Testing of location systems using WiFi in indoor environments
GUENTHER RETSCHER, ESMOND MOK
WiFi Signal Strength to Distance Conversion
In order to integrate WiFi positioning determination with other location techniques it might be interesting to convert the measured signal strength values at one location to a range or distance to an access point. Then it would be possible to perform a trilateration using distances to several access points or radio transmitters. An approach for combined WiFi positioning and GNSS was presented in Mok and Xia (2005) and Mok et al. (2006). In this approach the distances to WiFi access points are combined with pseudorange observations to GPS satellites to determine the curent user’s position. In the following the relationship between the measured signal strength and the distance to the corresponding access point is investigated.
Figure 6 shows the test site at the Podium level of the Hong Kong Polytechnic University. At one end of the 100 m long line either a Linksys or 3com access point was positioned. The signal strength was determined at 5 m intervals along the line. The observations were performed twice in both forward and backward direction. Figure 7 shows a graphical representation of the measured signal strength values on the top and the trend on the bottom. As can be seen from the Figure 7, the signal strength degrades with the distance from the access point. However, the ratio of decrease in signal strength is not the same along the whole straight line. In the first 10 metres the signal strength decreases very fast, followed by a slower decrease when the user moves away from the access points.
Results of further tests reported by Mok et al. (2006) have shown that the signal strength can be converted successfully to a distance. The signal strength quality, however, varies significantly under different environmental conditions. Errors are mainly caused by radio interference and multipath effects. For environments with less environmental interference a least squares polynomial fitting may be able to establish a reasonable signal strength to distance conversion relationship. For more affected areas an approach was developed for the conversion of the signal strength to the corresponding distance. This algorithm has been verified in an unfavourable site condition and has proven to be successful with a 90 % success rate in a 20 m radius area around the access point with the accuracy threshold set to 5 m. If only the determined ranges to the access points are used in the location process, it would not be necessary to perform calibration measurements in the beginning as it is the case for the fingerprinting method (see section 2.1). This would be a major advantage of this approach.
The performed tests have shown that WiFi positioning systems based on fingerprinting are able to determine the current location of user inside a building with an average standard deviation of ± 3 to 5 m. To achieve this level of positioning accuracy observations of the signal strength values to at least 3 to 5 access points are required. The main disadvantage of the employed fingerprinting method, however, is the required calibration of the system in the beginning which is time consuming and very costly. The system requires the observation of the signal strenght values at known location inside the selected areas where the user has to be located. In this calibration usually signal strength values for at least four directions on each calibrated point are measured and stored in a database.
If the signal strenght is converted to a range or distance to the corresponding access point and the location of the user is determined using trilateration, no calibration of the system would be required. Therefore the relationship between the measured signal strength values and the corresponding distance was investigated and a new approach for conversion of the signal strength to the distance was investigated. In future an integration of different location techniques using radio signals with GNSS will be achieved if the range to the access point or radio transmitter is determined. This would lead to an ubiquitous location method where the signals of different radio transmitters can be used to obtain an optimal location determination of the user.