Jonathan_S wrote:If you could pack 40 bits per pulse, like you suggested, then even a 1 pulse signal could carry 1 trillion different values -- even after you deduct quite a lot of that to leave room for some robust error correcting codes that still seems to leave vastly more codes that you'd need for any amount of "most likely threat parameters". I suggest that it's nowhere near that efficient at packing data into pulses, not at this early stage, or else it wouldn't take 4 pulses just to fit a custom code list for a reasonable number of just the most likely threat parameters.
Quite agreed. From Honor's description of the uses they were envisaging, it sounds like no more than 2 or 3 bits per pulse, giving a full "message" of 4 pulses at best 12 bits. If you build in 2 bits for error correction, that's a mere 1024 different messages.
It's also possible she was talking nonsense. She's not a comms specialist.
I'd also note that these RDs have a FTL transmission range of 4 light hours - while the newer really high bandwidth FTL like Keyhole II and Hermes have far shorter ranges; like around 40 times shorter (Keyhole II being good for around 5-6 LM). It's possible that the grav signals "smear" over distance as they travel along the Alpha wall and to get the maximum range you might need not only a much more powerful pulse but you also might need to use simpler encoding so it's still intelligible at those extended ranges.
But it does seem that due to the reduced transmission ranges the newer really high bandwidth FTL should needs far less powerful grav generators -- which presumably would be correspondingly smaller but also be easier to engineer for high pulse rates.
And the 1903 PD FTL tech might well have been capable of higher bandwidth that we see from these RDs if they'd been willing to engineer less powerful, shorter ranged, transmitters.
Or they stopped trying to transmit for over light-hours. The passage you quoted talks about how it had been mankind's dream to have FTL comms over interstellar range, so the R&D team might have been pursuing that first and 4 light-hours was the best they could, with the signal attenuation that is created by that distance. That would explain why they were mounting a huge power source.
But then they realised they were never going to get interstellar ("insurmountable limitations") and decided to focus on tactical uses only. At which point some young person who was not part of the project pointed out that tactical uses don't need light-hours: almost all combat happens within half a light-hour. So if you're not trying to send a message that will be received by something 8x closer, if it's a simple squared attenuation that's 64x better signal at the receiver. But in fact, most attenuation approximations are actually to the third or fourth power, so it could be even better than that.
Then you add the Fifth Imperium posts Dahak linked to above. It looks like the biggest improvement in pulse cycle time was the fact that they didn't need to charge a capacitor to send each.
Then I could add that the biggest breakthrough was finding out just
how to modulate signals. Remember this is a medium that no one has ever transmitted on before. There is no technology for how to finely control the pulse you're sending. Before it exists, you're doing a basic ASK or FSK (Morse code can be thought of as a very crude form of FSK). But once you do know how to do that, you can apply 2200 years of theoretical and applied development of information transmission. That could be a jump equivalent from going from Morse code to a modern
4096-QAM (see
also) or
OFDM) overnight.
In fact, our
own gravcoms are very similar to that. We know that the medium exists and we know there are pulses in it (see the LIGO and Virgo confirmations of the existence of gravitational waves). But right now, transmission is wholly impractical.
PS: Larry Niven and Greg Benford's third book on the
Bowl of Heaven trilogy, "Glorious," talks about how an advanced, K2-level civilisation uses gravitational waves for interstellar communication.
