Combining multiple sub-1 GHz frequencies in Radio Tomographic Imaging

Conventional localization systems require the target to carry a tag, which can be highly impractical for some individuals, such as the elderly, or in some situations, such as in emergencies. This requirement can be alleviated by an emerging set of tagless or device-free localization systems, of which Radio Tomographic Imaging (RTI) is a common example. Most variations of this technique assume the use of radio frequency (RF) signals in the 2.4 GHz band, or combinations of lower frequencies with that band; however, using only lower frequencies might decrease the power consumption and the influence of the environment, and increase the range of the system. We tested the combination of 433 MHz and 868 MHz in a single RTI system. We studied two approaches to combine the RTI images generated by one frequency with the other: an approach based on literature and a newly developed approach based on probability theory. This paper compares the result of both approaches. We found that the result based on literature has a root-mean-square error (RMSE) of 1.09 m, while our approach has an RMSE of 0.54 m. While also improving the state-of-the-art fusion of two frequencies, we proved the feasibility of combining only frequencies under 1 GHz in an RTI system.