Indoor Tracking and Localization of non-authorized Cellphones in Prisons Using Uplink Signals

Abstract

In prisons, cellphones are often used by inmates to communicate with the outside world creating a serious problem for public safety. We developed a system for indoor localization of non-authorized active cellphones operating in the 2G, 3G or 4G communication standards. The system is based on a fingerprinting technique of signals from 12 antennas covering the building under surveillance. The fingerprints were extracted from antenna signals using Welch’s classical short-term PSD estimator. To obtain relevant fingerprints background noise reduction was performed through spectral subtraction and separation of signals from different cellphones was achieved by k-means clustering. The resulting cellphone specific signals strengths provided relevant fingerprints for a final classification by the Mahalanobis distance. The validation performed in a four-story prison building with over 100 localization cells showed high performance yielding a localization error smaller than 1.8m for the 2G, 3G and 4G communication standards in 70% of all measurements