

Printing process uses carbon nanotubes










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RE: How's that gonna work?
By Solandri on 3/21/2010 5:37:14 AM , Rating: 2
No. A FFT just transforms a signal from the time to frequency domain or vice versa. If you have one signal of a certain frequency buried in noise of million different frequencies, a fourier transform can extract it...but if you have several thousand transmitters all on the same frequency, it's not going to help.

Some combination of discrete time signal processing and adaptive beamforming on the receiver end might work in theory, though...essentially taking advantage of the fact that each tag has its own unique position in 3D space.

Actually, he has a good point. A FT will also do spatial convolution/deconvolution. e.g. If you have the amplitude and phase information for light passing through a window in a certain direction, a FT applied to it will act like a lens, and "focus" the light to create an image of what the scene looks like through that window. I hadn't considered tackling the problem in the spatial domain (since there is no concept of a spatial domain in computer network communications).

With that mental barrier lifted, I can think of yet another way to solve this problem: Use two or (preferably) more receivers. Then you can use tomography to figure out the locations of each transmission, thus separating out each transmission. We did something similar with underwater acoustics (using both tomography and the phase of the arriving signal to distinguish between difference sources).

But those were just a handful of signal sources and the tomography computations took several hours back in the mid 1990s. I'm not sure how viable this would be with tens or hundreds of thousands of RFID tags. Maybe treating it more like phased array radar would simplify the whole thing. With a phased array receiver, you could "aim" it at a certain spot in the store, essentially picking up only the RFID responses form that location. Since the aiming point is determined mathematically (no moving parts), you could rapidly scan through the entire store, "listening" for responses one by one.

RE: How's that gonna work?
By porkpie on 3/21/2010 9:04:19 AM , Rating: 2
That's what I suggested above, using adaptive beamforming and discrete time signal processing to locate tags spatially.

