The part of the satellite being "software-defined" is probably the phased-array antennas of the transponder/payload. I think it is unlikely that all the GHz RF processing is realized as SDR. Maybe someone can dig up more information?
That makes it sounds like any other telco-satellite with beamforming.
Edit: More information from [1]
> Guilleux said the first Quantum satellite will have eight downlink beams that can range from a minimum diameter of 600 kilometers to a maximum of one-third of the Earth’s surface (the larger the beam, the more dispersed the signal power), and eight independent uplink beams. Customers can split any of those eight beams into smaller sub-beams and follow assets such as ships and planes, he said. O’Connor said the satellite will have a total of 3.5 GHz of capacity, from which any individual downlink beam could support up to 1 GHz. The first Eutelsat Quantum will function just in Ku-band, but Guilleux said future iterations could support other frequencies, including both military and civilian Ka-band.
[0] - https://www.esa.int/Our_Activities/Telecommunications_Integr...
[1] - https://spacenews.com/eutelsat-adding-two-more-quantum-satel...
My impression is that this is more of a tech demonstration satellite (to test the new payload, resource management software, etc) rather than an actual commercial satellite, as the total throughput seems to be much lower than state-of-the-art spacecraft from other companies.
[1] https://spacenews.com/eutelsat-adding-two-more-quantum-satel...
I think this was answered by your link: "Quantum was made possible by the British government, which is funding 90 percent of ESA’s commitment of 71 million euros in the Quantum program"
It's the government wasting a tremendous amount of money.
If your software/processor/bus can’t keep up with that data rate, you can always go lower in hardware and still leave enough headroom for software to tune and process.
I haven’t looked into SDR much though it’s in my “someday” list, so I don’t know how much is typically expected to be handled in software. I suppose it varies depending on the spectrum of interest.
My last project, the GHz spectrum was downcoverted in analog to the hundreds of MHz range, and digitally downconverted further with low pass filters and frequency shifting and then piping out over pcie to a ring buffer for software to do the interesting signal processing.
Xilinx has an RF Zynq FPGA in the works, or maybe available now, that has the analog front end conversion capabilities paired with an fpga with an embedded arm[1]. (No affiliation)
1. https://www.xilinx.com/products/technology/rfsampling.html
Btw, you cannot use standard fpgas in space. They are a special kind hardened for radiation and redundancy.
https://www.xilinx.com/applications/aerospace-and-defense/sp...