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It’s hard to bet against human determination but on the face of it, it seems out of reach to put satellite voice calls in a package the size of an iPhone. The

It’s hard to bet against human determination but on the face of it, it seems out of reach to put satellite voice calls in a package the size of an iPhone. The power requirements and thus the bulky batteries of existing satellite phones could be reduced by switching to protocols which trade off bandwidth for higher error correction abilities through encoding and I don’t think existing satellite phones use the most bandwidth efficient encoding available so that could be another improvement. There’s still the free space path loss between the surface of earth and a Globalstar or Starlink which my best guess estimate is in the order of 130db (can anyone more knowledgeable share a better estimate? I assumed bottom end of L-band, a height between Starlink and Globalstar, and used unverified dBi values for antennas at both ends). Then there’s latency but I don’t think that’s a show stopper for many common use cases of voice and data.

lxgr 3y ago
> The power requirements and thus the bulky batteries of existing satellite phones could be reduced by switching to protocols which trade off bandwidth for higher error correction abilities No, you can’t trade bandwidth against error correction. What you can do is use channel coding/error correction to trade effective signal-to-noise strength against data rate (at constant power). But since your data rate demand is fixed for voice, and the uplink transmit power is fixed too (by 4G/5G specs), you need to raise the received signal strength. The only thing that can help you there are more directional receive antennas on the satellites, which is what these new generations are doing.
- AnarchismIsCool 3y ago
  To be clear, what's happening here is people are accustomed to using "bandwidth" to mean bits/second but bandwidth has a much more specific meaning in the RF world, akin to calling Ethernet "SSH". Bandwidth is the actual number of kilo/mega/gigahertz the signal occupies on the spectrum and is largely a function of symbol rate, but the actual data rate has many more variables.
  - lxgr 3y ago
    I’m aware of the distinction and I’m using the terms accordingly. More bandwidth does not help you get a better data rate if you’re power limited. In the case of satellite communications you basically always are, and bidirectionally so when it’s to mobile/battery-powered devices.
    - AnarchismIsCool 3y ago
      Yeah, I posted to clarify for the person you replied to and other non-rf people. You obviously know what's going on but you used a lot of language I figured should be defined for anyone who comes across this.
      - lxgr 3y ago
        Ah, sorry, I mixed you up with GP then and thought you were disagreeing with the "power-limited regime" statement!
    - cycomanic 3y ago
      That is in fact incorrect. Capacity scales as Blog2(1+SNR) so increasing bandwidth at the same power (which would decrease SNR linearly assuming AWGN) will result in increased capacity. Although pratically you can do that only so far, because you're going to be limited by the bandwidth of your receiver.
      - lxgr 3y ago
        It’s only theoretically incorrect. Practically, satellite communications are power-limited and accordingly will always be so close to the noise floor (i.e. S/N << 1) that the logarithm can be approximated with a constant and the bandwidth dependency is negligible and drops out of the equation. It only matters in the bandwidth-limited regime, i.e. for S/N >> 1.
        CraigJPerry 3y ago
        This thread is why I like HN. Thanks for sharing the insights both.
livueta 3y ago
You can talk to satellites over lora with a 5w transceiver (source: done it), which is on the order of magnitude of a cell phone transceiver. Opus goes down to .75kbps. Ham boomerisms don't cleanly apply to modern voice codecs and digital modulation techniques.
- wkat4242 3y ago
  How do you do LoRa over a sat? I thought the chirping makes that difficult because you exceed the passband on the sat? Interesting! Which sat did you use?
  - livueta 3y ago
    I used tinygs on a lilygo esp32-based board with a semtech sx1262. Before its untimely demise I used SDSAT, which would rebroadcast messages. I'm not currently aware of any active sats that do that, but you can receive telemetry from tons of them as listed on tinygs.
    - wkat4242 3y ago
      Thanks! I'll look into it! I live in a busy city now near the ground floor so the hobby is kinda on hold as I can't get any antennas up :( But this might be portable enough to bring out on a hike.
katbyte 3y ago
I would be quite happy with texting or very slow internet tbh

lxgr 3y ago

> The power requirements and thus the bulky batteries of existing satellite phones could be reduced by switching to protocols which trade off bandwidth for higher error correction abilities No, you can’t trade bandwidth against error correction. What you can do is use channel coding/error correction to trade effective signal-to-noise strength against data rate (at constant power). But since your data rate demand is fixed for voice, and the uplink transmit power is fixed too (by 4G/5G specs), you need to raise the received signal strength. The only thing that can help you there are more directional receive antennas on the satellites, which is what these new generations are doing.

AnarchismIsCool 3y ago
To be clear, what's happening here is people are accustomed to using "bandwidth" to mean bits/second but bandwidth has a much more specific meaning in the RF world, akin to calling Ethernet "SSH". Bandwidth is the actual number of kilo/mega/gigahertz the signal occupies on the spectrum and is largely a function of symbol rate, but the actual data rate has many more variables.
- lxgr 3y ago
  I’m aware of the distinction and I’m using the terms accordingly. More bandwidth does not help you get a better data rate if you’re power limited. In the case of satellite communications you basically always are, and bidirectionally so when it’s to mobile/battery-powered devices.
  - AnarchismIsCool 3y ago
    Yeah, I posted to clarify for the person you replied to and other non-rf people. You obviously know what's going on but you used a lot of language I figured should be defined for anyone who comes across this.
    - lxgr 3y ago
      Ah, sorry, I mixed you up with GP then and thought you were disagreeing with the "power-limited regime" statement!
  - cycomanic 3y ago
    That is in fact incorrect. Capacity scales as Blog2(1+SNR) so increasing bandwidth at the same power (which would decrease SNR linearly assuming AWGN) will result in increased capacity. Although pratically you can do that only so far, because you're going to be limited by the bandwidth of your receiver.
    - lxgr 3y ago
      It’s only theoretically incorrect. Practically, satellite communications are power-limited and accordingly will always be so close to the noise floor (i.e. S/N << 1) that the logarithm can be approximated with a constant and the bandwidth dependency is negligible and drops out of the equation. It only matters in the bandwidth-limited regime, i.e. for S/N >> 1.
      - CraigJPerry 3y ago
        This thread is why I like HN. Thanks for sharing the insights both.

livueta 3y ago

You can talk to satellites over lora with a 5w transceiver (source: done it), which is on the order of magnitude of a cell phone transceiver. Opus goes down to .75kbps. Ham boomerisms don't cleanly apply to modern voice codecs and digital modulation techniques.

wkat4242 3y ago
How do you do LoRa over a sat? I thought the chirping makes that difficult because you exceed the passband on the sat? Interesting! Which sat did you use?
- livueta 3y ago
  I used tinygs on a lilygo esp32-based board with a semtech sx1262. Before its untimely demise I used SDSAT, which would rebroadcast messages. I'm not currently aware of any active sats that do that, but you can receive telemetry from tons of them as listed on tinygs.
  - wkat4242 3y ago
    Thanks! I'll look into it! I live in a busy city now near the ground floor so the hobby is kinda on hold as I can't get any antennas up :( But this might be portable enough to bring out on a hike.

katbyte 3y ago

I would be quite happy with texting or very slow internet tbh