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PA2AGA > TCPDIG 19.09.96 04:37l 163 Lines 6304 Bytes #-10881 (0) @ EU
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Subj: TCP-Group Digest 96/193A
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From: pa2aga
To: tcp_broadcast@pa2aga-10
Subject: TCP-Group Digest 96/193A
X-BBS-Msg-Type: B
TCP-Group Digest Tue, 17 Sep 96 Volume 96 : Issue 193
Today's Topics:
Help: JNOS & BAYCOM
higher speeds, coding etc.. (3 msgs)
How to use a baycom modem with TNOS/Linux? (3 msgs)
Networking and People (2 msgs)
suggestion
wampes hpux.c compile
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----------------------------------------------------------------------
Date: Mon, 16 Sep 96 11:54:25 EDT
From: yv4fnc@yv5nsf.ampr.org
Subject: Help: JNOS & BAYCOM
Hi. A friend of mine is willing to run JNOS soft and BAYCOM modem. He know
the program, config files, but he's failed to get start it. Can anyone here
give enough info about this stuff..? Thanks in advance!
Jos (YV4FNC).
------------------------------
Date: Mon, 16 Sep 1996 02:34:29 -0700 (PDT)
From: Phil Karn <karn@unix.ka9q.ampr.org>
Subject: higher speeds, coding etc..
>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!
That's because you assumed free-space propagation loss. Real
terrestrial links over this distance are very seldom line-of-sight.
Path losses are more like 1/(r^4) than inverse square.
Nevertheless, much of the fading (chopping) that results could be very
effectively mitigated with the proper coding, interleaving, modulation
and power control. Our own good experiences at far lower power levels
on 900 MHz and 1.9 Ghz with CDMA demonstrate this quite well. If it
works up there, it ought to be a cinch at 2m.
Phil
------------------------------
Date: Mon, 16 Sep 1996 07:07:18 -0800 (PDT)
From: Glenn Elmore <glenne@hpsadr2.sr.hp.com>
Subject: higher speeds, coding etc..
Phil responded >
> >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!
>
> That's because you assumed free-space propagation loss. Real
> terrestrial links over this distance are very seldom line-of-sight.
> Path losses are more like 1/(r^4) than inverse square.
which is my point exactly. THe obscene part is actually that amateurs
manage to *lose* so much of the potential system capacity. However, for
higher speed data we can't afford this.
> Nevertheless, much of the fading (chopping) that results could be very
> effectively mitigated with the proper coding, interleaving, modulation
> and power control. Our own good experiences at far lower power levels
> on 900 MHz and 1.9 Ghz with CDMA demonstrate this quite well. If it
> works up there, it ought to be a cinch at 2m.
>
No doubt, but your experiences were with a carefully constructed
system where one end of the path was in a good location. You also only
needed enough performance to get an audio channel or two through. Also,
2M is much more forgiving then 2.4 or 10 GHz when one doesn't have quite
LOS.
For ham radio, the combination of the requirements for higher data
rates, poorer locations on *both* ends of the amateur link and
propagation in the microwave bands (to get spectrum enough to make it
happen) cause the *amateur* experience to be vastly worse. This is my
point.
I'll be spending a few minutes talking at DCC next weekend and I'll try
to bring some supporting slides and numbers to help further clarify.
I hope you'll be able to make it too.
Glenn
------------------------------
Date: Mon, 16 Sep 1996 17:02:28 -0700 (PDT)
From: Phil Karn <karn@qualcomm.com>
Subject: higher speeds, coding etc..
> No doubt, but your experiences were with a carefully constructed
>system where one end of the path was in a good location. You also only
>needed enough performance to get an audio channel or two through. Also,
>2M is much more forgiving then 2.4 or 10 GHz when one doesn't have quite
>LOS.
But this was my point!! Most of our cells were at low altitude, and
only rarely was there a LOS path. And the really interesting thing is
that the faster mobile flutter at high frequencies is actually
*easier* to handle when you put a bound on delay. That's because the
interleaver span need only cover some typical number of fades, and
that's less time at higher frequencies and higher vehicle speeds.
It is probably not exaggerating to say that interleaving (with coding)
is what has finally made mobile communications practical at microwave
frequencies.
At low speeds, the automatic power control system kicked in and took
out most of the fades. In the middle (around 20-25 mph on 900 MHz)
there was a slight "bump" in the error rate curve as the power control
started to fall behind and before the interleaver could take over.
To be continued in digest: tcp_96_193B
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