A Software Radio-based UHF RFID Reader for PH／YMAC

RFID

A Software Radio-based UHF RFID Reader

for PHY/MAC Experimentation

Michael Buettner University of Washington buettner@cs.washington.edu

David Wetherall University of Washington djw@cs.washington.edu

Abstract—We present the design and evaluation of a ?exible UHF RFID reader that enables new PHY/MAC designs to be prototyped and evaluated.Our reader is built using the USRP software radio platform in conjunction with software we developed in the open-source GNU Radio framework.We believe it is the?rst inexpensive tool that readily enables changes to the physical and MAC layer of RFID systems.We evaluate our reader and show that it can inventory commercial tags out to6meters,which approximates the range of a commercial reader with com-parable transmit power.We then show two applications of our reader.The?rst evaluates the real-world performance of the EPC frame selection algorithm and?nds that it performs better than expected.Second,using the Intel WISP programmable RFID tag,we implement and evaluate an extension to the Gen2standard that results in up to a ?ve-fold increase in sample rate for streamed sensor data.

I.I NTRODUCTION

Radio Frequency IDenti?cation(RFID)is an emerging wireless technology that allows small,inexpensive com-puter chips to be remotely powered and interrogated for identi?ers and other information.While there are many kinds of RFID,e.g.,HF RFID in credit cards,recent advances in RFID have largely focused on passive UHF RFID as standardized by the EPC Class-1Generation-2 (Gen2)speci?cation in2004[1].

The application space for UHF RFID is rapidly ex-panding,and the ability to experiment with realistic sys-tems is becoming increasingly important.For instance, the reliability and performance of RFID readers is of interest,especially in dense,?ne-grained settings(such as item-level tracking)and for demanding applications (such as searching over the states of objects in a ubi-comp application).However,there is very little published information on low-level RFID performance in these settings.Prior work on the topic concludes that current reader systems suffer various performance degradations and there is ample opportunity for improvement[2],[3]. Additionally,privacy and security issues are central to any application that tracks objects and people directly or indirectly.Yet almost all work on RFID security using lightweight cryptographic schemes has been done via paper designs and analysis rather than experimentation; we are aware of only one exception[4].

Looking ahead,as RFID devices advance to include computation,storage and sensing[5],[6],[7],alternative MAC and PHY layer designs will inevitably emerge and need to be evaluated.This is because the Gen2protocol was designed with simple object identi?cation in mind and is a poor?t for gathering sensor data or supporting complex interactions with devices[8].

The dearth of low-level experimentation with RFID systems is a consequence of the current lack of tools available to researchers.Existing RFID readers are gen-erally black box systems which provide only limited con-?guration and return high-level results simply indicating tags that are in range.They do not provide the?exibility to observe or modify the MAC or PHY layer behavior. This makes the study of existing protocols dif?cult,let alone experimentation with alternative designs.High-end RFID test equipment is available in the form of protocol analyzers and systems have been built to measure tag performance[9].While well suited for conformance testing,they are closed source,expensive(>$80K),and generally do not allow complete control over the PHY and MAC layers.

In this paper,we present what we believe is the?rst open-source platform for low-level UHF RFID experi-mentation that gives users control of the PHY and MAC layers.It consists of the Universal Software Radio Pe-ripheral(USRP)and software we have developed using GNU Radio that implements a EPC Class-1Generation-2reader.Because all signal processing is performed on a standard Linux PC,MAC and PHY functionality can be modi?ed simply by re-writing user-level software. Our platform provides a high degree of?exibility while costing less than$1500.

We make two main contributions in this work.First, we present the design and implementation of our reader and show how a low-cost SDR platform and open-source software can be used to create a powerful tool for UHF RFID experimentation.We describe how we overcame the limitations inherent to the hardware which made implementing the reader dif?cult,and evaluate the reader performance,?nding that it can inter-operate with standard Gen2tags at a range of up to6meters.This

RFID

closely approximates the range of a commercial RFID reader when using the same transmit power of27dBm. Second,we present two applications of our reader that demonstrate its usefulness as a research tool.The ?rst application uses our reader and commercial RFID tags to study the performance of the frame size selection algorithm described in the EPC Gen2standard.We?nd that the algorithm’s performance is within7%of the maximum rate achieve using an oracle based algorithm. Also,because of RF capture,the algorithm achieves a 44%slot ef?ciency which is well above the the best expected ef?ciency of37%using the standard collision model.This has implications for how such algorithms are designed and analyzed.

The second application shows that,when paired with an equally?exible RFID tag,our reader can be used to develop entirely new MAC protocols.We demonstrate this capability by prototyping an extension to the Gen2 standard to accommodate high-rate streaming data for sensor-equipped RFID tags.Through experiments we show that our extension can provide a5X improvement in sampling rate compared to the normal Gen2mechanism for reading data from tags.

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