Time-of-flight positioning using the existing wireless local area network infrastructure

The ubiquity of IEEE 802.11 WLAN as well as the advancements in location based services and applications make locating a device more important than ever. While proprietary localization systems exist, extending an already deployed WLAN infrastructure with localization capabilities would lead to lower investment costs. For implementations based on commercial off the shelf (COTS) hardware, most research concentrates on using the received signal strength indicator (RSSI). The RSSI is inherently nondeterministic in dynamic environments, which limits its use in real world scenarios. This paper presents three measurement methods to obtain the time-of-flight of a WLAN frame exchange; more specifically the round-trip-time (RTT). Once the RTT is known, simple algorithms such as multilateration can be used to determine the position of the user. All three methods have been implemented. The first method uses the "time synchronization function", through which WLAN devices add timestamps to the received frames. The second method employs a higher resolution timestamp based on the CPU-clock. This timestamp is triggered directly by the hardware interrupt that is generated when sending or receiving a frame. The third and new method reads the clock of the WLAN chipset. Our implementations show how the measurements can be implemented on any WLAN device.