[Ground-station] Experimental Channels

[Mike Parker]

That’s somewhat like what we are planning to do on the CatSat HF experiment.  The entire HF band will be digitized.  Then in the on-board FPGA a number of narrowband tuners will be synthesized.  Analogous to a bank of USB radios, these will be tuned to WSPR, FT-8, etc. frequencies on 40, 20, 17, 15, 12, and 10 meters.  The result will be digitally multiplexed and stored.
Then the data stream will be played back at a higher rate, modulated onto a single 3 cm (10 GHz)  carrier using DVB-S2. and transmitted to ground stations.  There the multiplexed signal will be broken back into audio channels several kHz wide.
Since CatSat will be in a LEO orbit, it will be up to the user to apply inverse Doppler to correct for frequency shift between the station being copied and the satellite.  It is also likely that frequency shifts caused by the ionosphere will have to be compensated for.  Then the desired signal can be demodulated using WSJT-X software.  Of course, just as on HF today, it is likely that many signals will be present within the several kHz of bandwidth.
For those who don’t want to experiment with building a 3 cm station that must point a dish at the LEO, we plan to put the demultiplexed audio signals on the internet.
 
If one had multiple antenna elements on their spacecraft, one might put a receive channel on each separate element.  With all the channels tuned to the same frequency, the downlinked multiplexed data stream could then be combined on the ground to form steerable beams and nulls.  Each user could have their very own phased array in space.  How cool would that be!
 
So yes, I think that I,Q data streams still have a place.
73,
Mike Parker, KT7D
P.S. More information about CatSat was presented at the AMSAT annual meeting last fall.  I understand that the paper will be republished in a future AMSAT Journal.  The University of Arizona team is currently running as fast as we can towards a launch early next year.
 
 

> On Feb 12, 2020, at 10:55 AM, Wally Ritchie via Ground-Station <ground-station at lists.openresearch.institute> 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?
> 
> 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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