Exactly. I don't think filters, directional antennae or more complicated filtering can help with in-band interference on a mobile spread-spectrum technology. Filters don't work when the interfering signal is by definition 'in band', and directional antennae have no meaning in a mobile system...
But then ideal systems all work in utopia, don't they ;+)

Incidentally, the whole "radio waves don't interfere, they pass right through each other" is only true in empty space (or any simple linear medium). As soon as the waves enter a non-linear region, which I think the detector portions of _all_ receivers _must_ be (in order to convert the radio waves into a useful signal), they _do_ affect each other. A sufficiently strong noise signal can prevent detection of the signal of interest.

There are several ways around this:

1. Put a tuned filter in front of your receiver's detector, so the detector only sees waves near the frequency you're interested in (this is how we mostly do things today, with assigned channels for different users).

2. Put a directional filter (i.e. a directional antenna, a horn or dish or phased array or whatever) in front of the detector, so you only see signals coming from the right direction. If you and the emitter are moving relative to each other, now you have to track the emitter in order to be able to hear its signal. This is how many radars work.

3. Use some more complicated filtering scheme; such schemes often involve non-linear circuits, and so are susceptible to being overwhelmed by a sufficiently strong signal.

Which is not to say that new filtering schemes can't be developed, but saying "radio waves don't interfere" doesn't have much bearing on the issues.

Edited by author 11-25-2002 08:54 AM
Seconded. Somehow I find it difficult to believe a couple of things about the whole 'open spectrum'/wifi is the future meme:

1. That incumbent operators will ever buy into allowing other users on the spectrum which they have just paid huge amounts of money to use.

2. That any technique which shares spectrum (look up the underlaid concept) can do so without producing intereference to other users - add energy to a CDMA system and you get interference, end of story. To re-engineer the 'cocktail party' analogy of spread spectrum, lots of people whispering can still add up to a lot of noise and as a radio network designer I've got an issue with people making my job harder than it already is.

3. That wffi will ever achive the critical mass and connectivity required to deliver the contiguous high quality coverage of a cellular operator. Manhattan may have wifi nodes on each corner, but they're not much good to Jo(e) Public unless they all allow public acess. At a very basic level people just aren't that altuistic- I have wifi in my home, but apart from a geeky love of networks what's my motivation to open my net up to everyone else? I certainly won't make any money out of it at the moent.

Rheingold describes himself as "ten miles wide and a quarter of an inch deep". That seems to be a problem with a lot of the people quoted around here. They seize on some big idea like cognitive radio and they don't think about the details at all. But that's what makes the difference between an idea that will work and one that won't.

It seems like Internet hype all over again. In fact I get the impression that a lot of these guys are former Internet hypesters hoping to get the old money machine going again. They long for the good old days when a good story was enough to make you rich and famous, and nobody really cared whether you could come through with a useful and practical technology.

Face it. Those days are gone. We need a lot more skepticism and critical analysis of the ideas which are being tossed out around here. The Internet shakeup taught us that reality can't be ignored, and 9/11 put the exclamation point on that lesson.

Let's see more quoting of people who are ten miles deep and a quarter inch wide rather than vice versa. Let's have links to critical commentary that casts doubt on what is conventional wisdom among the online community. We need to applaud criticism, encourage detailed and skeptical analysis, and turn away from the addictive but ultimately pointless drug of self-congratulatory hype.

ARGH!

Heard on NPR this morning that the Miss America riots in Nigeria were a Smart Mob phenomena, abetted by cell phone and text messaging.

I love the direction radio is heading. When I was a kid listening to shortwave, I always wondered why the signal had to be so bad. I mean, the BBC was broadcasting on several frequencies at once, right? And I could receive, with varying degrees of success, three or more of these frequencies here in North America at any given time. And since the frequencies were in different bands, the wave propagation was different for each, meaning that there was a pretty good chance that when one frequency faded out, another one wouldn't, and often they'd all be fairly strong at the same time. So why, I wondered, couldn't I punch more than one frequency into my digital radio, and have it sync them? Why couldn't it recognize the similar modulation, match it, and turn out audio based upon the intersection of the frequencies entered? Now as an adult I understand some of the complications with doing this, but it's nice to see something similar in Ultra-Wide Band.