"We have designed a super-flexible system that allows us to vary modulation scheme, carrier frequency, RF tag configuration, antenna types -- anything an engineer can dream up for making these systems perform with greater range and reliability," claimed professor Gregory Durgin at Georgia Tech's School of Electrical and Computer Engineering. "Companies have already partnered with us to use the testbed to test new tag antennas, and to retrofit existing RFID tags with new sensor capabilities."

RFID tags are used for everything from inventory management to toll collection to passport identification to tracking luggage. Most tags are passive, including a chip and an antenna that absorbs a radio signal to backscatter its identity to a nearby reader.

The biggest problem with testing RFID tags is the sheer volume--warehouses and store shelves often contain hundreds of tags within range of a reader, many hidden behind other tags. When multiple tags are within range of a reader, the usual protocol is to interrogate the tag with the strongest signal, then put it to sleep and proceed on to the next strongest signal. That serial process can be time consuming.

Instead, the Georgia Tech test bed uses an anti-collision system capable of transmitting multiple, unique signals. The system allows up to 256 tags to be interrogated simultaneously. Instead of requiring readers to be within about a foot of tags, the Georgia Tech test bed can communicate with RFID tags within 400 square feet of the tester. Along with collecting tag information, the system can also track their signal strength in real time.

"We also have a robotic positioning system that drags tags through space so that we can study spatial variability and characterize link fading," said Durgin.

Testing RFID tags with new antenna designs is also a problem. A physical prototype of the new RFID tag must be built to test the antenna, often involving the costly ASIC fabrication. To solve the problem, the Georgia Tech team created an emulator that substitutes for the ASIC in a new tag design, thus allowing new antennas to be quickly prototyped and tested.

"We use custom transmitter and receiver hardware to generate and receive an arbitrary waveform of our own design so we can rapidly test unique antenna configurations and multiple antennas without actually constructing new tags for each experiment," said Durgin. "Instead, we use our custom microwave circuits to emulate the presence of an ASIC--it just clips onto the antenna. With it, we can emulate realistic RFID tags at any carrier frequency with any modulation scheme--even new ones of our own design."

The current test bed is limited to measurements at 915 MHz, the most common frequency for backscatter RFID applications, but it is currently being upgraded to test antennas at frequencies of 2.4- and 5.7-GHz. These higher frequencies will enable tags to operate over wider ranges while using smaller antennas.

The Georgia Tech RFID test bed was funded by the National Science Foundation. Georgia Tech engineering graduate students Anil Rohatgi and Joshua Griffin conducted the research.