Device-Free Localization: A Review of Non-RF Techniques for Unobtrusive Indoor Positioning

Accurate, reliable indoor localization or positioning is the key enabler for location-based services. Indoor localization can be broadly classified into two distinct categories. Active localization entails tracking a tag attached to or carried by the target. Passive localization, on the other hand, involves positioning a device-free or untagged target. While passive or device-free localization is comparatively more difficult to achieve, it is the preferred option for many applications. Vision-based techniques can accurately localize an untagged target. However, privacy is a significant concern and thus have limited usability for many applications, especially in noncommercial and residential settings. Passive localization using radio frequency (RF) or wireless sensing is an unobtrusive option and has seen extensive research efforts in recent years leading to a saturated research field but no consensus solution. Researchers have been investigating alternative solutions that can facilitate robust passive localization. The rapid proliferation of Internet of Things (IoT) is bringing ubiquitous networked devices and ambient sensors into modern buildings. The consequential pervasive ambient intelligence and signals of opportunity can enable unobtrusive device-free positioning. This article presents a comprehensive review of non-RF solutions covering visible light-, infrared-, physical excitation-and electric field sensing-based techniques. Limitations of the state-of-the-art and potential future research directions are also outlined.