Unsynchronized OFDM network positioning in multipath

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 Wi-Fi 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 firmware and hardware to enable round trip delay measurements by two-way transmission. 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, which enables to estimate the position of a device in a wireless network without requiring any modifications in the BSs. The solution approach is based on using a reference node with known position, which also receives the BSs transmissions, and deriving an approximate maximum likelihood direct 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 Wi-Fi 802.11g routers, which are unsynchronized and do not include any dedicated protocol for range or location estimation. The novel algorithm attains a relatively low position root mean square error of 1.6m.