Home Internet of Things Aerospace Apparel Energy Defense Health Care Logistics Manufacturing Retail

Access This Premium Content

Options To Access This Article:

What Subscribers Are Saying

  • "Probably the best investment I've ever made."
    Steve Meizlish, President & CEO, MeizCorp Services, Inc.
  • "I have found that RFID Journal provides an objective viewpoint of RFID. It you are looking for a resource that provides insights as to the application and implications of deploying RFID, RFID Journal will meet your needs, It gives you a broad perspective of RFID, beyond the retail supply chain."
    Mike O'Shea, Director of Corporate AutoID/RFID Strategies & Technologies, Kimberly-Clark Corp.
  • "No other source provides the consistent value-added insight that Mark Robert and his staff do. In a world dominated by press release after press release, RFID Journal is developing as the one place to go to make the most sense out of the present and future of RFID in commerce."
    Bob Hurley, Project Leader for RFID, Bayer HealthCare's Consumer Care Division
  • "RFID Journal is the one go-to source for information on the latest in RFID technology."
    Bruce Keim, Director, Hewlett-Packard
  • "RFID Journal is the only source I need to keep up to the minute with the happenings in the RFID world."
    Blair Hawley, VP of Supply Chain, Remington Products Company

Readers That Sense Distance

By measuring the distance between RFID interrogators and passive tags, companies can better differentiate items as they pass through portals.
By Hao Min
Feb 01, 2010—Many companies that have deployed RFID systems in warehouses, distribution centers and retail stores receive tagged pallets and cases through portals positioned next to each other. But passive tags vary in sensitivity and other characteristics, and it's often difficult for an interrogator to read all the tags—and only the tags—passing through its portal. A high-sensitivity interrogator may inadvertently read some tags passing through an adjacent portal, and reducing the sensitivity of the interrogator could result in some tags being missed entirely.

The underlying problem in these scenarios is that there's no distance information between the tag and the reader antenna. Some advanced interrogators use received signal strength indicator (RSSI), a circuit that measures the strength of an incoming signal, to estimate the distance. But RSSI also determines many other factors, such as antenna gain, impedance matching and backscatter co-efficiency, so its distance measurement is not specific.

At the Auto-ID Lab at Fudan University in Shanghai, researchers are developing a distance-sensing reader technology based on time of flight. It measures the time it takes for a radio wave to travel from the reader antenna to the tag antenna and back to the reader. The distance from the reader to the tag is calculated based on the travel speed of a radio wave in the speed of light (approximately 300,000 kilometers per second).

But it's not as easy as it sounds. It takes only 3 nanoseconds for the wave to travel 1 meter. So if we need to measure a distance of 10 centimeters, the accuracy of the time measurement should be 0.3 nanoseconds, which is difficult to reach using current technology. Another challenge is to differentiate the weak backscatter signal of a particular tag from the backscatter signal of other tags and the environment.

To address these problems, we use a modulation technique called direct-sequence spread spectrum. Instead of sending a continuous wave during reader-to-tag talk, the reader sends a pseudorandom sequence signal to the tag. The selected tag modulates the signal using backscatter. We measure the time between sending the sequence and receiving an echo signal from the tag. As the radio-wave speed is a constant, we can calculate the distance of the tag based on that delay. We can then set readers to interrogate only tags within this distance.

We recently tested a prototype of a distance-sensing RFID system and achieved distance measurement accuracy of about 0.5 meter. That's not sufficient for a live application, but we're working with the pseudorandom sequence to achieve 20-centimeter accuracy, which we believe we can reach this year. Commercial Gen 2 interrogators and tags with distance measurement capability could be available one or two years after that.

Hao Min is the research director of the Auto-ID Lab at Fudan University in Shanghai, China.
To continue reading this article, please log in or choose a purchase option.

Option 1: Become a Premium Member.

One-year subscription, unlimited access to Premium Content: $189

Gain access to all of our premium content and receive 10% off RFID Reports and RFID Events!

Option 2: Purchase access to this specific article.

This article contains 461 words and 1 page. Purchase Price: $19.99

Upgrade now, and you'll get immediate access to:

  • Case Studies

    Our in-dept case-study articles show you, step by step, how early adopters assessed the business case for an application, piloted it and rolled out the technology.

    Free Sample: How Cognizant Cut Costs by Deploying RFID to Track IT Assets

  • Best Practices

    The best way to avoid pitfalls is to know what best practices early adopters have already established. Our best practices have helped hundreds of companies do just that.

  • How-To Articles

    Don’t waste time trying to figure out how to RFID-enable a forklift, or deciding whether to use fixed or mobile readers. Our how-to articles provide practical advice and reliable answers to many implementation questions.

  • Features

    These informative articles focus on adoption issues, standards and other important trends in the RFID industry.

    Free Sample: Europe Is Rolling Out RFID

  • Magazine Articles

    All RFID Journal Premium Subscribers receive our bimonthly RFID Journal print magazine at no extra cost, and also have access to the complete online archive of magazine articles from past years.

Become a member today!

RFID Journal LIVE! RFID in Health Care LIVE! LatAm LIVE! Brasil LIVE! Europe RFID Connect Virtual Events RFID Journal Awards Webinars Presentations
© Copyright 2002-2016 RFID Journal LLC.
Powered By: Haycco