Hi Brian
WRT the pilot levels, - other OFDM HF modes use 3dB. That David's
implementation is only 1dB pilot boost is excellent, and shows a high
skilland best practice.
Have a look at the specification for another HF OFDM mode like DRM.
There are many boosted pilots, Tthe power boost for DRM is 3dB, so in
many respects, David's implementation is excellent for a pilot boost of
only 1dB- very impressive.
I think the opinion from your QAM friends may illustrate they are
il-informed about HF OFDM techniques.
Download ETSI ES 201 980 and take a look, section 8.4 pilot cells. 3dB
and walking pilots are used . DRM is in general transmitted up to 20kHz
bandwidth and more pilots are required.
In DVBT- same power is used but the pilot data is repeated and the demod
averages over multiple periods.
-glen english
On 25/08/2021 12:03 am, Brian Morrison wrote:
On Mon, 2021-08-23 at 10:43 +0930, david wrote:
The BPSK pilot is used for acquisition (e.g. estimating the initial
frequency offset) and frame sync. More power is allocated to the
BPSK
pilot to make those two functions fast and reliable (an important
requirement for PTT digital voice). As there is only one BPSK
carrier,
the overhead in terms of total power is about 1dB IIRC.
Yes, I thought acquisition might be a reason. The 1dB power overhead is
not much, but for some other systems using QAM a designer would be told
that 1dB is unacceptable.
The FreeDV 1600 waveform (the FDMDV modem) was an open source
implementation of an earlier waveform (see the History section on
http://freedv.org) that was known to work well for this application.
So it was something of a starting point for FreeDV, and a stepping
stone to later waveforms.
There are Octave and C versions of the modem (
https://github.com/drowe67/codec2/blob/master/README_fdmdv.md), plus
various blog posts, if you would like to experiment with the modem or
inspect it's implementation.
OK, I now know where to look if I decide to have a play.
Thanks for your answer, very illuminating.
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