Leveraging the Movements of Occupants to Generate Indoor Maps Using RF Signals

In this paper, we propose FreeMap, an approach that uses RF signals to generate accurate indoor maps of the line of sight as well as the non line of sight parts of buildings. To generate the maps, FreeMap leverages the movements of occupants in buildings. When an RF signal reflects from an occupant, the reflected signal arrives back at the receiver both directly and as multipaths, i.e., the copies of the signal that reflect from further walls before arriving back at the receiver. These direct and multipath reflections form a triangular geometry that enables FreeMap to locate various parts of the walls in the environment. As time progresses and the occupants move around, FreeMap localizes more and more parts of all the walls. It eventually obtains the locations of enough parts of all the walls that it can then connect them together to generate the indoor map. We have implemented FreeMap using a commercial FMCW radar (which provides in-home monitoring services to elderly) and have extensively evaluated it at eight measurement sites. Our results show that FreeMap generated maps with average precision, recall, and positioning error of $\mathbf{9 7 . 3 \%}, \mathbf{9 5 . 5 \%}$, and $20.3 \mathrm{~cm}$, respectively.