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Achieving Perfect Readability

By developing new chip and antenna technologies, we can improve the read rate of item-level tags.
By Hao Min
Jun 01, 2006—Companies that are using RFID in the real world have found they can't read every tag every time. If you're tracking goods on pallets and cases, that's not an insurmountable problem because a missed tag can be read later in the supply chain. But in many item-level applications, missing a tag can be a huge issue. For instance, if you can't read the tag on an item at checkout, a retailer could lose a sale. In a library application, missed tags may cause the loss of books.

At the Auto-ID Lab at Fudan University in Shanghai, China, we have been conducting systematic research to learn why interrogators fail to read some tags in many situations. We found that the reasons are very complicated, but most missed reads are caused by the following: Tags are located in a null spot, or "hole," where there is not enough energy to operate the microchip; tags are detuned by water or metal or other tags nearby; or the radio waves to or from the tag are interfered with or blocked by radio waves from other electronic appliances or electromagnetic energy given off by machines.

We've developed several innovative technologies to improve read rates:

Tag and Interrogator Emulators
We created a tag emulator and an interrogator emulator that can record their communications with each other and store them in memory with time stamps. They can also record valuable data, such as incoming signal strength and wave shape.

The emulators can be used in real-world environments to run read-rate tests. The recorded data is transferred to a personal computer, and the results analyzed. The tool lets you view the RF signal from an interrogator antenna or the reflected signal from the tag when a read was lost, revealing the reasons why a tag wasn't read.

Ultralow-power Tag Chip
The ability to read a tag consistently is closely related to the power consumption of the microchip in the tag. A chip that requires more energy will be read less often because it is difficult to get energy to the tag when the waves must travel through materials or when there is interference in the environment. We are developing sub-threshold circuits, adiabatic circuits (which don't gain or lose heat) and high-efficiency charge pumps that could reduce the tag's power consumption by a factor of 2 to 10. This would increase the read range and reduce the number of times tags aren't read.

Wideband-tuning Rectenna
It's well known that ultrahigh-frequency tag antennas are detuned when the tags are applied to objects made of metal or containing water. UHF tag antennas can also be detuned when they are put close to one another. We created a rectifier-and-antenna combination (rectenna) that performs equally well in all areas of the UHF operating band and is less sensitive to detuning by the material to which the tag is attached. It should improve read rates on difficult materials and in regions that use different areas of the UHF spectrum for RFID.

We can significantly improve tag performance with these technologies. Perfect read rates are now within our reach.

Hao Min is the director of the Auto-ID Lab at Fudan University in Shanghai, China, and a professor of ASIC and systems at State Key Laboratory of Fudan University.
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