My comment certainly generated some replies!
COFDM seems to be one of the 'flavours of the month'. It does have a lot to
offer where multipath is a problem, viz. digital broadcasting over HF through
to UHF and mobile comms. But it does suffer the peak to mean ratio as
discussed by several contributors. When G4GUO was developing hte waveform
for our digital voice experiements a few years ago, he found a paper that (as
far as I can visualise) developed a way of applying a set of start up phases to
each of the tones such that the peak to mean ratio was a minimum - but it was
still not very power efficient.
For LF, multipath is not a problem but power amplifier efficiency is. The
professionals realised this years ago, hence the old GBR 16kHz transmission
used originally FSK, then converted to MSK. In fact it was the nices, most
'perfect' MSK signal I have ever received off air, it was a beauty to watch it
on a vector gram display. Any mode that maintains an absolutely constant
amplitude and varies only the frequency or phase of the transmission will do.
As MSK is such a problem to demodulate properly, perhaps we ought to steer
clear of that, particularly as weak signal working is so important.
A few years ago I remember making a few experiments using a scheme that has
rarely been used - gradual PSK - where instead of ramping from 0 to 180 degrees
directly the phase moved progressively over, say 10% of the bit period. Seem
to remember trying it wirh the VE2IQ Coherent software, but not sure if this
was ever over teh air, or just across a bench.
Perhaps it is worth revisiting, with a soundcard implementation of PSK at, say
1 B/s and a gradual ramp. And why not include a GPS locked timing option
too..
Andy G4JNT
|
