On Fri, 20 Mar 2009 19:46:53 -0700, "Joel Koltner"
wrote:

Say I have an antenna that I know happens to provide an SNR
of 60dB...

Returning to one of the few quantifiables, it would be instructive to
judge why it is so astonishing as a point to begin a dive into the
discussion of noise figure. In other posts related to deep space
probe's abilities to recover data from beneath the noise floor, much
less cell phones to operate in a sea of congestion, I encountered the
economic objection that such methods cost too much - expense of
bandwidth.

Well, not having seen anything more than yet another qualification -
how much is "too much?" It is time to draw back and ask how much is
enough? What would NOT be too expensive? Replacing qualitative
objections with quantitative objections sometimes evokes a horse laugh
when the magnitude of the qualitative issue ceases to exhibit much
quality.

I won't open this round of enquiry with exotic Spread Spectrum which
portends the objection of phase issues with clocks (even knowing that
such modulation techniques automatically incorporate slipping to
adjust for just such problems). Instead I will slip back some 60
years to the seminal paper published by Claude Shannon who figured
this all out (with H.W. Bode) and quote some metrics for various
coding (modulation) schemes. Search for "Communication in the
Presence of Noise." When you google, search in its image data space
for the cogent chart that I will be drawing on, below. Obtaining the
paper may take more effort (or simply email me for a copy).

Starting with BPSK and a S+N/N of roughly 10.5 dB, the bit error rate
is one bad bit in one million bits. This is probably the most
plug-ordinary form of data communication coming down the pike; so one
has to ask:
"is this good enough?"
If not, then "SNR of 60dB" is going to have to demand some really
astonishing expectations to push system designers to ante up the
additional 49.5 dB.

Well, let's say those astonishing expectations are as wild as
demanding proof that you won't contribute to global warming if you
chip an ice cube off of a glacier - such are the issues of scale when
you chug the numbers for BPSK.

OK, so as to not melt down the planet, we step up the complexity of
modulation to better than the last solution for "good enough." Let's
take the Voyager probes of the deep planets where at a S+N/N of 2.53
dB (in what is called 8 dB coding gain) the same error rate of 1 bit
in 1 million is achieved. One has to ask:
"is this good enough?"
If not, then "SNR of 60dB" is going to have to demand some really
astronomical expectations.

OK, perhaps this is a problem demanding really deep pockets that
exceed the several $Trillion being spent on the past 8 years of
Reaganomic neglect. (Why else pound the desk for that extra 57 dB?)
Let's go the full distance to the Shannon limit. It will give us that
same 1 bit error for every 1,000,000 at -1.5 dB S+N/N. If this isn't
below the noise floor, then the problem demanding 60 dB will never
find the solution to positively answer:
"is this good enough?"

73's
Richard Clark, KB7QHC