I am hoping there will be some people on this list far more experienced
than me to be able to advise on this!
Am playing with VHF voice, etc, and I am thinking what is the best way
to get ultra-reliable (super-sensitive) performance on a VHF modem.
I am thinking for VHF, based on a radio where only a normal FM modulator
with a Class C PA is available, so it would have to be something like
4FSK. Bandwidth isn't a huge problem (say 12.5khz is available which
gives loads of headroom for 1000 baud Codec2).
Is it better to run 4FSK at a low baud rate (say using one of the low
baud rate versions of the codec), then have just a little error
correction, on the basis that low baud = narrow IF = sensitive, or is it
better to run at a high baud, really heavy on the error correction, such
as 4800 baud and then have loads of error correction such that the
actual throughput is only 1000 baud?
Basically which would work better - 4800baud air with 1000 baud after
error correction, or 1200 baud air with 1000 baud after error correction?
This would be assuming 4m band (70mhz) where Rayleigh fading isn't very
predominant.
Also, I know it becomes the law of diminishing returns between 4FSK,
8FSK, 16FSK, etc, and it is generally felt that 4FSK is the most
reliable, but is there any advantage to spacing the tones further apart
except from a small "guard band" would help prevent interference between
the tones. Presumably if the tones start to get really spaced out then
there is absolutely no advantage because you just waste spectrum with
huge guard bands for the 4 filters on receive.
I know that the work so far on Codec 2 has been based on low baud, low
error correction, but I am wondering if this equals best sensitivity or
if this is more about bandwidth efficiency.
I just wondering best way to get really sensitive comms, so a 5W HT
could go for miles and miles.
Sam
M1FJB
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