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In the last decade, the indoor localization is gaining increasing interest thanks to the wide diffusion of low-cost wireless sensors. It can be fruitfully employed to determine the spatial location of people or objects in large area such as airports, companies, hospitals (e.g. Ambient Assisted Living). Several technologies can be employed: WLAN, WSN, RFID, UWB. Localization techniques typically employ data related to the received complex signal (Received Signal Strength Indicator-RSSI and Phase Difference of Arrival-PDOA) the Time of Arrival (TOA) / Time Difference of Arrival (TDOA), or the Angle of Arrival (AOA). The main goal is to get a spatial resolution comparable to the size of the targets and high robustness with respect to multipath and fixed/mobile clutter phenomena in typical crowded indoor scenarios.


Among others, the passive UHF RFID technology is reaching much attention to develop Real Time Location Systems (RTLS), since it can provide a low-cost and easy-deployable solution.


In this framework the research unit at the MRL developed a novel phase-based localization technique to localize items moving along conveyor belts or rails by exploiting the synthetic-array radar principles: SARFID - Synthetic Aperture Radar approach for RFID tag localization [1]. The technique takes advantage of the fact that the tagged items move along a conveyor belt (Inverse Synthetic Aperture Radar approach) whose speed and path are known and it can be employed for a fully automatic management of items in airport handling systems, parcel distribution centers, and assembly lines with a high automation level. The belt/rail shape can be completely arbitrary [2]. The technique works with off-the-shelf readers, antennas and tags compliant with the EPC C1 Gen2 protocol. Due to the anti-collision algorithms implemented in UHF RFID protocols and also considering that each backscattered signal contains the tag unique identifier, the parallel processing of multiple tags can be easily implemented, so allowing for a real-time localization. If an RFID gate already exists at the belt section, the SARFID technique can be simply implemented with either an upgrade of the reader firmware or an ad-hoc middleware. Only one reader antenna is required, without the requirement of simultaneous detection of the tagged objects by a set of spatially separated antennas. Experimental results in real scenarios demonstrated that UHF-RFID tags can be correctly sorted and localized with a centimeter order accuracy [3]. The SARFID technique is also able to discriminate tagged items on the selected conveying line and any other tagged object, either static or not, located in the coverage area of the antenna reader [4].


SARFID also finds application in any scenario where a relative movement between the reader antenna and the tagged objects does exist and can be exploited for free: high automation production lines, handling systems on rails, overhead cranes, automatic storage system in warehouses, and so on. As an example, SARFID can be used to localize tagged objects on a shelf, if the reader antenna can be moved in front of the shelf through a proper mechanical system (Synthetic Aperture Radar approach). If a scanning along two perpendicular directions can be performed, a bi-dimensional localization is allowed [5].


Definitively the SARFID technique is a low-cost, easy-deployable solution that can be employed in any system including automatic conveying lines (belts, rails, etc.), to get an accurate spatial localization of tagged objects that are moved along the line.