel propagationcharacteristics. Satellite resourcesare finite and must be carefullycontrolled.The channelpropagationcharacteristics coupled with thepower control process dictates theamount of diversity gain to beachieved by employing multiplesatellites, the peak power requiredby a satellite and the total powerconsumed by the satellite. Ofparamount interest is thedetermination of propagation stateprobabilitiesas a fhnctionof satelliteto user telephone geometry in avarietyof differentlocations.

low LewdDesign

The low level design of theGlobalstarsystemis dependentuponthe characteristics of the receivedsignal after the application of thepower control process. The residualchannel characteristicsare used forthe design of plethora of differentissues. In our application, thechannelcharacteristicsareprimarilysuperiorto those encounteredin thecellularsystem.

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Sufnmary

In this paper, an overview of theGlobalstarsystemwas providedwithemphasison designissuesassociatedwith the channel propagationcharacteristics of the land mobilesatellite channel. Based upon acomparisonof the terrestrialcellularpropagationchannel,the landmobilesatellite channel provides manyadvantages.However, fhrther workis required to completelydefine thepropagation channel for theGlobalstardevelopmenteffort.

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