Cellphone 'radar' tracks traffic flow

New Scientist ^ | Oct 3 | Staff

[_Jim]

This sheds new light on the technique. Apparently, they are using a mobile station (or maybe I should say a remote station) - not the base station

Nop ... they simply used a pair of converted phones to do the acquisition of the RF signal and convert that to some lower IF for digitization by the data acquistion board they have listed ...

What they appear to be doing is simple TDOA (Time Difference Of Arrival, between the 'direct ray' from the base station and the 'reflected ray' from some target) with that equipment ...

IMO that was early proof-of-concept work; it worked for an aircraft some distance out. A 'crowded' urban environment is going to require A LOT more processing hardware from multiple 'observers (receivers and data processors).

[_Jim]

furthermore, the ground clutter - reflections from nearby metal buildings, billboards, transmission towers, etc. - would almost certainly overwhelm the tiny reflections from cars on the ground.

Correlators, multiple 'correlators' tracking various 'reflections' in software and only working on those with (or known to have) motion.

Call it MTI (moving target indicator) if you will; a simple technique of 'clutter elimination'.

[_Jim]

However, cellphones have their own Achilles' Heel. All makes and models of cellphones use a PLL to generate and control the transmitted frequency and the primary frequency generating circuit of the PLL is the VCO, which have notoriously low Q's and consequently poor spectral purity.

You're straining for gnats and glossing over more important considerations; for instance, the early work on this (the field trials) look to be more dependent/using TDOA techniques. This is a whole new ball game.

#2. This system uses base station emissions, not individual subscriber emissions ...

I dare say the spectral purity of the PLL's in these devices is several times (not quite an order of magnitude - but close) more pure than that found in your average 2M & 450 HT nowadays as well as the scanners curently avialable; and these sanners are markedly improved over the previous generation, but again, some mis/disinformation contributed by this article is leading some to think this is a mobile-based and not a base-station based bi-static modulated-CW RADAR system ...

BTW - I'm doing just fine considering ...

[_Jim]

so it would be incorrect to suggest that you can't have both spread spectrum and frequency hopping.

This argument seems to be continually made by those who do not wish to lose the SS versus FH argument. You can't have it both ways either, and it isn't. (Pssst: Look at their IF filter bandwidths.)

[_Jim]

PS. show me a commercial implementration of a FH-CDMA system.

Remember, the key here is 'multiple access' - multiple users using the same spectrum simultaneously ... TRUELY achieved with DS-CDMA (in the form of IS-95 cellular) where each 'channel' is 1.25 MHz (or so) in width WITH different subscribers assigned a *different* orthagonal spreading Walsh 'code' that allows each subscriber to be 'pulled out' of that 1.25 MHz wide 'channel' while differentiating that subscriber from 63 (or so) others ...

[snopercod]

Thanks and thanks.