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Increasing Read Ranges

An EPC-compliant Gen 2 active tag is on the horizon.
By Hao Min
Nov 10, 2008Using radio frequency identification technology to manage products in the supply chain includes tagging and tracking items, cases, pallets, containers and vehicles. Each requires a different read distance. Passive Gen 2 tags have a read range of about 5 to 10 meters (16 to 33 feet), suitable for item-, case- and some pallet-level tagging. For other pallets and for container and vehicle identification, a longer read range—up to 30 meters (98 feet)—is necessary.

Some battery-assisted (also called semi-passive) tags can use the Gen 2 protocol, but even with a high-sensitivity interrogator, their read range is about 20 to 25 meters (66 to 82 feet), unsatisfactory for certain applications. Active tags have a longer read range, but they're not compliant with the Gen 2 protocol and require a dedicated RFID interrogator.

It would be ideal if end users could buy Gen 2 interrogators that could read passive, battery-assisted and active tags, rather than having to purchase two or three different readers. At the Auto-ID Lab at Fudan University, we're designing a Gen 2 active tag that will be readable by an ordinary passive Gen 2 reader. To do that, we're examining the factors that restrict the read range.

According to the backscatter mechanism, which tags based on the Gen 2 protocol use, the power available to the tag is divided into two parts: the input power that runs the tag's circuitry, and the energy that is backscattered to the reader. The tag needs enough energy to supply the circuit and enough power to create a signal that the reader can detect. These two factors affect each other. Typically, the read distance of a passive tag is determined by the power consumption of the tag's chip. If the chip uses less power, more power is available to send back to the reader.

Can we improve the read range by increasing the sensitivity of the reader? It's difficult, because a combination of factors—thermal noise from electronic components inside the reader and low RF energy from the tag—would make the communication from the tag to the reader unstable.

The alternative is to increase the power transmitted from the tag. Since the Gen 2 protocol is based on backscattering technology, the RF signal transmitted by the tag must be at the exact same frequency as the receiving signal. We're designing a new active tag architecture that has a phase lock loop (PLL) with memory to lock the oscillating frequency of the circuits to the incoming RF signal from the reader during receiving mode. The frequency would be remembered, so the transmitted frequency would be exactly the same as the received signal.

We estimate the read range of this active tag would be more than 100 meters, without any change in current Gen 2 reader infrastructure. What's more, passive, battery-assisted and active Gen 2 tags could co-exist in the same field and be identified by an ordinary passive Gen 2 reader.

Hao Min is the director of the Auto-ID Lab at Fudan University in Shanghai, China, and a professor with the university's ASIC & System State Key Laboratory.
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