Check out things like "Olivia_MFSK" on wikipedia for something more current. It can transmit even with 14dB below the noise floor.
Seems like a any sane communication method for any galaxy spanning civilization would be: * encoded in such an advanced way that we wouldn't see it, even if using signals within our physics. * be highly directional
Maybe something like 1% of the asteroid belt would be converted into communication sats that could form a swarm that would over a few ns significant outshine the sun by focusing a signal on whoever you want to communicate with.
Of course typically the cloud of sats would be handling within the solar system communications or nearby stars that didn't require the entire cloud.
I do wonder if you could capture say 0.01% of the solar radiation for 24 hours and with 50% conversion frequency generate a single for 1 ns on the ideal frequency... could you outshine the galaxy on that frequency?
Of course the ideal frequency would be minimally blocked by dust and minimally generated by stars. The "Water hole" as mentioned by Seti (among others) would be a good start.
If a signal carries the “this looks like a signal” information in addition to the source information itself... isn’t that a waste of bandwidth? Wouldn’t an ideally encoded stream be indistinguishable from noise?
I can understand for practical reasons leaving some bandwidth on the table to make it easier to detect signals, but when talking about advanced alien life forms we have to assume they are playing closer to the limits of physics.
If they use frequency hoping, the changes in the frequency are even more regular. It's more difficult to measure the exact signal, but all the hops are inside an interval.