A bi-modal ad-hoc localization scheme for wireless networks based on RSS and ToF fusion

Localization of persons or equipment in ad-hoc scenarios, e.g., after an earthquake still poses many challenges. The popular received signal strength (RSS) based localization methods usually cannot provide the needed accuracy especially if sensor equipment or tools are to be localized in a static environment with non line of sight conditions. On the other hand, time-of-flight (ToF) measurements have shown more accurate results but have difficulties especially if low power hardware or narrow-band wireless systems are used. Inadequate timing capabilities of common low power clocks make synchronization between different nodes in a network difficult. To solve this problem, usual ToF methods use bilateral round trip measurements and high numbers of packets. Compared to one-way RSS measurements, this approach results in a much higher channel load and thus poorer scalability if a large network is assumed. In this paper we present a bi-modal scheme for hybrid RSS/ToF localization in order to tackle this problem. The main idea of the developed approach is to find an equilibrium between inaccurate but efficient RSS measurements on the one hand and precise but more resource-consuming ToF measurements on the other. The goal is to achieve superior localization accuracy in comparison to the use of one method alone. The proposed concept is outlined and achievable results are shown based on experimental data and simulative evaluation. We conclude that the proposed scheme offers good possibilities for improved localization accuracy by keeping the channel usage low.