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PA2AGA > TCPDIG 19.09.96 04:10l 152 Lines 6442 Bytes #-10882 (0) @ EU
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Message-Id: <tcp_96_191>
From: pa2aga
To: tcp_broadcast@pa2aga-10
Subject: TCP-Group Digest 96/191
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TCP-Group Digest Sat, 14 Sep 96 Volume 96 : Issue 191
Today's Topics:
higher speeds, coding etc..
Send Replies or notes for publication to: <TCP-Group@UCSD.Edu>.
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Problems you can't solve otherwise to brian@ucsd.edu.
Archives of past issues of the TCP-Group Digest are available
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We trust that readers are intelligent enough to realize that all text
herein consists of personal comments and does not represent the official
policies or positions of any party. Your mileage may vary. So there.
----------------------------------------------------------------------
Date: Fri, 13 Sep 1996 07:59:22 -0800 (PDT)
From: Glenn Elmore <glenne@hpsadr2.sr.hp.com>
Subject: higher speeds, coding etc..
Phil responded (in part):
> 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.
>
to a previous post of mine:
> > 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.
>
Phil again:
> 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.
We're agreeing here. It would be (is) utterly obscene but I believe it
is not so unrealistic. Please understand I wasn't saying we *need* that
much C/N but that amateurs can easily lose many 10's of dB and they may
not even know it. For hams, this loss can be greater than what we might
hope to gain with diversity of any kind.
To follow your above request for some link budget numbers, consider a 10
watt 2M nbfm transceiver working a hilltop repeater from 10 miles out:
+40 dBm +10 dB antenna gain gives +50 dBm ERP
Path loss at 10 miles on 2M is ~100 dB
10 dB receive antenna gain gives +50-100=-50 dBm at the receiver.
NBFM has ~20 KHz bandwidth. KTB in 20 KHz is ~-131 dBm.
Throw in typical noise figure and call the noise floor -128 dBm.
That's C/N = -50/-128 = 78 dB.
It's obscene, all right, but the truly amazing thing is, in amateur
practice sometimes there is still not full quieting in a similar
scenario!
> 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.
Again, I am much in favor of good coding choice and modulation
methods. I'm convinced we want to have them to deal with the realities
of the paths amateurs try to use. However, those same realities that
cause distortion and increased BER are also to *large* losses in the
fundamental C/N. The same link sites/paths that bring us multipath also
contribute to an incremental 30-80+ dB of signal loss; this is the part
which makes multipath a significant issue. In almost every real
situation, there is no indirect path which even holds a candle to what
pure LOS can do.
If my above numbers sound bleak to you, also please understand that I
present them after several years of working at both microwave and UHF
with other amateurs and real environments and moderate speed, wider RF
bandwidth radio networks. A well designed system, with suitably short
paths (small cell size), engineered sites and approriately chosen h/w,
coding and modulation would be *vastly* different. Unfortunately this
is not the current status quo in the amateur world. I don't know that I
believe it will ever be. If we are all to have a radio network of any
capability we have to deal with these realities.
Glenn n6gn
------------------------------
End of TCP-Group Digest V96 #191
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