[Ground-station] Experimental Channels

[KC9SGV]

Interesting requirements.
I would imagine that a pure "bent pipe" transponder would require less power consumption than an on-board digitizer and multiplexer add-on.

Bernard,
KC9SGV


Sent from my iPad

> On Feb 12, 2020, at 12:56 PM, Phil Karn via Ground-Station <ground-station at lists.openresearch.institute> wrote:
> 
>> On 2/12/20 09:54, Wally Ritchie via Ground-Station wrote:
>> 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?
> As you know, I really like this idea too. It could even be used for
> *digital* experimentation with different modulation and coding schemes
> not implemented on the spacecraft itself. But it's a rather inefficient
> use of downlink capacity so it should be enabled only when it is
> actually used.
>> 
>> 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.
> 
> 16 bits seems a bit (!) excessive, especially if a user wants a wider
> "transponder" than just 4 KHz. What's a typical uplink SNR, and what SNR
> is actually needed by the user?
> 
> There's also my idea of having some number of uplink channels that can
> demodulate and digitize analog FM, compress it onboard using a decent
> codec (e.g., Opus) and multiplex that into the downlink stream. This
> would not be as flexible as a digital uplink (e.g., the downlink
> metadata could only identify the uplink channel frequency, not the
> callsign of the user) but it might appeal to people who still want to
> keep an analog "feel" in the system, like a squelch tail. Maybe an
> analog FM uplink transmitter on 5 GHz would be easier to construct than
> a digital transmitter, though it would probably require more uplink
> power to close the link.
> 
> The same thing (digitization with Opus before inclusion in the downlink)
> could be done for *any* analog voice mode uplink, including linear SSB
> for anyone still crazy enough to insist on it for that wonderful "analog
> experience". And there probably will be some challenges like frequency
> stability. This would be more economical than a pure uncompressed "bent
> pipe" mode, but it would be an attribute of the virtual transponder
> instance when it is created.
> 
> But as far as the downlink is concerned, we're two decades into the 21st
> century. It should be 100% digital, period.
> 
> Phil
> 
> 
> 
> 
>