Time domain indeed (Score 2)

Forget the USA Today article; the author is clueless. The technology on the other hand sounds very much for real.

The fourier transform and the concept of "spectrum" is so deeply ingrained in so many engineers that they have lost touch with the time domain, essentially the "real world". Many things are a lot simpler in the frequency domain, like AM and FM. But for many probelms the frequency spectrum is the wrong tool. I see so many people who don't understand even simple transistor circuits because they try to think in the frequency domain.

So stop talking about spectrum - it is the wrong tool for understanding this technology. Maybe the power spectral density is useful to figure out if you're polluting radio or TV signals, but that's it.

And don't talk about bit synch either. While that's important in conventional communication systems, it isn't here. With a correlation detector, you just sit waiting around until the output of your correlator jumps up - that's the detection of the pulse - then you read your picosecond stopwatch. No need to expect the signal, just detect it. So simple!

If you're unfamiliar with correlation, and want to hear some math, here goes. The correlation of two functions (say f(t) and g(t)) is the integral S(f(t)*g(t))dt. Essentially multiuply the two signals together and then integrate. For two signals that don't look at all alike, the correlation is small. But when the two signals are very close, the integral is much much larger.

So imagine that you want to detect a pulse with very high resolution. At every instant in time you use a little integrator in some electronics to integrate your incoming signal with the expected signal (whatever shape the pulse has). When there's no pulse, the output of your correlator is very small. When the pulse comes along and lines up with the pulse in your integrator, your correlation gets really big relly fast and then small again as the pulse passes.

Try the math yourself: do the integral S(f(t)*f(t-d))dt where f(t) is the e^-3x pulse and notice that there is a huge peak at d=0.

Since the autocorrelation of that pulse (the pseudo gaussian e^-3x) they are using is very very high, you can detect this pulse with very high precision using correlation techniques. Sub wavelength resolution even.

(For all you communications engineers out there, this is the legendary matched filter technique. Except the typical use of the matched filter samples its output at the middle of the bit interval, when its output is supposed to be biggest(for a one) or smallest (for a zero). Here you do the opposite: when the matched filter output is maximum, that's the middle of the bit!)

So all you need is a correlation detector, a really accurate timer, and a pseudorandom noise generator to whiten up your spectrum and allow multiple channels. And if you do some dsp, your timer allows you to turn reflections off of objects into a pretty good radar image. (Except it's more like typical sonar sounding than typical radar).

If Fullerton's correlator is as good as he says, this stuff is very much for real. Believe it!