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Re: DSP software on Sunsat SO-35



Hi Johann,

Some years ago, I tried running two off-the-shelf 14.4kbps (V.32bis)
telephone modems over a full-duplex UHF radio link.  It worked!

Unfortunately the modems were 2-wire only, so we had to build an external
2-wire to 4-wire hybrid in order to interface to the transmit and receive
audio ports of the radios.  The modem's audio was passed through the
standard voice path, including pre-emphasis and deviation limiting on the
transmitter, and through standard 16kHz wide IF filters and de-emphasis at
the receive end.  For various (non-technical) reasons we deliberately
avoided special modifications, or even using flat audio inputs and outputs
on the radio.

The modems were placed in leased-line mode, and as per normal for a V.32bis
modem, they had a fairly long training session (about 1 minute?), after
which they connected at 14400 (top speed back then).

By fading the link (switched attenuators), we could hear the modems
renegotiate up and down in speed.  Full quieting was needed (>-107dBm) to
achieve 14400.  From memory, the link slowed to 4800bps at -112dBm, and
then failed below that.

We never got around to trying it again with the new V.34 modems, but in
discussions with the R&D section of the modem company, it seems we would
have achieved 28800bps if we'd tried (link exhibited an essentially flat
end-end response over 200Hz-3300Hz, with 43dB S/N)

However...  I'm not sure how successful this would be for a LEO
environment, due to:

1.  The rapid fading experienced at low elevations (and the sat is at low
elevations (<10 degrees) for approx 50% of total availability)

2.  Off-the-shelf modems would fail without being able to negotiate over a
full-duplex path (but you could establish a full-duplex connection,
couldn't you?  .. although this requires implementation of a V.34
demodulator as well as modulator, on the spacecraft)

A V.34 broadcast mode at 33.6kbps would be most interesting from a
satellite, especially if a receive-only modem could be derived for the
groundstations.  It is especially interesting for a relatively slow-moving,
high-elevation satellite such as P3D! (although the power budget required
to achieve full quieting is slightly sobering).

The V.34 standard is available from the ITU-T website for approx $30 or so
(should be free, but it's not).

Not sure if this helps at all, but it may be of interest to you.


Regards,


Chris vk6kch




At 18:23 13/02/2000 +0200, Johann Lochner wrote:
>[SNIP]
>constellations would be most welcome.  Also ideas on the suitability 
>of ITU recommendations for telephony modems (such as V.34).  Can 
>these protocols be used for simplex transmissions?  Are there maybe 
>other ways to increase the data rate?
>
>Regards,
>Johann Lochner, ZR1CBC


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