

Stanford Wireless Breakthrough - thewonggei
http://drdobbs.com/mobility/229218939?cid=nl_ddjupdate_2011-03-01_html

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coolgeek
"Levis said a researcher even told the students their idea was "so simple and
effective, it won't work," because something that obvious must have already
been tried unsuccessfully."

That's going up on the wall above my monitor.

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dublinclontarf
Yeah, ask the Romans why they didn't invent the hot air baloon.

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nimrody
Some more information here:

<http://sing.stanford.edu/fullduplex/>

The problem with full duplex wireless comm. on the same frequency is that the
transmitted signal is much stronger than the received signal. Therefore, echo
cancelling is usually impractical.

Not sure how these guys overcame the difficulties with transmitter
nonlinearities and other unknown impairments.

EDIT: they claim to do partial cancellation by placing the receiver antenna at
the null of the two transmitting antennas (getting 50 dB reduction in echo
power). Whether this is practical in a commercial low cost radio remains to be
seen.

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VladRussian
it may be practical in the office setup where 6+ antenna phased arrays Wi-Fi
switches are "cost-practical". With phased array you have much more
geometrical flexibility - you may "narrow beam" transmission to a client while
better adjusting the transmitter's null onto the receiver's physical location
to the current conditions, reflections, etc...

~~~
reemrevnivek
No, I think that the parent post was referring to the transmitters on the
radio device. The transmitting and receiving antennas are different, and they
place the receiving antenna in a null of the transmitters.

This is easy and practical because they're in a location which is fixed by the
PCB/housing. There's no phased array required.

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zwieback
Sounds very interesting but I wonder if the null of the transmitting antennas
is stable in the presence of multipath reflections.

This scheme also works only if all nodes have full-duplex setups, which is
impractical for small devices.

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lutorm
No, reflections can have any phase, so you can't possibly cancel them out with
a fixed geometry. But given the 1/r^2 falloff, unless the reflections are from
very nearby, they will be a lot weaker than the direct path.

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JoeAltmaier
The claim that this doubles the amount of information you can send is
nonsense. It simply allows you to send and receive simultaneously.

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regularfry
...which, assuming you're sending and receiving equal amounts, doubles the
amount you can send in a given time period.

<http://en.wikipedia.org/wiki/Duplex_(telecommunications)>

~~~
JoeAltmaier
Don't be trite. Receiving is completely different from sending. "You" doesn't
include the other guy, in my dictionary.

It may raise the amount that can be sent over the network in toto, assuming
that data demand is the same in all directions. Which it almost never is.

So what they've done is, they allow the data uplink to occur simultaneously
with the downlink.

A valid claim is, the latency of data is improved - you don't have to
wait/schedule your sends around the receive traffic. Which can matter quite a
bit in the bulk of cases e.g. downloading http while jabbing buttons/sending
commands up to the server.

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rmrm
it's fairly standard terminology...you've doubled the amount of data that can
be sent over a given chunk of spectrum per unit time, without changing the
modulation/encoding scheme (bits/hz). A normal full duplex FDD system takes
twice the bandwidth to pass the same amount of information as this
theoretically can.

Rather than think about bursty, asymmetric Ethernet bits, consider passing
something like a fully allocated T1 over it. Symmetric TDM links aren't
_totally_ dead, there are a lot of T1 radios out there. What used to take X Hz
of bandwidth, theoretically can be done in X/2...without changing the
modulation.

I don't think it will actually be used that way, but it is a breakthrough of
sorts to be able to do so.

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lutorm
I almost stopped reading after the first sentence: _Radio traffic can flow in
only one direction at a time on a specific frequency._ Right...

To be fair, the article improved after that.

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johngunderman
It was just poorly phrased. by "direction" they meant "from receiver to
transmitter" or "from transmitter to receiver".

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mikeknoop
Not to belittle their announcement, but what textbook says you can't filter a
signal? Or is the novelty in the degree to which they had to filter the
outgoing signal?

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signa11
> ... but what textbook says you can't filter a signal ...

i don't think any textbooks makes that claim. from the article: ...a
researcher even told the students their idea was "so simple and effective, it
won't work," because something that obvious must have already been tried
unsuccessfully.

~~~
mikeknoop
I can certainly understand that. My confusion arose from the quote:

> "Textbooks say you can't do it," said Philip Levis, assistant professor of
> computer science and of electrical engineering.

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powertower
Can't each transmitter/radio add some type of a 2-nd order frequency onto the
transmitting signal (imagine a sin-wave, except it's line is not smooth, the
line has its own "frequency") and detect that to filter the incoming signal?

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VladRussian
what you proposing is equivalent to increasing the frequency of the channel.
If both do the same "2-nd" order frequency - we're back to the same
"1-frequency" channel problem. If different - that's outside of the
"1-frequency" channel problem.

~~~
powertower
Each radio would have a unique 2-nd order frequency for it's own filtering
use, as a "tag" of it's signal. The primary frequency is known and used for
the transmition, and the 2-nd order one is piggybacked on it. I'm not saying
there are 2 transmiting frequencies.

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thisrod
> I'm not saying there are 2 transmiting frequencies.

200 years ago, people might have believed you. Then Fourier showed that your
statements contradict each other. He said a lot of other interesting things -
Bracewell's book is a good introduction.

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amitraman1
Sweet...2Gbps wireless bandwidth!

