El 14/03/18 a las 20:40, Daniel Cussen escribió:
I would hesitate to recommend this system, as it is vapour-ware at the moment, and all the previous kits/hardware released have been dropped and made obsolete by changes to the broadcast. Previous broadcasts were 12Ghz geostationary, but decoded using a DVB-S tuner demodulator, then they changed to L-band low earth orbit immarsat and now they are back on geostationary, this time with a new modulation scheme making pointing easier. They are using the "LoRa" standard, basically just using a protocol that allows low signal margin decoding ( Chirp Spread Spectrum modulation (CSS) which trades data rate for sensitivity within a fixed channel bandwidth. ), similar to WSJT/PSK31 and other low data rate weak signal modes.
It is interesting they thing that a bare LNB (about 80 degree beam width) that seems to be doing the trick, although they also show pictures of patch antennas which presumably are designed for narrower beam width more suited to this.
I also find it a bit hard to believe that the current Outernet goal can be made to work: a 30kbps stream from a GEO Ku-band satellite that can be received with a bare LNB or small patch antenna. This is not necessarily impossible if you run the numbers, but its feasibility is pretty borderline. I'll believe it when I see it working.
As I understand, one of the main issues they're having is co-channel interference. This goes as follows: if you look at link budget alone (free space path loss, the gain of an LNB and so on), maybe things can look OK. However, in the real world what happens is that your LNB has a wide beam, so you receive the signals from over a dozen different GEO satellites. The signal you want to receive is now interfered by DVB-S transponders (or other signals) from many different satellites and now you have a problem (as before the main contribution of noise was the LNB noise figure, and now you notice that the noise floor is much higher due to interference).
This is not a problem when using a dish, since the beamwidth is rather narrow and your dish only sees a few satellites at a time, so interference is unlikely. But when you look at many satellites instead, the spectrum is extremely crowded.
The fun thing about this story is that they claim that the are experimenting with LoRA to fight co-channel interference (since LoRA is spread spectrum). In my last talk about Outernet I commented that this is nonsense and that they don't understand properly how spread spectrum works.
If you think about it, spread spectrum (in comparison to a narrowband signal) works very well against narrowband interference, but it doesn't make any difference against wideband interference. In this case, the co-channel interference is DVB-S and other wideband signals. For all practical effects, they just look as an elevated noise floor and there is no way to fight against them, spread spectrum or not.
Since the Outernet topic has come to this mailing list once again, I take the liberty to remind you that while Outernet can be interesting from the technical point of view, they have always kept secret their modulation, coding, protocols and so on, and the key parts of the receiver are closed-source. This is no good for Amateur Radio and experimentation in general.
Thanks to my work and the help of some other people, now there is a fully open-source receiver for the (now defunct) L-band signal, as well as public specifications for everything. This was done by reverse engineering, without any support from the Outernet team (which don't seem to like this open-source receiver).