
WiFi: “beamforming” only begins to describe it (2014) - Tomte
https://apenwarr.ca/log/20140801
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willis936
Interesting read. I’m already familiar with these concepts, so I’m not really
the target audience. I think mentioning (and showing) some antenna radiation
patterns would help the viewer. The transceivers are not point sources. Their
3D orientation matters quite a lot, especially when multipath is a dominant
factor (ie inside).

Also don’t be fooled by the last sentence. There is no way to cheat Shannon’s
limit. These are all just tricks to try to get closer to Shannon’s limit.
TANSTAAFL. You need lots of transistors to implement these complicated tricks
and you are always limited by the channel (the signal from transmitter to
receiver, including all antenna, multipath, noise, and circuitry effects).

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

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ampdepolymerase
You are still not cheating entropy with that.

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

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derefr
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.)

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Filligree
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.

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jrockway
Previously:
[https://news.ycombinator.com/item?id=8313470](https://news.ycombinator.com/item?id=8313470)

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madengr
This is a good intro on MIMO. The spatial multiplexing was first used in the
50’s in microwave towers using quadrature splitters:

[https://www.microwaves101.com/encyclopedias/mimo-an-
historic...](https://www.microwaves101.com/encyclopedias/mimo-an-historical-
tutorial)

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shaklee3
Does Wi-Fi use the same frequency for a transmitter and receiver? In general,
this statement is not true:

"Relatedly, even if there is no explicit beamforming feedback, in theory you
can calculate the phase differences by listening to the signals from the
remote end on each of your router's antennas. Because the signals should be
following exactly the same path in both directions, you can guess what phase
difference your signal arrived with by seeing which difference his signal came
back with, and compensate accordingly."

But maybe the frequencies are close enough such that the channel matches.

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hinkley
I think it helps to think about the problem from a multiparty standpoint. On
wired ethernet the simplest case is one wire going between two machines. In
coax everyone shares one channel and so RX and TX have to interlace instead of
overlap. You go to twisted pair and it’s unambiguous that full duplex is
available and desirable.

But, now you cant just wire three machines together, because everyone’s TX has
to connect to your RX and vice versa and how would that work? How about with
fifty machines. You’d need some sort of star arrangement, or a little box in
the middle to juggle every outbound to every inbound.

With wireless there is no box in the middle (except, very loosely, in two-tier
mesh networks where infrastructure nodes talk on two channels and leaf nodes
on one), so it’s really like wireless coax with the occasional router. If you
want more bandwidth you could run several channels everywhere, or segment
fewer machines into separate channels and connect them with more routers.

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madengr
You actually can overlap RX and TX bandwidth on coax, though you need magic-
tees and very good impedance matching in either end, and of course constant
impedance cable. That limits it to about 100 Mbps, but you can buy the
(magnetic) magic tees to do it. These are specifically for replacing analog
video cameras with IP over the existing coax.

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NelsonMinar
Is there any data on real-world performance improvement from beamforming with
consumer gear? I know anecdotally 802.11ac is way better than 802.11n (much
less .11g) but there's so many related improvements in it that I don't
understand how helpful the beamforming part is.

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namibj
In our hackerspace, using 3 antennas in an ubiquity "UFO" danling in front of
a shelf, communicating 2~3m to a thinkpad with afaik 2 antennas, using an
80MHz 5GHz 802.11ac configuration, we get consistently >500Mbit/s (iirc
hovering at roughly 600Mbit/s) (unidirectional) to the NAS (1000BASE-T wiring,
1-2 switches in-between) when mounted via NFS. A single stream can't get more
than ~420Mbit/s _in the lab_.

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autonoshitbox
This is the worst way to learn RF physics.

