Toward Passive Sensing Large Crowds Through User-Generated Electromagnetic Activity

Abstract

The emerging field of Joint Communication and Sensing (JCAS) will have a substantial impact on future wireless communication systems and how people interact with them. The investigation of sensing large crowds with these communication systems still lacks thorough exploration. Current studies are generally based on small-scale experiments with few participants. However, these studies are not representative in comparison to large crowds. By passively sensing an environment, it is possible to link the captured electromagnetic (EM) activity within that area to the crowd size without applying any demodulation or decoding of packets and preserving privacy. Measurements were conducted in an exclusive area at a large music festival, using ten Software-Defined Radios (SDRs) to capture EM wave energy across several Wi-Fi and LTE bands. The collected data were compared to a validation data set. The study shows that the short-time Fourier transform (STFT) and the magnitude extraction technique are two viable techniques to apply to the captured data sets. Additionally, the correlations found between the validation data set and the LTE data sets are significantly stronger than those in the Wi-Fi data sets. This indicates that further research is needed on specific frequency band selection, advanced digital signal processing methods and scalable data-capturing strategies.