4 ms·

There's a ton of very clever stuff going on with modern RF modulations. For example, each client terminal will have a unique pseudo-randomization key, and trans

There's a ton of very clever stuff going on with modern RF modulations. For example, each client terminal will have a unique pseudo-randomization key, and transmissions are modulated by that key. That evenly distributes the transmission across the spectrum. To someone without the key, it appears to be random noise. Multiple transmissions can occur simultaneously on the same spectrum with different keys, without necessarily interfering with each other. Also, that 22km beam isn't going to be uniform, but rather higher gain at the center, and below some power threshold at the peripheral.

wmf 4y ago
CDMA doesn't increase capacity because the N codes have to each use 1/Nth the rate. It fell out of favor around 15 years ago because it doesn't allow one transmitter to temporarily use the whole channel, which is advantageous for data traffic.
jcrawfordor 4y ago
Spread spectrum methods are not magic, they do not manufacture new spectrum. As you get a larger number of simultaneous spread spectrum users the apparent "noise floor" (actually other spread spectrum transmissions) becomes higher and higher until the achievable data rate approaches zero. This isn't to say that the use of spread spectrum methods doesn't have benefits, but it doesn't change the fundamental concepts. Bluetooth, for example, uses FHSS but will still suffer from contention in very noisy environments.
midoridensha 4y ago
>That evenly distributes the transmission across the spectrum. To someone without the key, it appears to be random noise. I wonder if this is why SETI hasn't found any alien communications: they invented spread-spectrum communications and it all looks like noise to us.
- ncmncm 4y ago
  SETI participants recognized early that we would only ever be able to pick up signals meant for us. So, those would not look like noise. It is why few take it seriously. Why would anybody be putting that much effort into trying to get our attention?
- gridspy 4y ago
  It could also be that there are a series of different communication technologies. By the time we move onto the 3rd, noone is listening for the first. For instance AM radio, FM radio, TV, Spread spectrum analogue, digital, ... hypersuperfastwave ... Since each tech spreads from the source in a small sphere-shaped shell depending on when that technology was invented, you need a huge co-incidence for someone else's shell to hit you WHEN you are listening for that type of communication.

wmf 4y ago

CDMA doesn't increase capacity because the N codes have to each use 1/Nth the rate. It fell out of favor around 15 years ago because it doesn't allow one transmitter to temporarily use the whole channel, which is advantageous for data traffic.

jcrawfordor 4y ago

Spread spectrum methods are not magic, they do not manufacture new spectrum. As you get a larger number of simultaneous spread spectrum users the apparent "noise floor" (actually other spread spectrum transmissions) becomes higher and higher until the achievable data rate approaches zero. This isn't to say that the use of spread spectrum methods doesn't have benefits, but it doesn't change the fundamental concepts. Bluetooth, for example, uses FHSS but will still suffer from contention in very noisy environments.

midoridensha 4y ago

>That evenly distributes the transmission across the spectrum. To someone without the key, it appears to be random noise. I wonder if this is why SETI hasn't found any alien communications: they invented spread-spectrum communications and it all looks like noise to us.

ncmncm 4y ago
SETI participants recognized early that we would only ever be able to pick up signals meant for us. So, those would not look like noise. It is why few take it seriously. Why would anybody be putting that much effort into trying to get our attention?
gridspy 4y ago
It could also be that there are a series of different communication technologies. By the time we move onto the 3rd, noone is listening for the first. For instance AM radio, FM radio, TV, Spread spectrum analogue, digital, ... hypersuperfastwave ... Since each tech spreads from the source in a small sphere-shaped shell depending on when that technology was invented, you need a huge co-incidence for someone else's shell to hit you WHEN you are listening for that type of communication.