Regarding the d value used in Wi-Fi AP matching, we mentioned in Sect. 4.1 that it is currently a fixed value. From the experimental result shown in Fig. 9, we find it a viable solution to make the d value equal to 2r, given that the measured RSSI values follow the normal distribution [20, 21]. Doing so, we can make the indoor location sensing more flexible. That is, the d value can change with the distance, which will be useful especially for locations that are slightly far from the Wi-Fi AP, with the measured RSSI values fluctuate severely. However, this requires extra work to be done during the establishment of an indoor location—monitoring the RSSI for a longer period of time in order to compute the r value.