Exploiting Shadow Fading to DEtect CO-Moving Wireless DEvices

We present the DECODE technique to determine whether a set of transmitters are co-moving, i.e., moving together in close proximity. Co-movement information can find use in applications ranging from inventory tracking, to social network sensing, and to optimizing mobile device localization. The positioning errors from indoor RSS based localization systems tend to be too large making it difficult to detect whether two devices are moving together based on the inter-device distances. DECODE achieves accurate co-movement detection by exploiting the correlations in positioning errors over time. DECODE can not only be implemented in the position space but also in the signal space where a correlation in shadow fading due to objects blocking the path between the transmitter and receiver exists. This technique requires no changes in or cooperation from the tracked devices other than sporadic transmission of packets. Using experiments from an office environment, we show that DECODE can achieve near perfect co-movement detection at walking-speed mobility using correlation coefficients computed over approximately 60-second time intervals. We further show that DECODE is generic and could accomplish detection for mixed mobile transmitters of different technologies (IEEE 802.11b/g and IEEE 802.15.4), and our results are not very sensitive to the frequency at which transmitters