Toward sensor localization using WiFi-AP anchors: realtime AP-RSS monitoring using sensor nodes

Sensor networks play an important role to enhance indoor mobile applications. The sensor network for human detection in a house, for example, cooperates with a mobile application and automatically controls home appliances. To realize such enhancement in a vast indoor environment such as in a building, we face a sensor localization problem. We need to get location of huge number of sensor nodes since location of sensor nodes is important itself. Although there are many studies on sensor localization [1], these works still require so many anchors, namely reference nodes, to improve accuracy. We tackle the sensor localization problem by developing a sensor localization system that requires no newly deployed anchors. Our main idea is quite simple: we utilize WiFi APs as anchors for sensor localization. WiFi APs are already used as anchors in WiFi localization systems. We extend these WiFi-AP anchors to a sensor localization system. Several challenges come up with this idea. 1) How to detect WiFi-AP signals on sensor node? : Sensor nodes cannot demodulate WiFi (IEEE802.11) signals since sensor nodes are equipped with IEEE802.15.4 (ZigBee) modules. We need to pick WiFi-AP signals out from WiFi signals sent from many WiFi devices using a ZigBee module. We then measure received signal strength (RSS) of the AP signals. 2) How to distinguish WiFi APs? : Using the RSS value, we can calculate distance between a sensor node and an AP. For sensor localization, we need to figure out the AP that sent signals. To overcome these two challenges, we utilize a signal folding scheme presented in ZiFi [2] and configure each AP to have different beacon intervals. The folding scheme detects periodic WiFi-AP beacon frames. Using the different beacon intervals, we can separately detect WiFi APs. We then measure RSS of each AP and estimate sensor location. 2. SYSTEM OVERVIEW