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Due to recent advances in wireless networking, WiFi-based localization has been attracting significant attention. WiFi deployments are ubiquitous in public and private places including offices, malls, and hospitals.
WiFi-based localization techniques use the existing ubiquitous WLANs to provide accurate indoor localization without any additional hardware. It can be classified into two categories: device-based (Bahl & Padmanabhan, 2000; Youssef & Agrawala, 2005), and device-free techniques (Youssef, Mah, & Agrawala, 2007; M. Seifeldin & Youssef, 2009; Kosba, Saeed, & Youssef, 2012a; M. A. Seifeldin, El-keyi, & Youssef, 2011). Device-based systems track a WiFi-enabled device such as a cell-phone, based on the received signal strength (RSS) at this device; while device-free systems track entities that do not carry any devices based on their effect on the RSS at the infrastructure devices. A typical device-free system will consist of one or more signal receivers which are called monitoring points (MPs) such as laptops; signal transmitters such as access points (APs); and also an application server, which is usually one of the monitoring points, to collect data from monitoring points. Applications for the device-free systems include intrusion detection, smart homes, and sensor-less sensing. To overcome the complex propagation characteristics of WiFi signals in indoor environments (Youssef & Agrawala, 2003), typically both device-based and device-free systems require a calibration phase to construct a fingerprint or a radio-map that stores the RSS characteristics at different locations in the area of interest. Device-based systems use active-radio maps, where each stream represents the signal strength from an AP to the tracked device; while device-free systems use passive-radio maps, where each stream represents the signal strength from an AP to a MP capturing the effect of the tracked entity on the fixed streams. Figure 1 explains the difference between active and passive radio maps construction.
Figure 1. Difference between device-based ((a) Active) and device-free ((b) Passive) radio-map construction
Traditional methods of radio-map construction require the use of manual calibration, which is a tedious and a time consuming process. Therefore radio map construction has been based on simple scenarios, usually involving one entity in a specific environment.