Current passive UHF RFID technology involves two types of physical devices: tags and readers. As for purely passive tags, the inventory process comprises three logical functions:
- transmission of query commands to tags;
- transmission of a Current Wave (CW) to energize tags;
- reception and decoding of tag replies.
In conventional RFID systems, all such functions are concentrated at a single physical device, i.e. the reader. We envision an alternative approach where they are distributed among multiple devices. More specifically, we decouple the transmission functions (Query and CW) from the reception ones, and consider a scheme where transmitter and receiver are physically separated. We refer to the receive device as “RFID Listener”. It should be remarked that, in order to decode the messages from the tags, the Listener must decode the signals from the transmitter as well: this allows to decode the Query and to identify the time/frequency of the tag reply.
The separation of transmission and reception functions enables novel operational schemes where a single RFID transmitter coexists with multiple RFID listeners, and all such devices cooperate to discover, read and localize the tags in a distributed fashion. In such a vision, RFID listeners can be seen as “tag sensors”. They could be integrated into the low-cost low-power nodes of a Wireless Sensor Network (WSN) operating e.g. with IEEE 802.15.4 radios. Reception by multiple listeners brings multiple advantages over the multi-reader scenario, especially in case of dense deployments. Note that the UHF RFID Class-1 Generation-2 (Gen2) protocol requires a tight coordination between the transmitter and receiver, mainly to support explicit acknowledgments. Therefore, transmitter and listener cannot act independently as far as the Gen2 protocol is used. The distributed scheme we envision, where independence between transmitter and listener is a key feature, requires the definition of alternative protocols.
In order to carry out experimental research in the new scenario and test different protocol options, it is desirable to have a flexible implementation of both transmitter and listener(s). Software-Defined Radio (SDR), and particularly the open-source platform GNUradio, offers a cheap and versatile solution to researchers and practitioners to setup up a test-bed environment.
In our published works listed below, we make three main contributions. First, we discuss the potential pros and cons of the envisioned distributed scenario, where RFID transmitters and listeners are physically separated. Second, we present the initial implementation of a RFID Listener based on GNU-Radio, which is freely available on the linked CGRAN archive. Our implementation is fully compliant with the Gen2 standard, and represents a starting basis for developing novel versions of Listeners going beyond the current specifications, for experimental pre-standard research. Since our RFID Listener is able to decode both the queries from the reader and the replies from the tag(s), it can be also used as a sort of passive “protocol analyzer” for conventional Gen2 systems. As a third contribution, we present several experiments aimed at assessing the Listener performance and illustrating its operation as a flexible Gen2 protocol analyzer.
 D. De Donno, F. Ricciato, and L. Tarricone, “Listening to Tags: Uplink RFID Measurements with an Open-Source Software-Defined Radio Tool,” IEEE Transactions on Instrumentation and Measurement, vol. 62, no. 1, pp. 109–118, Jan. 2013.
 D. De Donno, L. Catarinucci, L. Tarricone, V. Lakafosis, and M. M. Tentzeris, “Performance Enhancement of the RFID EPC Gen2 Protocol by Exploiting Collision Recovery,” Progress in Electromagnetics Research B, vol. 43, pp. 53–72, 2012.
 D. De Donno, V. Lakafosis, L. Tarricone, and M. M. Tentzeris, “Multipacket-Reception MAC Schemes for the RFID EPC Gen2 Protocol,” 2012 International Symposium on Wireless Communication Systems (ISWCS), pp. 311–315, Paris, France, Aug. 2012.
 D. De Donno, V. Lakafosis, L. Tarricone, and M. M. Tentzeris, “Increasing Performance of SDR-based Collision-Free RFID Systems,” 2012 IEEE MTT-S International Microwave Symposium Digest, pp. 1–4, Montreal, Canada, June 2012.
 D. De Donno, F. Ricciato, L. Catarinucci, and L. Tarricone, “Design and Applications of a Software-Defined Listener for UHF RFID Systems,” 2011 IEEE MTT-S International Microwave Symposium Digest (MTT), pp. 1–4, Baltimore, MD, USA, June 2011.
 D. De Donno, F. Ricciato, L. Catarinucci, A. Coluccia, and L. Tarricone, “Challenge: Towards Distributed RFID Sensing with Software-Defined Radio,” 2010 16th ACM International Conference on Mobile Computing and Networking (MobiCom), pp. 97–104, Chicago, Illinois, USA, Sep. 2010.
 GNUradio-based RFID Gen2 Listener project page [Online]. Available: http://www.cgran.org/wiki/Gen2Listener/.