6 ms·

Can someone explain how does Starlink go about using the electromagnetic spectrum efficiently? 4G (and 5G even more so) relies on small cells to spatially parti

Can someone explain how does Starlink go about using the electromagnetic spectrum efficiently? 4G (and 5G even more so) relies on small cells to spatially partition the spectrum, therefore increasing the maximum bandwidth available in the system. Need to serve more users without compromising per-user throughput? Split the cells. Now, the cell of a satellite is enormous, and there's no way to reduce the tx power without getting out of range of the surface of the planet altogether. Not to mention that those satellites are pretty damn fast so they stay over any particular user for a very short amount of time. What am I missing? Or is this supposed to be a low throughput system (when added up over all concurrent users)?

wmf 4y ago
All modern satellites use multiple narrow spot beams; in Starlink's case each beam is 22 km wide. This allows spectrum to be reused.
- H8crilA 4y ago
  Oh, ok. But 22km is still a lot more than a typical 4G/5G urban cell. It's basically the size of a city. Whereas I can spot multiple stations in my city if I go for a 5-10 minute walk.
  - wmf 4y ago
    Yes, the density of satellite is far lower than urban cellular which is why no one is pitching satellite for urban customers.
    - colejohnson66 4y ago
      That, and latency. Or has that improved in the last decade?
      - Dylan16807 4y ago
        I wouldn't care about 40ms of latency for my phone. If by "last decade" you're comparing against geostationary, then yes. The satellites are roughly a hundred times closer.
      - deepdriver 4y ago
        Starlink latency is somewhere around 50ms on average according to friends, more than adequate for e.g. online multiplayer first-person shooters.
        Faark 4y ago
        Sounds about right. Here are some other stats, in case someone wants sth more authoritative than someones friends: https://www.ookla.com/articles/starlink-hughesnet-viasat-performance-q2-2021 https://www.ookla.com/articles/starlink-hughesnet-viasat-per...
      - smoldesu 4y ago
        Starlink gets 40-150ms, more often erring on the faster side. If they can get their peak load under control, I don't think I'd ever pine for hardwired internet again.
      - krisoft 4y ago
        > Or has that improved in the last decade? Yes. Starlink satelites are at low earth orbit, much much closer than the geosyncronous orbits communication satelites a decade ago had to be at. Distance is latency because the radio waves can “only” propagate at the speed of light. The shorter the trip the faster the signals can make it.
      - PeterisP 4y ago
        Latency is big issue for geostationary satellites - the ones at which you can aim a dish at a fixed point in the sky - which have to be very far away from Earth to be geostationary, 35000 km above surface, unlike Starlink satellites which are just 550km above ground.
        4y ago
  - mhandley 4y ago
    In principle you've got spatial diversity at both ends - on the ground and in space. I don't mean full duplex - I mean that the same area on the ground can in principle be covered by multiple Starlink satellites simultaneously using the same frequencies, so long as the satellites are not close together. The receiving phased array can separate the multiple signals just as it could if you used a steerable parabolic dish, but in software. Of course there may not be enough satellites launched yet to take advantage of this, but eventually there should be.
    - wmf 4y ago
      I don't think the FCC will ever allow that config due to interference with GSO.
      - krisoft 4y ago
        How will two starlink satelite communicating with a single ground receiver cause interference for GSO communications?
        wmf 4y ago
        They're on the same frequency band and traditional satellite dishes can pick up some amount of off-axis interference. Assuming constant power per beam, more beams (Nco) eventually overwhelm traditional receivers.
  - sgtnoodle 4y ago
    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?
pclmulqdq 4y ago
Phased arrays and beam steering are pretty magical. Both the satellite and the ground station can aim at each other by using hundreds of antennas as a "lens", providing very efficient use of the spectrum. If I had to guess, I would assume that Starlink's beam size is about 10-50 km in radius.
- MichaelCollins 4y ago
  There are limits to this magic. While sparse arrays of antenna can get you very good angular resolution when receiving signals, they're not so magical for transmission; as your array of transmitting antenna gets sparser, the power density of the beam also decreases proportionally. This is why aperture synthesis is great for radio astronomy, but next to worthless for far-field power transmission.
  - amelius 4y ago
    Why would the array get sparser? Isn't the point of an array to use more antennas, not fewer?
    - ncmncm 4y ago
