WMPS: A Positioning System for localizing legacy 802.11 devices

Abstract

The huge success of location-aware applications calls for the quick development of a positioning system alternative to GPS for indoor localization based on existing technologies such as 802.11 wireless networks. In this paper we propose WMPS, the Wireless MAC Processor Positioning System, that is a localization system running on off-the-shelf 802.11 Access Points and based on time-of-flight ranging of users’ standard terminals. We prove through extensive experiments that propagation delays can be measured with the accuracy required by indoor applications despite the different noise components that can affect the result, like latencies of the hardware transreceivers, multi- path, ACK jitters and timer quantization. Key to our solution is the choice of the Wireless MAC Processor architecture that enables a straightforward implementation of the ranging subsystem directly inside commercial cards without affecting the basic DCF channel access algorithm. On top of the proposed measurement framework, we develop a simple and effective localization algorithm able to work without requiring any preliminary calibration or device characterization. Finally, our architecture allows to adjust the measurement methodology as a function of the network load or propagation environments at run time, without requiring any firmware update.