Re: garage remote jamming

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

Dave Houston, Jeff Volp and Robert Green wrote:

JV>>Spread-spectrum not as complex as you may think.  It does not involve
JV> >multiple transmitters running on different frequencies.  The
pseudo-random
JV> >digital pattern is used to modulate a single transmitter, causing its
output
JV> >to look a lot like noise.  The receiver has a copy of the pseudo-random
JV> >pattern, and uses that as an input to the correlator.  When the
patterns
JV> >exactly line up, the correlator produces a valid output.  All you get
is
JV> >noise if just one bit off.  As I recall, the P-code bit rate is
10.23MHz, so
JV> >each bit is only 98nS.  Data is modulo-2 added to the pseudo-random
code,
JV> >and the result modulates the transmitter.  So, when the codes are
aligned at
JV> >the receiver, the data pattern comes out the correlator.

There are a number of spread spectrum techniques.  The one I am familiar
with:

http://en.wikipedia.org/wiki/Frequency_hopping

clearly switches between frequencies.  They go on to say that "Adaptive
Frequency-hopping spread spectrum (AFH) (as used in Bluetooth) improves
resistance to radio frequency interference by avoiding using crowded
frequencies in the hopping sequence."  That's a question I was asking
earlier - does Lutron "switch around" channels when it determines they have
high levels of interference?

I was trying to find out some more information about the subject and came
across the FCC rules about frequency hopping:

http://www.fcc.gov/oet/info/rules/part15/part15-2-16-06.pdf

and reading through that makes it clear that in spread spectrum
communications, more bandwidth is required.  I assume that means multiple
frequencies, although, as you point out, not multiple transmitters.

The most interesting tidbit I found at Wikipedia was this:

"The most celebrated invention of frequency hopping was that of actress Hedy
Lamarr and composer George Antheil, who in 1942 received patent number
2,292,387 for their "Secret Communications System." This early version of
frequency hopping used a piano-roll to change between 88 frequencies, and
was intended to make radio-guided torpedoes harder for enemies to detect or
to jam."

DH> Philips uses a simpler method with their RF-capable Prontos. They
modulate
DH> the 418MHz carrier with both the bit pattern and with ~36kHz. The
receivers
DH> use an Atmel U2538B chip to demodulate the 36kHz after the RF receiver
DH> demodulates the 418MHz. It's similar to how Consumer IR distinguishes a
DH> signal from ambient IR. But I suspect it, as well as the method you
DH> describe, could be jammed by a sufficiently strong RF signal just as
DH> Consumer IR fails in a strong ambient IR field (e.g. fluorescent
lights).

Which gets us back to the original question.  Can RadioRA be jammed with the
same sort of signal that is affecting garage and car door openers near Elgin
AFB and Quantico?  Its clear spread spectrum techniques may help make
communications more immune to the standard "noise soup" in the modern RF
environment. That could simply be a result of the FCC allowing SS
transceivers to operate at higher powers than single frequency devices.

Spread-spectrum clocking distributes the energy so that it falls into a
large number of the receiver's frequency bands, without putting enough
energy into any one band to exceed the statutory limits, at least according
to Wiki. But there's no telling whether the door openers in question had
such enhancements and still failed or whether Lutron RA will fail if
operating in the same geographical area.

An interesting comment at:

http://en.wikipedia.org/wiki/Spread_spectrum

says:

"The usefulness of spread spectrum clocking as a method of actually reducing
interference is often debated, but it is probable that some electronic
equipment with sensitivity to a narrowband of frequencies will experience
less interference, while other equipment with broadband sensitivity will
experience more interference."

DH > AFAIK RadioRA doesn't do anything out of the ordinary. At least, they
don't
DH> mention it in their literature or FAQs. I believe their repeaters repeat
in
DH> (near) real-time (as opposed to the sequential repeaters used by
Z-Wave).

There's a good selling point - "we're *much* faster than Z-wave!"  That's
like MS's advertising of XP: "much more reliable than previous versions!"

DH> The slight smearing this causes would probably preclude using the method
you
DH> describe.

Part of the reason RA may(?) work so well is that they are very conservative
about the radio range, saying that the units work for 30' in any direction a
worst case scenario with lots of metal in the walls.  If your transmitters
and receivers are close, they are more immune to noise.  But those low
limits mean (according to them) a 10,000 sq. ft. home will require the
maximum of 4 repeaters.   Even so, there's still a maximum of 32 loads and
that's just not enough for me so it doesn't really matter to me if it works
during an EMP blast from an airburst neutron bomb.  LutronRA comes up short
in other places, at least for me, no matter how reliable it's claimed to be.

--
Bobby G.