3 ms·

For others like me who are wondering how much bandwidth could be realistically achieved between sattelites, here is one real-world product from the company “Tes

For others like me who are wondering how much bandwidth could be realistically achieved between sattelites, here is one real-world product from the company “Tesat”: > The laser terminals supplied by Tesat needed less than 25 seconds on average to lock onto each other and begin transmission in both directions at 5.6 Gbit/s. https://www.laserfocusworld.com/lasers-sources/article/14104017/laser-links-will-link-small-satellites-to-earth-and-each-other https://www.laserfocusworld.com/lasers-sources/article/14104... Napkin math: that’s 56 clients using constant 100 MBit/sec. With overbooking coefficient of 0.01, that’s 5,600 clients on 100 MBit/sec plan per sattelite with 1 MBit/sec guranteed.

modeless 6y ago
To be clear, that refers to a demo system unrelated to Starlink. I don't think it is known where the Starlink lasers come from or what their capabilities are. All I know about them is that a reason they weren't deployed in the first batches of satellites is their original design included parts that did not burn up completely on reentry, and SpaceX wants Starlink satellites to always burn up completely, since there are thousands of them.
Robotbeat 6y ago
Another company that is doing 10Gbit/s. Considering a single Starlink satellite will have 4 laser links plus has a total radio bandwidth of about 20Gbit/s, this is sufficient. https://www.reddit.com/r/Starlink/comments/en7dtx/comment/fdx379d?utm_source=share&utm_medium=web2x https://www.reddit.com/r/Starlink/comments/en7dtx/comment/fd... ...however, this is nowhere near any fundamental limit on optical transmission. SpaceX is operating around 20-40GHz radios, and let’s just say a bandwidth 1% of the frequency with a SNR of 1, giving them a bit rate per channel of 200-400Megabits/s. The same calculation with near IR optical (which is optimistic obviously) gives 3 Petahertz frequency, 30THz bandwidth, and 30terabits/s/channel. So optical is nowhere near any kind of fundamental limit, unlike radio which needs many channels (ie phased array, MIMO, spatial multiplexing generally, etc) to saturate the bandwidth of the satellite bus. (Also, note that broadband users have a capacity factor of like 1-2%, so on average 20Gbps can give service to like 10,000 subscribers per viewable satellite using 100Mbps peak service.)
- grkvlt 6y ago
  3 PHz is way into the UV part of EM, near IR is about 30 THz so 300 Gbps max on a NIR channel using your numbers?
  - Robotbeat 6y ago
    Hah, oops. 3e-15s period, not 3e-16s. You’re right. What’s an order of magnitude between friends? (Won't let me edit the original post.)
  - Robotbeat 6y ago
    So you are correct, of course. I meant to use 1000nm, not 100nm. However, the thing about being in space is you actually COULD use vacuum UV (including 100nm) which doesn’t propagate in fiber and is even blocked by air. Not only does that have the bandwidth advantage but also allows even smaller apertures (or greater gain for the same aperture). Of course, vacuum UV transceivers aren’t really available (you can get coherent UV lasers down to 126nm) but in the long term it is possible.
lgats 6y ago
The lasers are sat-to-sat not sat-to-ground. This is a big deal when it comes to bandwidth as there is no atmosphere attenuating the signal

modeless 6y ago

To be clear, that refers to a demo system unrelated to Starlink. I don't think it is known where the Starlink lasers come from or what their capabilities are. All I know about them is that a reason they weren't deployed in the first batches of satellites is their original design included parts that did not burn up completely on reentry, and SpaceX wants Starlink satellites to always burn up completely, since there are thousands of them.

Robotbeat 6y ago

Another company that is doing 10Gbit/s. Considering a single Starlink satellite will have 4 laser links plus has a total radio bandwidth of about 20Gbit/s, this is sufficient. https://www.reddit.com/r/Starlink/comments/en7dtx/comment/fdx379d?utm_source=share&utm_medium=web2x https://www.reddit.com/r/Starlink/comments/en7dtx/comment/fd... ...however, this is nowhere near any fundamental limit on optical transmission. SpaceX is operating around 20-40GHz radios, and let’s just say a bandwidth 1% of the frequency with a SNR of 1, giving them a bit rate per channel of 200-400Megabits/s. The same calculation with near IR optical (which is optimistic obviously) gives 3 Petahertz frequency, 30THz bandwidth, and 30terabits/s/channel. So optical is nowhere near any kind of fundamental limit, unlike radio which needs many channels (ie phased array, MIMO, spatial multiplexing generally, etc) to saturate the bandwidth of the satellite bus. (Also, note that broadband users have a capacity factor of like 1-2%, so on average 20Gbps can give service to like 10,000 subscribers per viewable satellite using 100Mbps peak service.)

grkvlt 6y ago
3 PHz is way into the UV part of EM, near IR is about 30 THz so 300 Gbps max on a NIR channel using your numbers?
- Robotbeat 6y ago
  Hah, oops. 3e-15s period, not 3e-16s. You’re right. What’s an order of magnitude between friends? (Won't let me edit the original post.)
- Robotbeat 6y ago
  So you are correct, of course. I meant to use 1000nm, not 100nm. However, the thing about being in space is you actually COULD use vacuum UV (including 100nm) which doesn’t propagate in fiber and is even blocked by air. Not only does that have the bandwidth advantage but also allows even smaller apertures (or greater gain for the same aperture). Of course, vacuum UV transceivers aren’t really available (you can get coherent UV lasers down to 126nm) but in the long term it is possible.

lgats 6y ago

The lasers are sat-to-sat not sat-to-ground. This is a big deal when it comes to bandwidth as there is no atmosphere attenuating the signal