      You can spread out a given collection of N antennas as wide as you like to get better angular resolution, but you lose signal strength at the target. Radio astronomers use big antennas and look for a long time. That doesn't work for communication. You can add more antennas, but those cost money. So, you use as many as you can afford, and space them out according to what you need. A too-broad antenna array on a satellite would have one set of problems, one on the ground others.
stagger87 4y ago
There are a lot of people that have written about this, here's one for instance. http://www.satmagazine.com/story.php?number=1026762698 http://www.satmagazine.com/story.php?number=1026762698
supertrope 4y ago
You’re not missing any spectral efficiency trick. Starlink maxes out at one customer per square mile or about one million total customers. It’s for areas beyond cellular coverage. Cell networks could improve infill coverage consistency with professional installs of roof mounted CPE. That would help people who have a weak signal inside their home but good signal outside.
- Retric 4y ago
  Starlink is effectively a point to point connection with each satellite. There is no technical reason they can’t have 1 billion customers with a large enough network and the right hardware. The issue is economic. Target 1 customer per square mile and most of the orbit over land is useful, target 100,000 customers per square mile and most satellites are only beneficial over megacities like NYC. Due to the short lifespan of individual satellites they can adjust to market forces and new technology fairly rapidly. If for example China lets them in the economics around density change significantly.
  - jcrawfordor 4y ago
    Directionality of antennas/electronically steered beams (off axis gain is not zero on the satellite or the terminal) and the relatively large bandwidth involved in these links imposes a limit that you'll hit even with an unlimited number of satellites. Calculating that limit is tricky but I don't think it's practical to cover urban areas for the reasons the parent brings up... cellular networks rely heavily on short effective range; the minimum achievable spot size for LEO satellites is still much larger than a typical urban LTE cell, and the terminal has the same problem of spot size in orbit being quite large.
    - rlt 4y ago
      The spot size is proportional to the phased array antenna size, right? Could SpaceX just keep scaling up their phased array antennas on the satellites as Starship reduces launch cost?
Retric 4y ago
There are two directions satellite to ground and ground to satellite, and they operate slightly differently. From the ground you can think of an array of satellites like a monitor showing a document. As you read every word is being displayed at the same time in the same color, but because the light is coming from different locations you can focus on a little area and ignore the rest of the screen. A traditional satellite dish uses a similar idea to just look at a single satellite at a time and ignore the rest of the sky, this is why you need to aim them. What starkink does differently is using a phased array to let it change focus without physically moving the dish, which is really really fast and lets them track satellites in LEO. Just like cellphones it can hand off from satellite to satellite plenty fast to keep up with the speeds involved. As soon as you can handle 1 transistion just repeat the same thing millions of times, computers are great at repetitive tasks. As to throughput, it’s supposed to be fast enough to watch 4K video which isn’t gigabit speeds but is vastly better than millions of people are stuck with.
- ncmncm 4y ago
  None of that makes any difference to the question. Phased array techniques save transmitter power more than bandwidth, because the beams just can't be very narrow with such a small array. It means one satellite gets your signal, not all in sight. Up top, one beam takes in many, many subscribers. Starlink will struggle to maintain more than a few tens of megabits. Sibling comment has the correct answer.
  - deleted 4y ago
    [deleted]
  - Retric 4y ago
    Power ends up being bandwidth, but I don’t think you understand why. A cellphone networks depend on proximity to a cell tower to alow frequencies to be reused across multiple cell towers. On top of this they also use a wide range of techniques to share each individual towers available bandwidth with multiple cell phones. But the most basic trick is signals get weaker with distance. Starlink also uses distance as a satellite can only see a small fraction of the total number of ground stations. However, weaker from distance is indistinguishable from a weaker signal due to a phased array antenna pointing somewhere else, so they can in effect simulate more cells than they would have from proximity alone. On top of this they can use all the existing cellphone tricks to maximise bandwidth. PS: Yes Starlink is limited to ~100 megabits per customer, but that’s vastly better than 1.5 Mbps DSL which is what many rural communities are stuck with.

wmf 4y ago

All modern satellites use multiple narrow spot beams; in Starlink's case each beam is 22 km wide. This allows spectrum to be reused.

