“…When it comes to indoor environments, the past decade has witnessed flourishing research on indoor positioning systems (IPSs) [ 4 , 5 ]. A variety of sensing technologies have been proposed, including radio frequency identification (RFID) [ 6 , 7 ], WiFi [ 8 , 9 , 10 , 11 , 12 ], acoustic signals [ 13 , 14 , 15 ], geomagnetic fields [ 16 , 17 ], pedestrian dead reckoning (PDR) [ 18 ] and Bluetooth [ 19 ], which utilize on-board sensors, such as the accelerometer, gyroscope, microphone, WiFi module and Bluetooth module. WiFi is the primary alternative to GPS to perform indoor localization, since existing WiFi network infrastructures have been widely available in the majority of commercial and residential buildings, and more importantly, nearly every commercial mobile device is WiFi enabled.…”