On Tue, 16 Sep 2014, David Lang wrote:
On Tue, 16 Sep 2014, Dave Taht wrote:
It would be very nice to get some TXOPs back:
Is this crazy or not?
http://web.stanford.edu/~skatti/pubs/sigcomm13-fullduplex.pdf
I start of _extremely_ skeptical of the idea. While it would be a
revolutionary improvement if it can work, there are some very basic points of
physics that make this very hard to achieve.
If they can do it, they double the capacity of existing wireless systems,
which helps, but it's not really that much (the multipath directed
beamforming helps more)
I'll read though the paper and comment more later.
Ok, they are working on exacty the problem I described. They do a significant
amount of the work in digital, which is probably why they get an 87% improvement
instead of a 2x improvement. This also will eat a fair bit of the DSP processing
capacity.
As they note, this only works with single antenna systems. They list support for
multi-antenna systems as future work, and that's going to be quite a bit of work
(not impossible, but very hard)
This will be a great thing for point-to-point infrastructure type links, but
isn't that useful for more 'normal' situations (let alone high density
environments)
MIMO multi-destination can provide as much or more airtime saving when you
actually have multiple places to send the data
think of it as the core frequency vs core count type of tradeoff.
David Lang
warning, radio primer below
the strength of a radio signal drops off FAST ( distance^3 in the worst case,
but close to distance^2 if you have pretty good antennas)
you loose a lot of signal in the transition from the antenna wire to the air
and from the air to the antenna wire.
The result of this is that your inbound signal is incredibly tiny compared to
your outbound signal.
In practice, this is dealt with by putting a very high power amplifier on the
inbound signal to make it large enough for our electronics to deal with. to
do this effectively for signals that vary wildly in strength, this amplifier
is variable, and amplifies all the signals that it gets until the strongest
one is at the limits of the amplifier's output.
Because of this, a receiver without a good input filter can get into a
situation where it cannot recive it's desired signal because some other
signal somewhat near the signal it wants is strong enough to cause problems.
digital signal processing is no help here. If you digitize the signal (let's
talk 8 bits for the moment, although 12-14 bits is more common in the real
world), and you have one signal that's 100 times as strong as the other
(which could be that one is 10 ft away and the other 100 ft away), the near
signal is producing samples of 0-255, while the far signal is producing
samples 0-2. there's not much you can do to get good fidelity when your only
hvae 3 possible values for your data.
Real radios deal with this by having analog filters to cut out the strong
signal so that they can amplify the weak signal more before it hits the
digital section.
But if we are trying to transmit and receive at the same time, on the same
channel, then we are back to the problem of the transmit vs receive power.
Taking a sample radio, the Baofeng uv-5r handheld (because I happen to have
it's stats handy)
on transmit, it is producing 5w into a 50ohm load, or ~15v (v=sqrt(P*R)),
while it is setup to receive signals of 0.2u volt.
being able to cancel the transmitting signal perfectly enough to be able to
transmit and at the same time receive a weak signal on a nearby frequency
with the same antenna is a HARD thing to do, and the tools to do so tend to
be very finicky (read temperature sensitive)
David Lang
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