Secure Distance Indicator Leveraging Wireless Link Signatures

Received Signal Strength (RSS) and Round Trip Time (RTT) are two common metrics for a wireless receiver to tell the proximity of a remote wireless transmitter. A large RSS or a small RTT normally indicates a close transmitter, and vice versa. Both metrics are effective in a benign environment. However, when the transmitter modifies the send time or transmit power to hide its real distance, they may fail to identify the actual proximity of the transmitter. In this paper, we propose a secure physical layer metric that not only reflects the distance between the transmitter and the receiver, but is difficult to manipulate. Our theoretical and experimental studies show that the proposed metric and the distance is inverse proportional, in both the ideal and practical scenarios with shadow fading and channel noise. We also create distance distribution profiles based on the proposed metric, and point out how such profiles can be used to enhance the reliability of the distance estimation.