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There are underlying physical limitations to spectral efficiency. The best modulation schemes in use today can hit about 60 bits per second per hz of bandwidth.

There are underlying physical limitations to spectral efficiency. The best modulation schemes in use today can hit about 60 bits per second per hz of bandwidth. Meanwhile, single fibers top out above 40 terabit. To duplicate the bandwidth of just a single fiber link with a radio downlink, you'd need over 600 ghz of available spectrum. That'd require advances pushing deep into the thz gap, but beyond that, the physics of propagation are so different between 6 ghz, 60 ghz, and 600 ghz that it's hard to conceive of a single system covering so much spectrum. And that's not even getting into issues with geometric limitations of radio beam widths. While change is constant, this isn't an argument that any change you can imagine is physically realizable. Fiber is here to stay for a very long time precisely because it starts out with such massive fundamental physical advantages. It's also a mistake to assume innovation only benefits one alternative. We're still learning how to make and run fiber cheaper too.

shaklee3 6y ago
For what it's worth, satellite is typically around 1-4 bits per second per Hz.
Tuna-Fish 6y ago
> 60 bits per second per hz of bandwidth Between one transmitter and receiver. Starlink is designed to cheat Shannon by utilizing SDMA on a truly massive scale. Every antenna in the system, both on the satellites and on the ground stations, is a large phased-array antenna, capable of simultaneously emitting different signals on the same wavelength in different directions, and capable of discriminating between multiple different signals on the same wavelength that come from different directions. This is how Starlink intends to push so much throughput over just ~50 GHz of bandwidth. Not by pushing more bits per hz per link, but by filling the sky with hundreds of simultaneously visible satellites, and having each of them to all use all of the available spectrum at the same time on every link between any of them and any of the ground stations.
- jasonwatkinspdx 6y ago
  Phased arrays aren't magic. The directivity of the beam scales logarithmically with array size / element count. The limitations are such that you should think of it as being able to send Nebraska one signal while you send Kansas a different one. You can't get much more specific than that. Elon has been very explicit about starlink never being competitive vs telcos in urban areas. And independent people have done the math too. The system geometry maxes out at less than a single fiber: http://www.mit.edu/~portillo/files/Comparison-LEO-IAC-2018-slides.pdf http://www.mit.edu/~portillo/files/Comparison-LEO-IAC-2018-s...
  - Tuna-Fish 6y ago
    Phased arrays are not magic, but they are better than you make them sound like. When the directivity of the beam scales logarithmically with the element count, it just means you need a whole lot of elements. The beam sizes are in the FCC filings, and they will be able to fully re-use the frequencies for ground stations that are >20km apart. (Not with the same satellite, you want adjacent beam spots on the ground to come from different satellites, but with literally hundreds above you at any time, that's not a problem.) Availability of bandwidth does not really limit them until the satellite counts are up to really hilarious numbers. The aggregate throughput numbers in the paper you linked are correct, for the initial FCC filing. Since then they have multiplied the size of the constellation by ~7 and the throughput per satellite by ~3 (for 2.0 satellites). (ISL's are still missing, though.)

shaklee3 6y ago

For what it's worth, satellite is typically around 1-4 bits per second per Hz.

Tuna-Fish 6y ago

> 60 bits per second per hz of bandwidth Between one transmitter and receiver. Starlink is designed to cheat Shannon by utilizing SDMA on a truly massive scale. Every antenna in the system, both on the satellites and on the ground stations, is a large phased-array antenna, capable of simultaneously emitting different signals on the same wavelength in different directions, and capable of discriminating between multiple different signals on the same wavelength that come from different directions. This is how Starlink intends to push so much throughput over just ~50 GHz of bandwidth. Not by pushing more bits per hz per link, but by filling the sky with hundreds of simultaneously visible satellites, and having each of them to all use all of the available spectrum at the same time on every link between any of them and any of the ground stations.

jasonwatkinspdx 6y ago
Phased arrays aren't magic. The directivity of the beam scales logarithmically with array size / element count. The limitations are such that you should think of it as being able to send Nebraska one signal while you send Kansas a different one. You can't get much more specific than that. Elon has been very explicit about starlink never being competitive vs telcos in urban areas. And independent people have done the math too. The system geometry maxes out at less than a single fiber: http://www.mit.edu/~portillo/files/Comparison-LEO-IAC-2018-slides.pdf http://www.mit.edu/~portillo/files/Comparison-LEO-IAC-2018-s...
- Tuna-Fish 6y ago
  Phased arrays are not magic, but they are better than you make them sound like. When the directivity of the beam scales logarithmically with the element count, it just means you need a whole lot of elements. The beam sizes are in the FCC filings, and they will be able to fully re-use the frequencies for ground stations that are >20km apart. (Not with the same satellite, you want adjacent beam spots on the ground to come from different satellites, but with literally hundreds above you at any time, that's not a problem.) Availability of bandwidth does not really limit them until the satellite counts are up to really hilarious numbers. The aggregate throughput numbers in the paper you linked are correct, for the initial FCC filing. Since then they have multiplied the size of the constellation by ~7 and the throughput per satellite by ~3 (for 2.0 satellites). (ISL's are still missing, though.)