On Wed, Aug 13, 2003 at 10:14:51AM -0500, The Fool wrote: > http://zdnet.com.com/2100-1103_2-5062542.html > > </p><!-- --><p>RFID functions as an evolution of the bar code, but is > more efficient and versatile because items can be identified wirelessly. > For the retail supply chain, this means, for example, that a box of goods > could be added to a shop's inventory system without opening the box and > scanning each item individually, since the RFID scanner could identify > all the items through the box.
I've been reading about RFIDs being able to do this for a while now, but I haven't seen an explanation of how it can scan multiple items in close proximity. Does anyone know what keeps all the transmissions (reflections) from the RFID chips from "overlapping"? I doubt they each have their own individual frequency within the 13.56MHz band, because to have millions of unique frequencies would require absurdly tight frequency discrimination. Is can't be simply distance/time of flight, because some of the tags may be the same distance from the reader (imagine a sphere with the reader at the center and the tags on the surface of the sphere).
Do they possibly use some sort of CDMA technology? If so, how can the reader decode the CDMA stream without knowing the pseudo-random code key for each RFID chip? (The code is what needs to be read, after all).
The only thing I can think of that might possibly work would be that each RFID chip delays a random amount of time before responding to a query from the reader (and this random time changes each time for each RFID chip that is queried). Then if the reader keeps querying over and over, eventually, by luck, it should get a clean signal from each RFID chip. But this seems that it would get very inefficient if you had hundreds of RFID chips in close proximity.
Anyone else have any ideas how it works?
Erik Reuter
Aw heck. Go here:
http://www.inf.ethz.ch/personal/vogt/pub/smc02rfid.pdf
I just skimmed it, but it looks correct. That's one way, but certainly not the only way. Well, I've read it more closely. The way the Fool's article reads, I'm assuming a pass through sensor box, which is what I'd do. Have separate broadcast and receive heads, separate pairs on each side of the box. One head broadcasts, the two receivers receive and compare results. The other head broadcasts.
Isn't that what they talk about, when a person pushes his shopping cart out the exit, "without paying"? I'm assuming they'd do plenty of tests, to make sure a can of soup leaning against the scanner, but not "inside" the scanner would not be picked up.
Then again, when I'm in my local new super market, which is all bar codes, and I see a manager walking around the store scanning shelves on low stock items......what a waste of data!
For my senior project I tried to develop a passive chip that would broadcast a code that changed WRT temperature, for data logging. The problem was I couldn't get a good response over a few hundred degrees F (if that), and the job I had at the time, we had 3k degree F ovens.
And my last job I was dealing with half and quarter waves for global tracking devices. Now I program in COBOL, and could not be happier.
Kevin T. - VRWC bike time
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