The subject area of this thesis is the locating of mobile users using the future 3rd generationudspread spectrum communication system UMTS. The motivation behind this work is twofold:udfirstly the United States Federal Communications Commission (FCC) mandated the provisionudof user location into services in the United States of America due to the increasing number ofudemergency calls originating from unknown locations. Secondly the user location can enableuda number of other potentially profit–making applications and services. These are generallyudthought to be the important new applications of the third generation mobile networks.udThe UMTS standard has now made provision for a time difference of arrival based mobileuduser location system in which the mobile measures time differences of arrival of received signalsudfrom surrounding base stations (BS’s). There are two main problems to such a technique:udfirstly the problem of detecting enough base stations to make a location fix, the so called ‘hearability’udproblem. In spread spectrum systems all base stations transmit on the same bandwidthudthus non–serving BS’s may not be detectable in normal operation. The second problem isudnon–line of sight (NLOS) propagation, in which time difference measurements (or any otherudmeasurement types) may be corrupted significantly, thus causing significant location error.udThe thesis of this work is that these two problems can be entirely overcome using spatial filteringudof measurements and location estimates. Two constraints that are placed on the filteringudalgorithms are that the operation should be real time and that the precise distribution of NLOSuderrors is unknown (though certain key characteristics are exploited).udA channel model is first developed, which specifically characterises line of sight and NLOSudtransitions as well as out of cell radio wave propagation. Several scenarios are then simulated.udSlow moving users, low hearability and heavily NLOS conditions pose the biggest challenge.udSpatial filtering is achieved by Kalman filters adapted to the problem, as well as simple averagingudfilters. Results show that improved location accuracy (to within FCC recommendations)udis possible in all considered scenarios with spatial filtering as well as improved robustness toudlow hearability. The detection stage of the receiver is also analysed in detail and methods toudimprove hearability are presented.udThe performance of a hybrid location system using angle of arrival measurements of the mobileudat the serving BS is also assessed. A fairly pessimistic model for the spread of NLOS errors isudused, however significant location improvement is noted in several scenarios. Worst performanceudoccurs in urban scenarios so finally a novel approach to user location is described which isudrobust to NLOS propagation conditions and also overcomes the hearability problem since onlyudmeasurements at the serving BS are required. The technique, termed Scatterer Back Tracingud(SBT), uses and requires multipaths to calculate the mobile location. Results suggest this SBTudcan provide extremely high location accuracy but is very sensitive to measurement noise.
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