H8crilA 4y ago
Oh, ok. But 22km is still a lot more than a typical 4G/5G urban cell. It's basically the size of a city. Whereas I can spot multiple stations in my city if I go for a 5-10 minute walk.
- wmf 4y ago
  Yes, the density of satellite is far lower than urban cellular which is why no one is pitching satellite for urban customers.
  - colejohnson66 4y ago
    That, and latency. Or has that improved in the last decade?
    - Dylan16807 4y ago
      I wouldn't care about 40ms of latency for my phone. If by "last decade" you're comparing against geostationary, then yes. The satellites are roughly a hundred times closer.
    - deepdriver 4y ago
      Starlink latency is somewhere around 50ms on average according to friends, more than adequate for e.g. online multiplayer first-person shooters.
      - Faark 4y ago
        Sounds about right. Here are some other stats, in case someone wants sth more authoritative than someones friends: https://www.ookla.com/articles/starlink-hughesnet-viasat-performance-q2-2021 https://www.ookla.com/articles/starlink-hughesnet-viasat-per...
    - smoldesu 4y ago
      Starlink gets 40-150ms, more often erring on the faster side. If they can get their peak load under control, I don't think I'd ever pine for hardwired internet again.
    - krisoft 4y ago
      > Or has that improved in the last decade? Yes. Starlink satelites are at low earth orbit, much much closer than the geosyncronous orbits communication satelites a decade ago had to be at. Distance is latency because the radio waves can “only” propagate at the speed of light. The shorter the trip the faster the signals can make it.
    - PeterisP 4y ago
      Latency is big issue for geostationary satellites - the ones at which you can aim a dish at a fixed point in the sky - which have to be very far away from Earth to be geostationary, 35000 km above surface, unlike Starlink satellites which are just 550km above ground.
      - 4y ago
- mhandley 4y ago
  In principle you've got spatial diversity at both ends - on the ground and in space. I don't mean full duplex - I mean that the same area on the ground can in principle be covered by multiple Starlink satellites simultaneously using the same frequencies, so long as the satellites are not close together. The receiving phased array can separate the multiple signals just as it could if you used a steerable parabolic dish, but in software. Of course there may not be enough satellites launched yet to take advantage of this, but eventually there should be.
  - wmf 4y ago
    I don't think the FCC will ever allow that config due to interference with GSO.
    - krisoft 4y ago
      How will two starlink satelite communicating with a single ground receiver cause interference for GSO communications?
      - wmf 4y ago
        They're on the same frequency band and traditional satellite dishes can pick up some amount of off-axis interference. Assuming constant power per beam, more beams (Nco) eventually overwhelm traditional receivers.
- sgtnoodle 4y ago
  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?

pclmulqdq 4y ago

Phased arrays and beam steering are pretty magical. Both the satellite and the ground station can aim at each other by using hundreds of antennas as a "lens", providing very efficient use of the spectrum. If I had to guess, I would assume that Starlink's beam size is about 10-50 km in radius.

