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?
As someone else posted there's also a need to consider Tx antenna
bandwidth if the SDR route is chosen, particularly on lower bands where
a tuner of some kind is likely to be in use. Even if that tuner is
automatic will it change fast enough, and will frequent changes cause
longer term issues with worn relay contacts in such devices for example?
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.
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.
Alan G0TLK
On 02/09/2021 07:32, Phil Karn via wsjt-devel wrote:
On 8/29/21 12:30 PM, Gordon Weast via wsjt-devel wrote:
Ed,
When I look at decodes on 20m today, I've seen multiple times when
signals offset by as little as 1 Hz are decoded just fine.
Basically, the protocol works fine with lots of overlap. There is NO
need to channelize transmission frequencies.
I don't see that that necessarily follows. It is always desirable to try
to make the channel utilization more uniform. The ultimate fix here is
to frequency hop between every *symbol*, not every transmission, and to
use FEC to fix the inevitable collisions. That's true CDMA. But it's not
backward compatible with FT8, so it's a separate topic.
Restricting transmissions to the channels you're proposing would
severely restrict the number of simultaneous users.
Why? The protocol would not necessarily inhibit transmission when it
thinks a channel is busy. It could look at the signal levels and
conclude that it wouldn't necessarily interfere with another
transmission even if it transmitted at the same time.
In any event, you can't avoid overlap since what looks like a clear
channel at your location will probably not look like a clear channel
at a different location.
Again, that's the point of my original MACA paper -- that simply sensing
whether or not the channel is busy doesn't really tell you what you want
to know. Collisions occur at receivers, not transmitters.
Phil
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