Research On Indoor Localization Using RFID System

Position information is becoming more and more important in many fields. Radio frequency identification (RFID) as a non-contact automatic identification technology has been widely used in asset management, personnel tracking, logistics and other fields. Localization of the tagged item, obtaining both the ID and the position, has significant value and wide application foreground. Localization of the passive RFID tags is hard to achieve in the indoor environment for the complex environment, multipath propogation and noise. Therefore, studying the RFID localization method is important.Currently, most location systems based on RFID technology utilize received signal strength (RSS) and reference tags to localizate targets. This dissertation presents how to localizate the RFID tags by using reflected tag signal phase at the background of warehouse management. First, several key problems on phase based RFID localization were studied, including phase measurement, phase stability and error suppression. Then a novel localization approach using phase variation was proposed. Angle of arrival (AOA) based RFID localization approach was proposed. Then a secondary localization approach combining AOA and RSS was proposed based on the error analysis. The main contents include the following aspects:1) RFID localization parameters including RSS, time of arrival (TOA) and signal phase were measured and analyzed in terms of position-sensitive, anti-interference ability and measurement implementation. The proposed Zero-IF quadrature phase calculation to be applied in RFID system made it easier to measure the signal phase. A tag phase measurement machine was designed and implemented to verify the signal phase. Phase stability was studied from tag reflection, reader circuits and space propogation. An error suppression approach was proposed.2) A novel RFID localization approach using phase variation was proposed. First tag-to-tag phase influences were measured and analyzed. Then method by using phase variations of nearby tags was utilized to estimate the positions of added pallets. An UHF passive RFID reader which can inventory and localizate tags was designed and fabricated. At last, the checking-in localization system was designed based on this approach. Experiments were carried out for verification.3) Angle of arrival (AOA) method was proposed for tag localization. The AOA was estimated by phase difference of arrival (PDOA). Then, multiple antenna pairs were used to localizate the tag. This method doesn’t need reference tags. Also the method is sensitive to tag position and not affected by the orientation of the tag antenna.4) RFID-based warehouse localization was studied. By experiments in typical warehouse environment, the impacts of environmental factors to localization were analyzed. Based on error analysis and statistics, a secondary localization approach combining AOA and RSS was proposed. Experiment results showed that the proposed method can suppress the affects of multipath, and effectively improve the localization accuracy. At last an inventory localization system was proposed.