Hi Take
Single carrier , ideally constant envelope is my preferred amateur radio
data transmission mode.
it has the least chance of being set up incorrectly and generating
substantial wideband interference.
HOWEVER !
linear modes, and multi carrier modes (and constant envelope spread
spectrum modes) have substantial advantages in multipath environments,
where the occupied bandwidth is greater than the channel coherence
bandwidth. like SSB on lower HF bands.
It is true that there are good multipath mitigation strategies for
single carrier waveforms , using time domain sequences and equalisers.
These work OK where the channel is moving slowly - ie quasi stationary ,
and I think that HF is a good candidate for this, a training sequence
can be transmitted periodically or interleaved continuously , and with
enough repetition rate to model the changing channel.
I do not know why hams have not embraced PAPR reduction. My guess is
that spectrum purity and adjacent channel energy is not much of a
concern to most.
The reason PAPR reduction is not in the standard specification for DVB
and DRM is that PAPR reduction was very much a 2nd thought, as DVB and
DRM came around the beginning of OFDM in widespread use (apart from DAB) .
And I think in early days, the computational capacity just was not
economic to do the extra PAPR work.
Certainly the PAPR reduction to around 6.5dB is fairly easy, and maybe
4.5dB is very good.
If I were doing a ham HF-DV mode, I think I would use a a single carrier
with time domain equaliser, and concentrate on minimum SNR operation.
73
glen english VK1XX
.
On 18/06/2020 10:50, Tsutsumi Takehiko wrote:
Hi Glen,
What is your position of Single-carrier FDMA or LP-OFDMA technology
compare to PARP reduction scheme which you have rich experience in
your memo?
I wonder why radio amateur society leaves PARP issue untouched when
they adopt OFDM technology.
Does it comes from SSB experience which has about 5dB PARP?
Regards,
take
de JA5AEA
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*件名: *Re: [Freetel-codec2] FreeDV Tx power
Hi David
I've been across OFDM PAPR reduction development for many years.
Most successful simple schemes involve some sort of selective mapping
or bit flipping.
-IE the PAPR is computed and an alternative trajectory is generated .
-These can be either tweaking the source bitstream slightly (many methods)
-and also providing overmapping of the constellation (say 64 QAM
points might only map to 48 unique symbols, giving the system a choice
of some other point that can convey the same data.
-and also deliberate FEC violation (modify the most encoded code and
remove/reduce the offenders)
-and also selective bit flipping - easy and fast.
I would encourage some form of PAPR reduction for any ham OFDM
development.
Without it, OFDM is often and (and ignorantly) overdriven creating
spectrum headaches for adjacent channel users
-glen
On 17/06/2020 07:33, David Rowe wrote:
Hello List,
I've been working with Peter, VK3RV on the surprisingly tricky problem
of measuring FreeDV Tx power.
Over the weekend I built an inline "sampler" and tried a few averaging
options to measure the power of a 700D waveform on my spec-an. I am
using my IC7200 with the drive adjusted so that the ALC just moves. I
am sampling the power between the IC7200 and my ATU, which is connected
to a dipole on 40M.
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CORTEX RF
&
Pacific Media Technologies Pty Ltd
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