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Comment by namibj | Hacker News Reader

MIMO can and does go beyond the single-path shannon limit. See e.g. DVB-S2 using circular polarisation duplex.

Your point is a good one, and I don’t think you should get down-voted. I’ve seen many people appeal to Shannon’s law for SINGLE channel as an argument to why things like MIMO or Starlink are limited to low amounts of bandwidth. More channels is absolutely an important way to get around the “but there’s only so much bandwidth” argument, and by increasing number of channels (and increasing size of reciever and transmitter), you absolutely can just keep increasing throughput for the same frequency bandwidth allocation.

Thank you for the support.

The case of StarLink (in difference to (indoor) 802.11n/ac) won't give you more than polarization duplex (capped at 2x, in practice you'd get up to (iirc) ~30dB isolation) for any individual point-to-point connection. Yes, including the satellite <=> ground-facility "backhaul".

Theoretically it _should_ be possible to get like 10x bandwidth via spatial MIMO when using <3μm (likely <1μm) wavelengths, but that might require optical heterodyne detection to get the MIMO streams separated from another in the speckle pattern the atmosphere caused.

I'd like to read that paper about optical heterodyne detection spatial MIMO for the link to/from a satellite in an 200~1000km orbit.

One terminal (which has beamforming capability on both ends with hundreds of elements) can connect to multiple satellites with the same frequency band.

Oh sure, that's a massive effect. It's just that for a single p2p link, there'd be at most minimal benefit from MIMO. As soon as there are either multiple satellites or multiple user terminals (or dare both), i'd expect nothing worse than the bandwidth scaling with the square root of the antenna count.

“Single-path” never has been and never will be synonymous with “channel”.

Thank you. I was confused by the above comment.

Not sure what you mean by that, but Shannon's law applies with MIMO as much as anything else. A single channel is still limited by the Shannon capacity, but you can do tricks to constructively interfere on different paths, or use multiple antennas to create multiple new channels.

You are still not cheating entropy with that.

I think when we come up with a way to cheat entropy it will be bigger news than wifi.

As an aspiring speculative-fiction author always on the hunt for a new premise, I’d love to know, what would be possible if we could “cheat entropy” in this sense?

(This doesn’t have to be possible in our universe; posit a universe where it is possible—maybe one inside a Matrix-like simulation inside our own universe, if you like.)

So many things would break.

The statement that we cannot cheat entropy is basically a statement that the volume of an evolving quantum system (in phase space) stays constant over time, but gets more complex.

Entropy increases because our maps of that volume increase in volume, because we can't follow all the little eddies and crinulations.

So, okay. Two options for you: Maybe we can follow it, because the universe turns out to be more predictable than expected. This is basically a statement that chaos theory is wrong, and entropy isn't increasing as fast as our current understanding of the rules say.

Alternately, maybe the volume of the actual phase space is decreasing over time. This would allow the volume of our estimates to stay constant despite increasing complexity, but breaks linearity and suggests the universe might be going away.

While information theory entropy and physical entropy are not identical concepts, they are coupled. I recommend Isaac Asmiov’s “The Last Question” if you haven’t read it already.

There are processes that decrease local entropy but increase global entropy. We call it life.

I didn't claim it did.

But you get diminishing returns from around 1bit/s/Hz if you just turn up the power. Your energy efficiency will go down, a lot. The more you can get spatial multiplexing to keep you away from that border, the less energy you'd need to dump into the air. And as computing get's cheaper, that energy tradeoff moves more and more towards MIMO. Back in 2005 we didn't have a need to think of 8x8 MIMO (Laptop <=> AP). Now it becomes realistic, so we got it specified and standardized in 802.11ac to allow everyone to join in the testing/experimental setups.

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