System measures multiple tags and tests new antenna designs

A system capable of simultaneously measuring hundreds of RFID tags and rapidly testing new RFID tag prototypes has been designed by researchers.

“This testbed allows us to measure the signal strength of tags hidden behind other tags and to rapidly test unique antenna configurations and multiple antennas without actually constructing new tags for each experiment,” said Gregory Durgin, assistant professor in the Georgia Institute of Technology’s School of Electrical and Computer Engineering in the US.

If several RFID tags are in the vicinity of a reader, the reader usually communicates with the tag transmitting the most powerful signal first and then puts it to ‘sleep’ to prevent it from transmitting repeatedly. Then the reader moves to the next most powerful signal, and so on.

“We designed a really inexpensive, simple anti-collision system that transmits multiple unique signals back to us simultaneously without this complicated back and forth process,” said Durgin.

The system includes three parts: a transmitter, receiver and emulator. The emulator simulates the activity of an integrated circuit. The transmitter sends a radio signal to the antenna. By attaching the emulator to an antenna, a unique spread spectrum signal is transmitted to the receiver.

Each antenna signal can then be separated from the others, allowing his team to simultaneously measure the signals from multiple tags. Their experiments have shown they can measure the power strength and phase of up to 256 antennas in the field of view, which is an area in front of the reader of approximately 6 x 6 m.

“To test new signalling schemes and frequencies, we just have to change the emulator’s signal — we don’t have to fabricate a new chip that could cost $100,000 in a silicon foundry,” Durgin said.

“We can also evaluate multiple custom antennas in numerous configurations in realistic tag environments for only a fraction of the time and cost of previous methods.”

Testing multiple configurations is important because RFID readability and antenna power strength can be affected by the relative position and orientation of the tag antenna and the reader.

The researchers designed the testbed for measurements at 915 MHz. They are currently expanding the system to test antennas at higher frequencies of up to 5.7 GHz.

“At higher frequencies, even though the tag is physically stationary, you are electromagnetically lifting the antenna signal off the object and it starts to work better,” said Durgin.

“Plus, at higher frequencies, smaller antennas can be constructed, which means more antennas can be placed on a tag to produce more energy for communications.”

The tags usually require a reader to be within a foot of the chip, but operating at higher frequencies could greatly improve the range and reliability of the RFID tags, according to Durgin.

“This testbed is just the beginning of our ability to characterise the performance of different RFID tag antennas in a real channel and push these technologies to higher frequencies, longer read ranges and overall higher reliability,” said Durgin.

The research, funded by the National Science Foundation and conducted with former graduate student Anil Rohatgi and current graduate student Joshua Griffin, was presented at the IEEE International Conference on RFID.