Recent advances in RF-based passive device-free localisation for indoor applications

Radio frequency (RF) based indoor localisation techniques have gained much attention over the past nearly three decades. Such techniques can be classified as active and passive while passive systems can have either device-assisted or device-free characteristics. Device-free localisation can be a prominent research field as it transcends other device-based approaches in certain application scenarios. Accordingly, we have witnessed an influx of IDFL research focusing on multiple disciplines including occupancy, positioning, activity and identity. However, despite the recent emergence of several exciting technologies and corresponding techniques, IDFL faces some important challenges and because of this, we haven't come across many mainstream commercial products using RF-based IDFL techniques. In this article, we survey the recent progress of IDFL prioritising on indoor positioning. We decompose the localisation dimensions into occupants, space and time, provide a detailed taxonomy and a comprehensive review of these techniques. We divide the state of the art mainly into Wireless Network-based and Radar-based, evaluate the respective technologies and the techniques qualitatively, discuss trends, limitations and also indicate future research directions relevant to this field.