Time-based radio localization in IEEE 802.11b wireless local area networks

Abstract

Position determination in indoor environments is a challenging task as satellite-based localization is not available and contemporary wireless standards have no dedicated support for localization.<br />Time-based localization using the propagation times of a signal between a transmitter and the receivers seems to be a promising approach as it offers good accuracy even for large distances. This thesis proposes a time-based localization system for IEEE 802.11 WLANs where each base station timestamps the signals received from a target. When using the difference of the observed arrival times no modifications of the targets are required. Therefore, any IEEE 802.11 compliant device can be located. This approach can be seamlessly integrated into existing WLANs and enables localization for billions of already deployed WLAN devices.<br />A key factor for any time-based locating system is the quality of the timestamps. Apart from the prerequisite of sub-nanosecond synchronization accuracy between the base stations, a Time of Arrival (ToA) estimator is required which is able to achieve the same accuracy.<br />In this thesis solutions for both highly-accurate synchronization as well as timestamping are presented. In contrast to standard receiver designs, we present a novel Fractional Delay Ranging Receiver (FDRR) architecture which estimates the ToA of the received signal with respect to the local clock with sub-100 picosecond accuracy. Additionally, it is shown that the position can be tracked by precise Doppler phase measurements, even if the transmission duty cycle is low.<br />As the intended area of operation of such a system is within indoor environments, the impact of multipath propagation on the location estimate is studied. The analysis reveals that multipath propagation may create ranging errors of several meters. It is shown that these can be mitigated by either using Doppler phase estimates or frequency diversity.<br />