[Ground-station] Experimental Channels

[Wally Ritchie]

At Hamcation, G4KLX and others expressed some very valid concerns that
Digital Multiplexed Transponders will not provide the opportunities for
experimentation with analog modes that are available through QO-100's
analog bent-pipe transponder. While we haven't been talking much about this
subject, the DMT can actually provide even greater opportunities for such
analog experimentation while still taking advantage of all of the
advantages of a pure digital design. How exactly can that be?

In parallel with its normal operations, the receiving system of the DMT
will have the ability to tune individual channels located anywhere in the
receiver's full passband(s). This can include signals in the standard
uplink digital channels, or channels in any supported auxiliary receive
band. Such a channel can be tuned, filtered, and converted to digital IQ
baseband. This is essentially the same process used in any SDR based
receiver - ultimately we end up with IQ samples.

Using a standard 4K voice channel as an example, we can filter to 3400Hz
wide, and nyquist sample with 16 bits at 8KHz, and we end up with a
synchronous stream of 32K Bytes per second or 256kbps. Such raw IQ channels
can then be relayed in real time through the standard quasi-error-free DMT
downlink using GSE encapulation of a UDP or RTP stream. If real time isn't
a requirements, the channels can be stored and compressed (with lossless
compression) using a smaller bandwidth and transferred as a compressed
file. In either case, the analog channel is essentially passed through the
satellite, even though digital techniques are involved and there is no
direct analog path in the satellite. The telephone network has been doing
this end to end for the last 50 years - the analog is only at the
end-points (if that).

Using such channels will require allocating an experimental IQ channel
through the DMT's standard protocols available to the most basic of
stations. This will allocate a general purpose tunable IQ channel and the
transport to relay it over the downlink or store it as a file that can be
later retrieved. But thereafter, the experimenting amateur can do just
about anything possible over the QO-100 narrowband bent-pipe transponder -
provided she complies with the rules and does not interfere with other
amateurs.

Such general purpose IQ facilities can also provide familiar waterfall
displays where the power spectral density of a band segment can be
transported over the downlink for a or 2D/3D waterfall display. So in
reality, the functions similar to a websdr can be made available for
experimentation with existing or new analog modes. In practice, the
spectral density display of the uplink will likely be an always available
feature available in the downlink stream.

WU1Y
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