MichaelCollins 4y ago
There are limits to this magic. While sparse arrays of antenna can get you very good angular resolution when receiving signals, they're not so magical for transmission; as your array of transmitting antenna gets sparser, the power density of the beam also decreases proportionally. This is why aperture synthesis is great for radio astronomy, but next to worthless for far-field power transmission.
- amelius 4y ago
  Why would the array get sparser? Isn't the point of an array to use more antennas, not fewer?
  - ncmncm 4y ago
    You can spread out a given collection of N antennas as wide as you like to get better angular resolution, but you lose signal strength at the target. Radio astronomers use big antennas and look for a long time. That doesn't work for communication. You can add more antennas, but those cost money. So, you use as many as you can afford, and space them out according to what you need. A too-broad antenna array on a satellite would have one set of problems, one on the ground others.

stagger87 4y ago

There are a lot of people that have written about this, here's one for instance. http://www.satmagazine.com/story.php?number=1026762698 http://www.satmagazine.com/story.php?number=1026762698

supertrope 4y ago

You’re not missing any spectral efficiency trick. Starlink maxes out at one customer per square mile or about one million total customers. It’s for areas beyond cellular coverage. Cell networks could improve infill coverage consistency with professional installs of roof mounted CPE. That would help people who have a weak signal inside their home but good signal outside.

Retric 4y ago
Starlink is effectively a point to point connection with each satellite. There is no technical reason they can’t have 1 billion customers with a large enough network and the right hardware. The issue is economic. Target 1 customer per square mile and most of the orbit over land is useful, target 100,000 customers per square mile and most satellites are only beneficial over megacities like NYC. Due to the short lifespan of individual satellites they can adjust to market forces and new technology fairly rapidly. If for example China lets them in the economics around density change significantly.
- jcrawfordor 4y ago
  Directionality of antennas/electronically steered beams (off axis gain is not zero on the satellite or the terminal) and the relatively large bandwidth involved in these links imposes a limit that you'll hit even with an unlimited number of satellites. Calculating that limit is tricky but I don't think it's practical to cover urban areas for the reasons the parent brings up... cellular networks rely heavily on short effective range; the minimum achievable spot size for LEO satellites is still much larger than a typical urban LTE cell, and the terminal has the same problem of spot size in orbit being quite large.
  - rlt 4y ago
    The spot size is proportional to the phased array antenna size, right? Could SpaceX just keep scaling up their phased array antennas on the satellites as Starship reduces launch cost?

Retric 4y ago

There are two directions satellite to ground and ground to satellite, and they operate slightly differently. From the ground you can think of an array of satellites like a monitor showing a document. As you read every word is being displayed at the same time in the same color, but because the light is coming from different locations you can focus on a little area and ignore the rest of the screen. A traditional satellite dish uses a similar idea to just look at a single satellite at a time and ignore the rest of the sky, this is why you need to aim them. What starkink does differently is using a phased array to let it change focus without physically moving the dish, which is really really fast and lets them track satellites in LEO. Just like cellphones it can hand off from satellite to satellite plenty fast to keep up with the speeds involved. As soon as you can handle 1 transistion just repeat the same thing millions of times, computers are great at repetitive tasks. As to throughput, it’s supposed to be fast enough to watch 4K video which isn’t gigabit speeds but is vastly better than millions of people are stuck with.

ncmncm 4y ago
None of that makes any difference to the question. Phased array techniques save transmitter power more than bandwidth, because the beams just can't be very narrow with such a small array. It means one satellite gets your signal, not all in sight. Up top, one beam takes in many, many subscribers. Starlink will struggle to maintain more than a few tens of megabits. Sibling comment has the correct answer.
- deleted 4y ago
  [deleted]
- Retric 4y ago
  Power ends up being bandwidth, but I don’t think you understand why. A cellphone networks depend on proximity to a cell tower to alow frequencies to be reused across multiple cell towers. On top of this they also use a wide range of techniques to share each individual towers available bandwidth with multiple cell phones. But the most basic trick is signals get weaker with distance. Starlink also uses distance as a satellite can only see a small fraction of the total number of ground stations. However, weaker from distance is indistinguishable from a weaker signal due to a phased array antenna pointing somewhere else, so they can in effect simulate more cells than they would have from proximity alone. On top of this they can use all the existing cellphone tricks to maximise bandwidth. PS: Yes Starlink is limited to ~100 megabits per customer, but that’s vastly better than 1.5 Mbps DSL which is what many rural communities are stuck with.