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PA2AGA > TCPDIG 14.09.96 10:45l 143 Lines 5808 Bytes #-10886 (0) @ EU
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Subj: TCP-Group Digest 96/190B
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Date: Fri, 13 Sep 96 22:48:00 MET
Message-Id: <tcp_96_190B>
From: pa2aga
To: tcp_broadcast@pa2aga-10
Subject: TCP-Group Digest 96/190B
X-BBS-Msg-Type: B
As a reality check, note how little it should take to make a voice
channel operate assuming 10 watt transmitters and 10 dB noise figures.
Compare this to what is often experienced in the amateur world.
Hardware which is capable of 60-80 dB in excess of the requirements is
not even always sufficient. Compounding the problem is that most
amateurs' experience with this is also with FM systems where the
phenomenon of quieting further masks the degradation. "Full quieting"
generally refers to C/N > 20 db and the fact that C/N *should be* 80 dB
and is only 25 can be totally missed.
> With coding you should be able to knock the Eb/N0 down to about 5 dB
> even for the high rate code, assuming BPSK or QPSK instead of
> noncoherent FSK. So if your rx temp is 1000K, that's a 20 dB savings.
> That's the difference between 10W and a kilowatt.
But we may often need the difference between 10W and 10 megawatts or
more. I'm not sneezing at the gains available by code coding/modulation
choices but they need to be put into context of the other system
problems.
Glenn
------------------------------
Date: Thu, 12 Sep 1996 22:51:20 -0700 (PDT)
From: Phil Karn <karn@qualcomm.com>
Subject: high speed coders/decoders
I'd like to see some actual link budgets. Or just the path loss
figures, antenna terminal to antenna terminal.
I think RF propagation is not as bleak as you make it out to
be. During our 900 MHz CDMA capacity tests some years ago, we found
the most common mobile transmit power (for 10 kb/s data rate) was in
the 1-10 mW range. This is for typical cellular mobile antennas (the
ubiquitous corkscrew magmount) and for typical mobile/cell ranges of a
few miles. Neither were the base stations all that massive; one was
admittedly on a high tower but most were the typically modest cell
installations you see at low altitude on buildings and phone poles.
Only rarely were paths line of sight. When they were, transmitted
mobile powers could go down to the microwatt range (often being
exceeded by the received power from the cell!)
We see similar numbers at 1.9 GHz.
If you scale those figures up to 230 kb/s, you are still well within
1W.
Yes, things get harder if you try to cover much greater distances
thanks to the empirical r^4 law typically seen in metropolitan non-LOS
paths, but that's actually a blessing. We need lots of densely packed
low power radios talking quietly to their neighbors, not a few
screaming at the top of their lungs to a mountaintop dozens of miles
away. Spectral efficiency goes way up, and multipath is substantially
reduced.
Phil
------------------------------
Date: Thu, 12 Sep 1996 23:06:20 -0700 (PDT)
From: Phil Karn <karn@qualcomm.com>
Subject: high speed coders/decoders
> As a reality check, note how little it should take to make a voice
>channel operate assuming 10 watt transmitters and 10 dB noise figures.
>Compare this to what is often experienced in the amateur world.
>Hardware which is capable of 60-80 dB in excess of the requirements is
>not even always sufficient. Compounding the problem is that most
>amateurs' experience with this is also with FM systems where the
>phenomenon of quieting further masks the degradation. "Full quieting"
>generally refers to C/N > 20 db and the fact that C/N *should be* 80 dB
>and is only 25 can be totally missed.
A C/N ratio of 80 dB in a FSK link would be utterly obscene and
totally unnecessary. If you're trying to keep a margin for fading,
there is a much better way: diversity through space, time or
frequency.
It's a well established fact that on a Rayleigh fading channel without
diversity, bit error rate decreases only linearly with Eb/N0 (SNR).
If you get 10^-3 at 20 dB, then to get 10^-4 you'll have to go to 30
dB. 10^-6 would cost you 50 dB. This is independent of the modulation
method; while binary DPSK is 3 dB better than binary FSK at any given
Eb/N0, for example, both BER curves are parallel straight lines.
But with diversity, the curves go down much more steeply. Any or all
forms of diversity (space, time, frequency) will work as long as the
individual components are statistically independent.
Space diversity usually requires extra antennas and RF hardware, and
frequency diversity generally requires spread spectrum. Only time
diversity can be added to an existing narrowband system with minimal
changes. And the easiest and most effective way to exploit it is with
error correction coding.
The proper choice of coding, interleaving and modulation can easily
buy you 30-40 dB of gain on a Rayleigh fading channel, far more than
the puny 7 dB or so that JPL sweat blood to obtain on the nonfading
Gaussian deep space channel. And it lets you exploit other forms of
diversity (space, frequency) as they become available, e.g., with
adaptive antennas and spread spectrum.
Phil
------------------------------
End of TCP-Group Digest V96 #190
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