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Internet of Things (IoT) has attracted much attention these days. As the key component of IoT, the Radio Frequency Identification (RFID) system has been growing in usage. An RFID network is composed of a backend system (including the database), one or more readers, and several RFID tags. One type of the RFID tags is the active RFID tag. An active RFID tag has on-board power source and is able to transmit signals actively. It may gather information from sensors and store the information for later delivery to the reader. To extend the coverage of the RFID network, some RFID multi-hop relay systems (using active RFID tag as relay tags) have been proposed. They are all designed in "Reader Talks First (RTF)" mode. That means multi-hop links may be built up only when the reader queries and the tags response. On the contrary, in "Tag Talks First (TTF)" mode, an active tag cannot report anything to the RFID system if it cannot find an available reader in its sense area. It may occur when the reader is located out of the sense area of the tag, or the reader is out of service (but the tag still needs to find out another reader to join in the RFID network). If these active tags are connection-lost from the system, some emergence messages may be lost, or at least be delayed. In this paper, we propose two multi-hop relay algorithms used in TTF mode, which are designed for Free2move and 802.15.4 respectively. Simulation results show that with the help of the proposed algorithms, the loss performance in TTF mode is improved a lot.
Wang et al. (Sun,) studied this question.