Richard Clark wrote:
On Wed, 23 Feb 2011 09:29:56 -0800, Jim Lux
wrote:

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)

This seems to be straying from what this is about: an antenna system.
You seem to imply that the 10W FPGA is a source of radiation to
confound things. How that arrived, I don't know.

No.. the original article was talking about low power 802.11/802.16
systems (which radiate less than a watt).. At that power level, the fact
that a suitably good linear amplifier is going to consume a fair amount
of power (20% efficiency would be doing well) is insignificant next to
the power consumed by the digital processing necessary to implement the
cancellation algorithm.



The presence of uncontrolled reflecting surfaces in proximity to the
antenna system is the objection, and the introduction of unintended
out-of-phase reflection transmission signals combined with intended
receive signals present at the receiving antenna yields the classic
problem of S+N/N degradation.

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

This does not answer the objection. Reality (conventional usage) will
bring these corrupting out-of-phase signals and the best that software
can offer is a regression of retries to obtain error corrected packets
sorted out a the cost of seriously depressed through-put rates.

No.. I would expect that this would actually work fairly well. The idea
is to allow full duplex operation, rather than the current half duplex
used in, e.g., 802.11b/g. That would double the throughput (if traffic
on the network were symmetric).

If you want to run full duplex, you have to have some way to "see" the
received signal in the face of a much larger transmit signal. Since
you're transmitting, you've got a copy of the transmit signal, so it's
really a matter of figuring out what the transfer function is from
transmitter to (self)receiver. If the external environment were fixed,
then one could probably do it with a coupler with adjustable gain and
phase. However, as you point out, the environment isn't constant, so
you need a way to adaptively adjust. You could do it with a single
transmit and single receive antenna, but that puts a tough dynamic range
requirement on the system. Say, -20dB coupling from Tx to Rx antenna,
+30dBm on transmitter, -100dBm received signal.. you need 110dB
instantaneous dynamic range.. that would need a 18-20 bit converter,
which doesn't exist at a reasonable price in the 10-50 MHz sample rates
needed.

The idea of using a pair of transmitters to reduce the dynamic range
requirement on the receiver is clever. I would think you could get 40dB
suppression without too much trouble, which gets your instantaneous
dynamic range requirement down to 70dB, which is starting to be in range
(there are inexpensive fast 14 bit converters, for instance)




This "invention" is more about being clever than useful and belongs in
a museum case next to the Babel Fish (whose utility is questionable
when you have to listen to Vogon poetry).

73's
Richard Clark, KB7QHC