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not original commentor, but chiming in here: > When you say there isn't enough, is that there isn't physically enough spectrum, or is it just that there isn't

not original commentor, but chiming in here: > When you say there isn't enough, is that there isn't physically enough spectrum, or is it just that there isn't enough licensable spectrum (or even enough available unlicensed spectrum)? Both. Or rather physical spectrum is a superset of licencable spectrum and already the physical spectrum only has so many quantum numbers to encode information in. We can pretty much rule out orbital momentum encoding for satellite transmissions; the effects the atmosphere has on polarization are just too strong, for orbital momentum to survive to a degree, that it remains usable for consumer grade equipment. That leaves as encodable quantum numbers: - frequency - amplitude - polarization - momentum (i.e. direction of propagation) The band usable for radio satellite communication, i.e. able to pass through the atmosphere is rather narrow (compared with the whole EM spectrum): About between 100MHz up to 20GHz. That leaves you with a total usable bandwidth of about 19.9GHz. Multiply that with the symbol density reachable through AM (under good conditions with a low noise floor you can encode about 5 to 6 bits). Polarization gives another bit. Spatial multiplexing gives you log2(n steerable directions) extra bits (usually not more than 4 beams you can control). Divide that by two, because existing transceiver systems (except for some lab systems) can't do full duplex on a given channel. If you multiply that up this gives you a theoretical limit of about 10TBit/s you can implement for a given satellite location (having several satellites in proximity means they have to share the spatial encoding bits).

bargl 11y ago
On top of that you have the licensing which is going on, so if one of these systems get put into place they will most likely dominate the spectrum so that the others, if deployed, wouldn't even be allowed to transmit. Good answer by the way. I can second that what datenwolf said is pretty darn accurate, but doesn't account for a lot of the inteference that happens to reduce usability of the specturm. You also get into beam widths so and antenna. So if you have 700 satellites up in the air given full spectrum that means they'd have a fixed limit of 10TBit/s. That can be divided between their users (but it won't be anything close to 10TBit/s). So this will really be a factor of how big their beams are, how many people are within their beams, and how much frequency they can legally use. While this is really cool and possible, I doubt this will open up a whole new internet in it's first pass. There is also the issue with, how are they going to get the signal to it's destination? They can't beam it all the way around the world for you, and from what I understand at least OneWeb isn't going to communicate from satellite to satellite. So they'll have to have a ton of ground stations to receive your link and then send it over fiber, or transmit up to GEO, then down again. OR they'll have to hop up and down between nodes to it's destination, in a similar fashion to TOR. I don't know how they plan to solve this or if SpaceX has a better solution.
VLM 11y ago
On a slight tangent Shannon had a lot to say about signal to noise ratios / bit error rates and you're not going to like the theoretical minimum transmitter power required to push 10 TB over 20 GHz of BW at a reasonable bit error rate into orbit. Now figure maybe twice that for practical implementation at that huge scale, and twice that again because ultra wide band amplifiers are not known for cheapness or efficiency... This theoretical satellite phone from 2050 is going to get somewhat warm in your hand...
- datenwolf 11y ago
  Oh, I'm fully aware of the information theoretical limit. I just made an back-of-the-envelope estimation on the absolute theoretical upper bound, assuming a system operating at 0K and zero noise floor. My intention was to show, how small the available bandwidth is already under those idealized circumstances.
sathackr 11y ago
Thanks for doing the math! Good to have a concrete number. They might be able to achieve parity with fiber on optical links between satellites -- basically the same technology that's used with fiber -- but of course atmospheric conditions won't support that sort of bandwidth at optical frequencies, so there would be no easy way to get it to/from the surface.

bargl 11y ago

On top of that you have the licensing which is going on, so if one of these systems get put into place they will most likely dominate the spectrum so that the others, if deployed, wouldn't even be allowed to transmit. Good answer by the way. I can second that what datenwolf said is pretty darn accurate, but doesn't account for a lot of the inteference that happens to reduce usability of the specturm. You also get into beam widths so and antenna. So if you have 700 satellites up in the air given full spectrum that means they'd have a fixed limit of 10TBit/s. That can be divided between their users (but it won't be anything close to 10TBit/s). So this will really be a factor of how big their beams are, how many people are within their beams, and how much frequency they can legally use. While this is really cool and possible, I doubt this will open up a whole new internet in it's first pass. There is also the issue with, how are they going to get the signal to it's destination? They can't beam it all the way around the world for you, and from what I understand at least OneWeb isn't going to communicate from satellite to satellite. So they'll have to have a ton of ground stations to receive your link and then send it over fiber, or transmit up to GEO, then down again. OR they'll have to hop up and down between nodes to it's destination, in a similar fashion to TOR. I don't know how they plan to solve this or if SpaceX has a better solution.

VLM 11y ago

On a slight tangent Shannon had a lot to say about signal to noise ratios / bit error rates and you're not going to like the theoretical minimum transmitter power required to push 10 TB over 20 GHz of BW at a reasonable bit error rate into orbit. Now figure maybe twice that for practical implementation at that huge scale, and twice that again because ultra wide band amplifiers are not known for cheapness or efficiency... This theoretical satellite phone from 2050 is going to get somewhat warm in your hand...

datenwolf 11y ago
Oh, I'm fully aware of the information theoretical limit. I just made an back-of-the-envelope estimation on the absolute theoretical upper bound, assuming a system operating at 0K and zero noise floor. My intention was to show, how small the available bandwidth is already under those idealized circumstances.

sathackr 11y ago

Thanks for doing the math! Good to have a concrete number. They might be able to achieve parity with fiber on optical links between satellites -- basically the same technology that's used with fiber -- but of course atmospheric conditions won't support that sort of bandwidth at optical frequencies, so there would be no easy way to get it to/from the surface.