Research On Indoor Wireless Positioning Technology Based On WLAN

With the significant development of WLAN(Wireless Local Area Networks) in recent year, more attention has been paid to location-based services and a range of related applications. The naissance of IEEE 802.11 WLAN series standard makes it possible for users to access the Internet at any time and any place via mobile and portable devices. Considering massive development of WLAN systems, the requirement for wireless position, especially for indoor position, has been increasing rapidly. However, because of the serious signal shadowing, fading, noise and multi-path interference, the accuracy of indoor location systems cannot meet the requirements of high-precision positioning in the indoor or densely built outdoor areas. Therefore, how to obtain accurate, reliable and real-time position information should be highly concerned. So it is of great significant importance for the research on indoor wireless location based on WLAN.This dissertation mainly focuses on WLAN-based indoor wireless localization technology. First of all, the background, purpose and significance of WLAN-based indoor wireless localization technology are briefly introduced, followed by the development and current study of this field. Then, the paper elaborated theoretical basis of WLAN-based indoor wireless positioning technology. On this basis, two existing RSSI(Received Signal Strength Indicator)-based indoor wireless localization technology schemes were illuminated and analyzed in depth, namely, maximum likelihood mixed with weighted centroid localization algorithm based on RSSI, and distance difference localization algorithm based on RSSI. On the other hand, issues such as signal shadowing, fading, noise and multi-path interference, etc. will pose significant change to propagation losses and may make the measurement values of RSSI suffer large random fluctuation. And the existing RSSI-based indoor wireless positioning technologies are very limited in improving positioning accuracy. In order to solve the problem, a novel scheme, self-adaptive distance correction positioning algorithm based on RSSI, is proposed in the paper. The scheme adopts Gaussian model to sift RSSI. Besides, a direction correction factor and a distance threshold are defined in the improved trilateration localization algorithm and the improved distance difference correction localization algorithm. Computer simulations are conducted. And the results illustrate that compared with the existing positioning algorithm based on RSSI, this scheme can reduce the positioning error caused by random fluctuation of RSSI to some extent in the presence of multipath fading and low SNR(Signal Noise Ratio). Because of the serious signal shadowing, fading, noise and multi-path interference, the measurement values of RSSI suffer large random fluctuation. As a result, it is difficult for the existing positioning algorithms to achieve high positioning accuracy. Meanwhile, positioning technologies based on RSSI only use information of the energy of signal field strength. For signals which comply with IEEE802.11 standard, there are a lot of statistical characteristics which are not used effectively. To solve the problem, a novel localization scheme is proposed, namely, indoor wireless positioning technology based on CP(Cyclic Prefix) for OFDM signal. The scheme uses the correlation value between the set of samples in CP and the set of data samples that are copied into the cyclic prefix instead of RSSI. Because the two sets have great relevance, the scheme can effectively reduce the impact of noise and multi-path interference. Computer simulations are conducted, and the results illustrate that this new scheme can improve localization accuracy much effectively than the existing schemes base on RSSI under the same conditions. And it can meet the requirements of high-precision positioning in most applications.