Location estimation in multipath environments with unsynchronized base stations

Position estimation of a target node based on time-of-flight measurements in multipath environments is challenging, especially when the signal bandwidth is limited, such as is the case in WIFI 802.11. Furthermore, the availability of location-based services in today's wireless networks is still limited, since it requires either the overhead of synchronizing the base stations (BSs) or applying modification in the BSs protocols to support round trip delay measurements. In this paper, an algorithm for location estimation of a target node in a multipath environment based on one-way transmission of OFDM signals from unsynchronized base stations is developed. The solution approach is based on using a reference node with known position, which also receives the BSs transmission, and deriving an approximate maximum likelihood position estimation of the target node based on the collected samples at both the target and the reference nodes. We demonstrate the algorithm's performance with off-the-shelf WIFI 802.11g routers, which are unsynchronized and do not include any dedicated protocol for range or location estimation. The novel algorithm attains a position root mean square error of 1.7m and a position norm error that is smaller than 3m in 90%.