Re: [Cerowrt-devel] full duplex wifi?

[David Lang <david@lang.hm>]

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.


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