Message-Id: <200411231919.iANJJOZF004834@smtp18.singnet.com.sg>
From: "Stewart Nelson" <sn@scgroup.com>
To: rsgb_lf_group@blacksheep.org
Date: Tue, 23 Nov 2004 11:19:24 -0800
Organization: SC Group
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Subject: RE: LF: Linear amplifiers , MSK, and 500kHz
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Hi Alberto,

One possible problem is that the ratio of peak to average
power is very high.  For example, if you had 64 carriers of
one watt each, the average power would be 64 watts, but you'd
need (in theory) a 4 kW amplifier to handle the case when
all carriers were in phase.  In practice, such a system could
use a PA with 400 W peak capability; the occasional clipping
would reduce the effective S/N only slightly.  An amateur
system might be ok with a 250 W amp.  But if that same rig
could be run at 250 W with slow PSK or FSK, it would negate
any advantage of COFDM.  However, if your output power was 
limited thermally, or by a one watt ERP regulation, then a
multicarrier system would offer some interesting advantages.
It's also possible that some clever precoding could reduce
the peak power required.

A cool stunt might be transmitting e.g. LPC-10 coded speech
with COFDM.  I suspect that using the same average power,
you'd get several times farther than with SSB.

73,

Stewart KK7KA


-----Original Message-----
From: Alberto di Bene [mailto:dibene@usa.net] 
Sent: Tuesday, November 23, 2004 9:34 AM
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Linear amplifiers , MSK, and 500kHz

I was playing with the idea (just the idea...) of trying COFDM on LF.
I have postponed any further work on this to next year for lack of time, 
but I am wondering
if anybody has words of wisdom on this subject. It would be unpleasant 
to spend on it a
few weeks only to find then a show-stopper that somebody else had 
already found in the past...

73  Alberto  I2PHD