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CELLULAR CDMA (IS-95) LOCATION, A-FLT (ASSISTED FORWARDLINKTRIANGULATION) PROOF-OF-CONCEPT INTERIM RESULTS

D. N. Nissani, 1 Shperling

Motoro la Israel Ltd.Kremenetski St 3 Tel Aviv 67899Email:bdn006@ email.mot.comT el: 972-3-5659210

AbstractThe A-FLT Location method for cellular CDMA (IS-95) is now incorporated into the IS-95standard either s a standalone method or s a companion complement o A-GPS (AssistedGPS). A Proof-Of-Concept project is reported which included the development of aprocessing platform (signal measurement and extraction, timing estimation, locationestimation), extensive urban (Tel Aviv center) field d t gathering, and processing andevaluation of the test results. Interim results on error and coverage are presented.BackgroundThe FCC has required by means of its E811 Phase I1 mandate that, s of October 2001,cellular subscribers in distress should be able to automatically report their positions with anaccuracy of 125 meter, 67 of the time. Other value-added services and applicationswhich require high precision subscriber location, such s Location Sensitive Billing andMobile Yellow Pages are emerging.

Technical solutions have been proposed for the different Air Interfaces (IS-95, GSM, etc.).From the cellular operator perspective, these techniques may be classified s such that donot require the replacement or enhancement of users handsets (and thus exclusively relyupon network equipment update- network based solutions) and those that are based onnew, location enhanced phones (handset based solutions). In particular, the IS-95standard has recently adopted A-FLT (as a standalone method or in conjunction withA-GPS), a handset based technique by which the handset estimates the time difference ofarrival (TDOA) of IS-95 pilot signal pairs emitted by the serving and neighbor base

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stations. The location best estimate is then calculated (at the handset or network) usingthese estimates s well s the participating base stationsknown locations.

A test configuration was set-up in order to assess the location error and coverageperformance of the A-FLT method. These s well s interim results are reported in thefollowing paragraphs.TestConfigurationA typical urban area of about 9 km 2, centered around the Motorola building in Tel Avivwas selected and 100random sites were marked. The precise (sub meter) location of the IS-95network (Pelephone) antennas (usually 3 antennas per base station, 15 base stations in total)located inside this area were measured by meansof a differentialGPS (dGPS) receiver and aLaser Ranger Compass (Figure 1).

. -a , .5 1 15 2 25 3

Figure 1.Tel Aviv center test area, base stations antennas (blue triangles), and random testsites (red circles)

An off the shelf high sensitivity, high resolution IS-95 Pilot Scanner was acquired. Thisprovided correlation results and allowed detection of pilot signals buried in (other basestations) interfering noise down to a level of Ec Io < -27 db where Ec is the energy perpilot spread spectrum chip and Io is the interference spectral density. The resolution of thismeasurement was chip 1 chip = 813.8 nsec in IS-95) thus allowing as per Nyquistsampling theorem) auto-correlation function alias-less reconstruction.

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A post-processing platform (Matlab based) was developed which provided for: a. pilotsignals detection, identification, interpolation and time-of-arrival estimation, b. TDOAcalculation, TDOA bias correction and TDOA variance allocation, and c. best locationestimate.

A data gathering campaign was launched, and data files including Pilot Scanner snapshots(about 100 snapshots per site, 100 sites), dGPS quality locations of test sites and basestation antennas were stored.Interim ResultsThe cumulative distribution function (CDF) of the location error samples is shown in Figure2.The results exhibit about 48 meter and 130meter at 0.67 and 0.9 probability levelsrespectively. It should be noted however, that sinF the TDOA bias correction processmentioned above artificially compensated for non line of sight and multi path effects s wells for systematic errors (such s inter base station synchronization errors) worse results may

be expected in realistic implementations.This issue deserves further investigation.C D F o f O u t d o o r E r r o r 1 0 0 si t es ) 5 M a x PN s , O8 - J u n - l 9 99 , A v = 54 l m . S t ddev-58 2m

1 I I

1

5 0 1 0 0 1 5 0 2 0 0 2 5 0 3met ers

Figure 2. CDF of 100 location error samples, center Tel AvivA precondition to carry out a location estimateis to possess timing estimates of pilotsignals originated in at least 3 different base stations (if a strict TDOA method is applied,or 2 base stations if complementarymeasurements are used).

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Plots depicting the probability to detect 1 , 2 or 3 base stations s function of pilotdetection sensitivity are shown in Figure 3. For Ec / Io< 26db (approximately the PilotScanner sensitivity threshold), the results (dotted lines) are extrapolated. The results showthat if at least 3 base stations are needed a sensitivity Ec Io < -30 db is required toachieve a probability of detection of 0.86.

Figure 3. Probability of detecting 1 , 2 , 3 base stations in at least 10 of snaps hots.SummaryInterim results of the feasibility of the A-FLT cellular CDMA location method both interms of error and coverage performance were reported. Several issues are under currentinvestigation, including the root cause(s) for the reported TDOA estimates bias.

The author is grateful to the many Motorolans who contributed to this effort includingYaniv Bargury, Ilya Chernyak, Baruch Altman, Hezy Sallach and Moshe Alon, all of themwith Motorola Israel.

References:[13T.S. Rappaport, J.H. Reed, B.D. Woerner, Position Location Using WirelessCommunicationson Highways of the future, IEEE CO. Magazine, 10/96, pp. 33-41[2] J.J. Caffery, Jr., G.L. Stuber, Overview of Radiolocation in CDMA Cellular Systems,IEEE CO. Magazine, 4/98, pp. 38-45

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