Wireless local area networks have been designed for wireless communication. Frames are acknowledged (ACKed) after a short and predefined MAC idle time. The MAC idle time varies with i) the physical distance between stations, caused by the delay of wireless signal propagation, and ii) the time to detect the ACK at the local station, which varies with the signal strength of the incoming ACK. We present CAESAR, CArriEr Sense-bAsed Ranging, that combines time of flight and signal-to-noise ratio measurements to calculate the distance between two stations. CAESAR measures the distance by estimating the MAC idle time in a data/ACK communication at a 44 MHz clock resolution and the ACK detection time on a per-frame basis. CAESAR is a software-based solution that is entirely implemented at the transmitter and it requires no protocol modifications and only a limited calibration in links with multi-path propagation. We implement CAESAR on commodity hardware and conduct extensive experiments both in controlled network conditions and dynamic radio environments. Our measurements confirm the accuracy of the solution and show the capability to track the distance to WLAN smartphones at pedestrian speeds.
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