On 9/2/21 2:26 AM, alan2--- via wsjt-devel wrote: > > Hi, I've been following this thread with interest and have a few > questions/comments please: > > If the proposed protocol frequency changes are set to use the wider > bandwidth of a SDR receiver linked to a T/R switch with a standard > voice rig as Tx as I saw in one post, those frequency changes will > presumably need to use CAT control. Will that be reliably fast or > stable enough with such relatively frequent changes, for all rigs? > You would not need this to implement my original idea, which is to hop each transmission *within* a single SSB bandwidth. The "hopping" would be done in the transmit waveform generation software, and it is only within the range that the receiver already demodulates everything anyway.
The only concern would be if the per-transmission hopping is performed over a new, wider FT8 allocation that requires a SSB radio to be retuned beyond a single SSB bandwidth. It would work best with a separate receive SDR able to demodulate the entire thing at once, but even then the transmitter would only have to move one or two SSB channels, and then only on each transmission. I don't see this as a problem for any modern radio. Whether it's a problem for very high-Q antennas I'll leave to the antenna experts. How many antennas, and for what bands, have to be retuned for a change of 3 kHz? > Is there a way of getting some indication of how many collisions are > currently occurring in any user session, firstly to try and obtain > some real world data on how big and frequent the issue might be, and > secondly if possible what the decode conditions were? I base that on > the vagaries of HF propagation that I suspect might be the principal > controlling factor and of course are entirely unpredictable. That's a *very* good question. We can infer some of this by looking at receive SNRs when many transmissions were needed. If you need 5 transmissions to get through, and then you do with a high SNR, the cause could be either a series of collisions or a very rapid propagation improvement. Note that you don't need to look only at the transmissions addressed to you; you can look at the SNRs of the same stations talking to someone else (or calling CQ). And you can look at those sent after your own QSO. I point out that there may actually be a more serious "collision" problem at a higher protocol level. If you call CQ, and then five stations answer you, you can pick only one to answer. The other stations may keep calling you, especially if they can't hear the station you're actually working, and they may collide and/or cause QRM to other users. Because they get no response from you, they may also think they're colliding when you're just ignoring them. They might even increase power (a bad idea). Per-transmission hopping will definitely reduce the pile-on effect that causes so much QRM when a sought-after station calls CQ. Remember, he'll hear you no matter what frequency you use within the SSB bandwidth. Think of my idea as something like automated, random split frequency operation. (There's a lot of wisdom and experience in conventional ham analog operation. We should use as much of it as possible). The pileup problem could also be addressed at a higher level by automatically answering each calling station in turn, or even better by answering them all *at the same time*, i.e., with multiple transmit waveforms on separate frequencies. This latter approach would require a *great* deal of care to avoid intermodulation distortion as the transmitter would no longer see a single tone with a 1:1 peak-to-average power ratio. I would only do this in a SDR transmitter carefully designed for the task. There's already too much distortion in the common setup using analog audio between computer and transmitter. > Are collisions an issue at VHF and above where propagation is different? > > If this gets implemented it would be good to have it switchable in and > out, so users who are interested can compare what's happening. > > I assume congestion is much lower at VHF, but repeated collisions on the same frequency can still be a problem if you answer on the caller's frequency, especially during a contest. So there's still a strong case to be made for randomizing the frequency of each transmission. And of course it should be a selectable option. Phil
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