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Comment by paul_f | Hacker News Reader

You can add in quadrature amplitude modulation. Reusing bandwidth is a a fascinating field. It's not as simple as 1 bit per Hz

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Current laser com systems are getting multiple bits per photon - it seems that the communications field will keep pushing the limits until there is simply no business case, and then push some more. Really, the only limit is Shannon's limit, but that's only concerned with data rate. The modulation schemes that sit on top of the raw data is where all the capacity magic happens.

I assume you're referring to something like this: https://www.sciencedaily.com/releases/2017/02/170203102740.h... but this isn't a scalable transmission system since it's based on single photon detection, and single-photon detection is not a high bandwidth communications ability because you have to discriminate the photons. It's a far cry from "current laser communications".

AFAIK the fastest IP network speed at the moment if 44.2 Terabits/sec for a single light source (https://www.sciencedaily.com/releases/2020/05/200522095504.h...).

I'm talking about the laser communication satellites that NASA et al have launched into orbit, which are supposedly around 3-4 bits per photon.

These are hitting upwards of 10Gbit speeds (not quite 44.2 Terabits) but still impressive nonetheless.

https://en.wikipedia.org/wiki/Laser_communication_in_space https://en.wikipedia.org/wiki/Optical_PAyload_for_Lasercomm_...

> the only limit is Shannon's limit, but that's only concerned with data rate.

But that limit, the capacity, is also the maximum possible limit of information transfer on the channel. Modulation schemes don't increase it or change it, they just push the achievable information rate of the system closer to the capacity.

Modulation is definitely the wrong word in this context. I was thinking about channel coding and compression algorithms that increase the efficiency of the system without requiring additional capacity.

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