Even though I do nothing but software these days, technically I'm an electrical engineer (that's why it's
Myelin Electronics). That means I'm meant to design things like computer hardware and radio transceivers. Which, of course, I don't actually do, but it's nice to think about sometimes. So, here goes:
If you're doing anything with radio, you usually need some sort of license to be able to transmit. This is so transmissions from multiple people don't stomp all over each other. Cellphone companies and TV/radio channels will buy big chunks of spectrum and exclusively use it for their own stuff.
If you're doing something as a hobby, you don't want to have to deal with this sort of thing, so you use the ISM band. I think ISM means "industrial/scientific/medical". You can transmit straight into the ISM band within certain limits - you can't send in too much power, and you have to be able to cope with all the crap that other people are transmitting.
I can never find the info I need about the ISM band, except today I finally found this:
FCC Rules for ISM Band Wireless Equipment
The useful link on that page doesn't work, but you can get to
FCC rules part 15 - Radio frequency devices via the
FCC rules (47CFR) link.
To make your devices actually work, you've got to be fairly clever. If you just transmit all your power in one narrow band, someone else transmitting equally powerfully will interfere with your transmission so badly that you won't get anything out the other end. So, you use spread-spectrum techniques - you modulate your transmission with a code (a pseudorandom sequence) which spreads the power out over a range of frequencies. A receiver using the same code can piece it all back together and extract your signal from the noise.
For further reading, here's a paper about spread-spectrum techniques:
A Condensed Review of Spread Spectrum Techniques for ISM Band Systems
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