Abstract: Methods and apparatus for estimating RFID tag locations in multipath environments are described. A plurality of RFID readers, sparsely placed reference tags, and constructed signal vectors can be used to estimate the location of RFID tags in a multipath environment.

Abstract: A radio-frequency identification (RFID) tag, or tag, is affixed to a particular item and stores unique identifying information about that item. It can be queried with a reader that transmits wireless signals to the tag and receive the tag's responses, which can be correlated with information in inventory records. Conventionally, when a reader stops receiving a tag's responses to these queries, the inventory records are updated to show that the tag and associated item have been removed from the inventory. But a tag can stop producing detectable response for other reasons, including being too close to other tags, so simply removing the tag and item can lead to inaccurate inventory records. Stateful inventory technology address this problem by maintaining and transitioning tags among different states, including a stale state for tags that have not been read recently, depending on when and where the tags were last read.