Software-based system for measuring location observables in IEEE 802.11 networks

Several techniques have been proposed for positioning nodes over IEEE 802.11 networks, but only few consist in time-based multilateration, mainly due to the protocol stack not supporting accurate timestamps. One frequent solution is to develop the hardware required for providing accurate timestamps. However, this approach tends to slow down the research and the ulterior deployment of the location techniques, since the performance of these techniques is bounded to a specific hardware design. This paper presents a measuring system aimed at providing location observables that can be used in time-based multilateration techniques. The system, which follows a software approach, is based on enhancing the SoftMAC layer of Linux with location-measuring capabilities. The system is conceived for supporting any kind of time-based measurements, by adding as many plugins as measured observables. Two plugins have been initially developed. The first one computes the round trip time of a message from a station to an access point and back again to the station. The second plugin calculates time-differences of arrival suitable for being used in the passive TDOA technique. This work provides the definition of the measuring system. Real data has been collected to test the system. The results indicate that the measurements provided by the system can be used for location purposes, i.e. they follow the physical laws in which metrics are based (e.g. they grow along with the distance between the nodes).