Unlimited Airwaves

Dan Gillmor has an article concerning the notion of scarcity of the airwaves, which has long been a testament of faith at the FCC. Recent advances in technology may render that testament false.

Comments

Re:Described before?

[dknj]

I seemed to have answered my own question, the article i was refering to was the Ultra Wide Band. Additionally, this article seemed interesting.

-dk

Re:New?

[Gordonjcp]

Bandwidth. Right, consider a *very* precise signal, at exactly 100MHz. Look at it with a spectrum analyser - you'll see a very very thin spike. Once you modulate it, it will spread out on either side. So, for example, a conventional voice radio channel, with a bandwidth of 300-3400Hz roughly (the same as an ordinary phone) will spread to 100MHz +/- 3.5kHz. To leave room for different channels you need a "guard band", so the channels are usually 10kHz apart.

This is one of the reasons Morse code is still so popular with amateur radio enthusiasts - you can send extremely narrow band signals that allow you to communicate fairly quickly. If someone is really good at it, they can communicate almost as fast as speech, over a channel a few *tens of Hertz* wide.

Re:New?

[SomeoneGotMyNick]

That's an understandable perception and theoretically will work. Consider this enlightenment and not a flame.

Current FM radio modulates the signal above and below the designated carrier frequency. Therefore a 20Khz signal (peak of human hearing) will modulate a 95.3MHz carrier between 95.28 and 95.32MHz. IIRC the full 40Khz deviation accounts for both channels of a stereo broadcast.

There's additional use for Broadcast radio. I forget where I saw it, but I believe there is an offset from the designated frequency to place a mono only 20Khz band away from the stereo part of the transmission for mono FM radios to pick up properly. This may however be an outdated use of the extra bandwidth.

Additional bandwidth can be used for other data/audio signals to be carried independent of the main broadcast for Broadcast FM plus 'padding' between stations.

Re:New?

[Gordonjcp]

The stereo system used on FM radio works like this. Your normal FM carrier has a mono signal (L+R), and modulated on a 38kHz subcarrier is the difference signal (L-R). The mono radio can hear only the L+R signal, the higher band L-R being filtered out. In a stereo decoder, the L-R signal is demodulated, and mixed with the L+R to give L only, which is then subtracted from L+R to give R only.

It's sometimes called M/S (mid/side), so we can express it like this:

M=L+R, S=L-R when transmitting.

L=M+S, R=M-L

Clear as mud, right?

Re:Radio kindergarten Part II

[Gordonjcp]

Because the higher the frequency of the modulation, the further from the centre frequency you go. For example, given the precise 100.000MHz signal above, if you modulated it with a 1kHz signal, you'd get a spike at 99.999MHz and a spike at 100.001MHz, as well as the original 100.000MHz one, right? If you then increased it from 1kHz to 10kHz, the spikes would be at 99.99MHz and 100.01MHz. If you increased the signal to 1MHz you'd have spikes at 99MHz and 101MHZ.

I recommend that you get hold of the ARRL handbook from your local library, or indeed the RSGB book if you're in the UK. These are the standard works on amateur radio, and explain all these things far better than I can....

Re:This will probably come out "ignorant"...

[Permission Denied]

To implement what you're talking about, you need some sort of time sync. Basically, what you're talking about is very similar to how ethernet works, but there are two fundamental differences between ethernet and radio at one specific frequency: with ethernet, you can both listen and broadcast at the same time (which means you can detect when you have a collision, thus CSMA/CD); with radio, you cannot listen while you're transmitting because your signal will drown out any incoming signal. This means you need some way of saying "OK, you can broadcast now." You could do this either on a time-slice basis (like 802.11) or with a token-passing scheme (and there are some wireless protocols that do token-passing). Another problem is that you might have three radios, like this:

A ---- B ---- C

where A is four dashes away from B and B is four dashes away from C. Suppose that a signal "lasts" for five dashes. That means A and C can't see each other, but B can see both. This brings up other nasty problems with simple protocols (and wireless protocols like 802.11 deal with this).

So, the simple packet-addressing scheme won't work for two-way communication. As for one-way communication, there's no need to "label" the recipient of a broadcast; radio is inherently broadcast, so everyone can hear everything anyway.

Re:i dont get it

[CarlDenny]

> Why cant we just use higher and higer frequencies? 2GHz full? Use 20GHz? Or 50GHz? Or a googlehertz?

Because, the higher the frequency, the shorter the wavelength.

And the shorter the wavelength, the less "penetrating power" the signal has, and the more the signal is absorbed by intervening walls/clouds/.../and eventually air.

In short, 100Ghz signals can't even make it across a room without getting in trouble.

Re:i dont get it

[bugg]

Your reasoning is slightly flawed. Visible light is 400nm-700nm which works out to be 7.5*10^14 Hz and 4.3*10^14 Hz- much larger than 100GHz. I have no trouble seeing the light from my lightbulb across the room.

Re:Ten percent of the spectrum needs to be open

[Kirkoff]

It's called Amateur Radio.

Which is generally not open to experimentation, testing or demonstration of new methods and technologies.

Umm, what Amateur Radio are you dealing with? The license I hold does allow me to experiment with new techniques. For example PSK-31 was invented in 199\8 or 1999 and is widespread. Yes, there are some limits, for example, you can't use more bandwidth then a voice channel on HF for new modes, but that's just common sense. Up above 3GHz, you're pretty much open to anything you want, including spread spectrum. You can do SS on everything above 70cm. If it's currently not allowed, the FCC does grant 6 month at a time experiment permits. If it works, the FCC will allow it. For example, see the ARRL's experiments in the 60m band.

--Josh