[Cerowrt-devel] full duplex wifi?
david at lang.hm
Fri Sep 19 01:55:58 EDT 2014
You don't really know what you are transmitting unless you attenuate your
transmitter output, and even then, the attenuator isn't going to be completely
linear, so you have more of a close approsimation than precise information.
The problem of also seeing reflections of your transmission can be especially
bad (and was not covered in the paper). If you have your AP sitting a couple
feet from a mirror (or a mostly solid chunk of metal, like a cubical partition),
you will get a strong reflection from that back into your AP, and this can
overwelm more distant stations (this cannot be solved in the digital relm
due to the problems I described earlier, and can only be solved in the RF relm
by avoiding the problem with directional antennas)
Cutting down the power will help this sort of thing significantly, but until you
cut the power down to _really_ low levels, not enough.
I can see cases where this would help, but I don't see this as something that
will help in the general case.
On Thu, 18 Sep 2014, dpreed at reed.com wrote:
> This is not completely crazy. A couple of grad students and I demonstrated
> this type of thing with USRP's in my lab at MIT. The problem you, David Lang,
> refer to is basically the key thing to deal with, but the physics and
> information theory issues can be dealt with.
> There's significant work in the RADAR (not radio) field that bears on the
> design of this. I am sure there is more of that that is currently classified.
> There are a lot of practical design issues in the front-end and the waveform
> design to be able to do this sort of thing well - especially in the field
> rather than the lab. Your receive antenna will receive echoes of your own
> transmission that have to be separated from your signal and the source you are
> listening to.
> Since this is full-duplex, there are only two signals involved and each knows
> its own signal's waveform pretty precisely - you can even attenuate the
> antenna output to get a precise measure of your signal.
> So I think in a few years this might be practical - but a protocol to exploit
> this capability optimally would be complicated because of the need to
> compensate for the propagation environment effects.
> On Tuesday, September 16, 2014 11:08pm, "David Lang" <david at lang.hm> said:
>> 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
>> 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
>> 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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