We study the problem of tracking a moving device under two indoorlocation architectures: an active mobile architecture and a passivemobile architecture. In the former, the infrastructure has receivers atknown locations, which estimate distances to a mobile device based on anactive transmission from the device. In the latter, the infrastructurehas active beacons that periodically transmit signals to a passivelylistening mobile device, which in turn estimates distances to thebeacons. Because the active mobile architecture receives simultaneousdistance estimates at multiple receivers from the mobile device, it islikely to perform better tracking than the passive mobile system inwhich the device obtains only one distance estimate at a time and mayhave moved between successive estimates. However, an passive mobilesystem scales better with the number of mobile devices and puts users incontrol of whether their whereabouts are tracked.

We answer the following question: How do the two architectures comparein tracking performance? We find that the active mobile architecture performs better at tracking, but that the passive mobile architecturehas acceptable performance; moreover, we devise a hybrid approach thatpreserves the benefits of the passive mobile architecture whilesimultaneously providing the same performance as an active mobilesystem, suggesting a viable practical solution to the three goals ofscalability, privacy, and tracking agility.

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