Mitigating the reader collision problem in RFID networks with mobile readers

Radio frequency identification (RFID) is a means to identify and track objects using radio frequency transmission. An RFID system consists of readers and tags. Readers use radio signals to communicate with the tags. The tags may be active (battery powered) or passive (powered by the reader's signals). RFID is increasingly being used in many applications such as inventory management, object tracking, retail checkout etc. The reader collision problem occurs when the signal from one reader interferes with the signal from other readers. Such interference can result in lack of communication between the readers and some of the tags in the vicinity leading to incorrect and inefficient operation of an RFID system. This problem is further aggravated when mobile/hand-held readers are used in the system. The evolving standards like ETSI EN 302 208 for RFID readers use a CSMA based protocol called "listen before talk" to reduce the reader collision problem. However, carrier sensing alone cannot solve the reader collision problem in RFID networks. For example, the signals from two readers that are out of range from each other, may still collide at the tags in the region between them. Although this aspect of the reader collision problem seems similar to the hidden terminal problem encountered in traditional wireless systems, existing solutions such as RTS-CTS mechanisms are not applicable. One of the reasons being: a reader may communicate with multiple tags simultaneously. Hence a separate collision avoidance is required between CTS sent by these multiple tags, making the protocol more complicated. In this paper, we describe Pulse, a distributed protocol to reduce reader collisions. The operation of the Pulse protocol is based on periodic beaconing on a separate control channel by the reader, while it is reading the tags. The protocol functions effectively not only with fixed RFID readers but also with mobile RFID readers. We show, using simulation in QualNet, that using Pulse protocol, the throughput (overall read rate) is increased by as high as 60% as compared to "listen before talk" (CSMA) and by 232% as compared to Colorwave. We also present an analytical model for our protocol in a single hop scenario.