[Ground-station] Experimental Channels

[Paul Williamson]

Allow me to expound on a few of Wally's points:

> Using such channels will require allocating an experimental IQ channel
> through the DMT's standard protocols available to the most basic of
> stations.

Yes. One of the key benefits of the all-digital DMT design is that uplink
users are subject to automatic authentication (proof of identity) and
authorization (permission to use the system) before they are permitted to
communicate through the system. Any pseudo-analog IQ relay channel would
have to be set up on demand by request of an authenticated, authorized
user, for a limited period of time. The requesting user would then become
responsible for the channel, sort of like the control operator on a
terrestrial repeater. In particular, the responsible user would be expected
to monitor the content of the downlink channel created, and shut it down if
abuse is detected.

That scheme means that Phil's proposal to accommodate FM uplinks for users
so inclined is much less interesting than it might first appear. The
hypothetical FM users would have to also operate a digital uplink to set
the channel up. It wouldn't replicate the experience of a plain analog FM
transponder. Of course, even without the auth+auth scheme, there's no way a
microwave GEO can replicate the key advantage of FM LEOs: the use of cheap
off-the-shelf VHF/UHF FM transceivers and low-gain hand-waved antennas. The
users already have to obtain specialized equipment to operate the DMT, and
we are already planning to do our best to cost-optimize it; they might as
well use it in the intended all-digital way and get all the advantages
designed into the system.

The experimental IQ relay channels are not about satisfying users who think
they want an old-fashioned "analog experience". It's about experimenting
with alternative uplink waveforms that the payload is not programmed to
demodulate and multiplex onto the downlink. Presumably the goal would be to
eventually implement the waveform in the payload, eliminating the gross
inefficiency of downlinking raw IQ data. With a reprogrammable payload,
that might be possible on the same mission. Depending on the details, it
might only be possible in a follow-on mission with more powerful hardware
on board.

> 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.

They could, but that's not the right way to do it. A waterfall display
needs only a tiny fraction of the bit rate an IQ relay would require for a
given bandwidth. The payload should do the work and the rate limiting.

> In practice, the spectral density display of the uplink will likely be
> an always available feature available in the downlink stream.

I'm not so sure about that. Someone with more of the link budget figures at
their fingertips will have to check me on this, but I'm guessing that
normal uplink signals at appropriate power levels should be pretty hard to
see on a waterfall. A full-bandwidth waterfall would only be useful for
diagnosing loud interfering signals. Such a tool would not need to be
always on. It might even encourage miscreants to transmit loud signals just
to show up. The payload managers would need access to a waterfall
occasionally, but it might be counterproductive to train users to watch it
routinely.

  -Paul KB5MU
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