Message-ID: <01C4D3B0.56A0CC60.actalbot@southsurf.com>
From: Andy <actalbot@southsurf.com>
To: "'rsgb_lf_group@blacksheep.org'" <rsgb_lf_group@blacksheep.org>
Date: Fri, 26 Nov 2004 12:06:18 -0000
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Organization: UKNWN(UK)
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Subject: RE: LF: Linear amplifiers and MSK
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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