Design and simulation of sensor networks for tracking Wifi users in outdoor urban environments

ABSTRACT We present a proof-of-concept investigation into the use of sensor networks for tracking of WiFi users in outdoor urban environments. Sensors are fixed, and are capable of measuring signal power from users WiFi devices. We derive a maximum likelihood estimate for user location based on instantaneous sensor power measurements. The algorithm takes into account the effects of power control, and is self-calibrating in that the signal power model used by the location algorithm is adjusted and improved as part of the operation of the network. Simulation results to verify the systems performance are presented. The simulation scenario is based on a 1.5 km 2 area of lower Manhattan, The self-calibration mechanism was verifi ed for initial rms (root mean square) errors of up to 12 dB in the channel power estimates: rms errors were reduced by over 60% in 300 track-hours, in systems with limited power control. Under typical operating conditions with (without) power control, location rms errors are about 8.5 (5) meters with 90% accuracy within 9 (13) meters, for both pedestrian and vehicula r users. The distance error distributions for smaller distances (<30 m) are well-approximated by an exponential distribution, while the distributions for large distance errors have fat tails. The issue of optimal sensor placement in the sensor network is also addressed. We specify a linear programming algorithm for determining sensor placement for networks with reduced number of sens ors. In our test case, the algorithm produces a network with 18.5% fewer sensors w ith comparable accuracy estimation performance. Finally, we discuss future res earch directions for improving the accuracy a nd capabilities of sensor network systems in urban environments. Keywords: sensor network, tracking, WiFi, urban, outdoor, maximum likelihood Distribution A. Approved for public release: distribution unlimited. Case Number: 88ABW-2017-1886. 1. INTRODUCTION 1.1 Motivation and background There are a number of situations in which it is useful to be able to track the motion of individuals in an urban environment. This ability can enable the determination of tr affic patterns; it may also enable surveillance, or providing directions and location-specific information to the individuals. Urban environments pose a challenge to location technologies such as GPS, because urban canyons between tall buildings shield users from satellite signals required to give them a position fix. Using WiFi signals to track individuals is a feasible alternative which has recently received much attention ([1],[2],[3],[4]). With the exploding popularity of smart phones, the percentage of individuals who produce WiFi signals is quite large. Furthermore, the signal produced by each individual is uniquely identifiable, since WiFi devices periodically transmit a probe request packet whic h includes the MAC address of the device [5]. Other researchers have studied tracking of targets along roads: see [6] for example, and the citations therein. However, we are not aware of any prior work that treats urban tracking with a WiFi sensor network. In this investigation, we propose, simulate, and verify th e performance of a design for a fixed sensor network dedicated to tracking outdoor urban WiFi users. Our design takes into account the possible effects of power control, which is not required for WiFi systems but may be implemented in some deployments. We also propose and evaluate an algorithm for reducing the number of sensors in the network which minimize the impact on the ne tworks performance.