Richard Clark wrote:
On Tue, 22 Feb 2011 09:54:53 -0800, Jim Lux
wrote:

Here we have three (3) antennas, and as we all know they are not in
isolation.

Somewhere, there's a nearby (or near enough) overlooked reflective
surface that disrupts that oh-so-absolutely-necessary symmetry.

All practical systems like this use some form of adaptive logic to fix
that. Usually, adaptive canceling is done in the receiver, because the
signal levels are lower, but in the 802.11 kind of world, with 100mW
linear transmitters, there's probably not much cost difference.

A different matter if you're running a kilowatt.

It only takes a couple of milliWatts (kiloWatts aside) to ruin your
day in competition for listening to microWatt signals. The desired
signal's transmitter antenna would have to be literally within the
near field of the active transmitter (and receiver's) antenna system.
At that point, we may as well use a land-line with hybrid bridges.

Software coming to the rescue for a hardware problem works only in
multi-million dollar projects (fly-by-wire avionics comes to mind).


In these sorts of systems (the ones alluded to in the original news
story), all the signals are in one FPGA (in digital form) and the powers
are fairly low so all the RF stuff runs basically linear (DC to RF
efficiency isn't a huge deal on a 50mW transmitter next to a 10 Watt FPGA)

So it *is* a wireline hybrid bridge.. but done with numbers instead of
transformer windings.

The point I was getting at is that in these MIMO systems, there's
already multiple receive and transmit channels with substantial signal
processing going on. So it doesn't really matter much whether you do the
cancellation/null forming in the Tx or Rx side. If you can do some
clever canceling with Tx, and make it possible to use a cheaper Rx (or,
run full duplex without needing huge dynamic range/linearity in the Rx)
then that's probably a net good.

The technique proposed is very, very similar to one used to create
increased stereo separation ("headphone sound") from conventional stereo
speakers. You send a part of the Left channel signal to the right
speaker that just cancels the signal arriving at the right ear from the
left speaker. It's a very, very impressive effect.



73's
Richard Clark, KB7QHC