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VE2HAR > MT63 11.03.05 01:04l 132 Lines 5649 Bytes #-7495 (0) @ WW
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Subj: Re: [MT63] 20 kHz wide Digital Proposal (Was: ARRL MUST GET
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> Walt said:
> You might see 56KBPs data but it would only be on-the-air for a short time.
> The "channel" would be shared.
Are you suggesting that a TDMA system be used to share a 20KHz wide channel
at HF? How would you allocate timeslots? Would you just use CS/MA? I'm not
even sure where to begin to explain how poor of an idea that would be.
> Right now we have MT63 at 200 WPM thoughput on a 2 KHz wide bandpass at a -5
> dB SNR on a poor CCIR channel. But we really need to push 800-1200 WPM at the
> same signal level. I don't see doing that with less than a 6 KHz channel even
> exploiting some of the new OFDM type modulation protocols being developed.
Why? Why do we 'need' to do this? Look at the popularity of the current digital
modes. It's pretty much proportional to the useability of the mode--as perceived
by those who use it. PSK31 is the clear leader because it fits the needs of
many people. MT63/olivia/MFSK16 are way back in the noise because the negatives
of the modes outweigh their benefits. Should a need arise, I would expect their
useage to rise as well. But, until there is a *need*, why do we 'need' to do
this?
What we have here is a proposal by a small group of people for a large chunk of
an internationally shared (and contested) resource.
Also, your throughput vs signal level vs bandwidth thinking needs to be clarified.
Here are some thinks to keep in mind when talking about the HF signal propogation
path:
1) if the baud rate of a carrier is slower than the reciprical of the channel
coherency time, you're going to have intersymbol interference, so there is a
practical minimum for baud rate--Peter M. has spoken on this topic a few times
saying that PSK31 is just a bit too slow for reliable usage on some paths (polar).
2) At least for the USA, there is a maximum speed of 300 baud.
3) channel throughput = baud rate * carriers * log2(symbols)
4) more symbols/carrier requires more Eb/N0
5) FEC only works on the high side of the Eb/N0 = -1.67db Shannon limit
This leads to a few design decisions:
1) baud rate should be <300 and >30
2) Fewer carriers at a higher baud rate beats more carriers at slower buad rate
3) More carriers are more resistant to fading than fewer carriers
4) TX power is *finite*
5) Spectrum is scarce
So, MT63 (63 carriers of BPSK and heavy FEC) in a 1KHz BW at -5db (in the same
BW?) SNR competing with a similar signal in 6KHz of BW will be -12.8db SNR (in
a 6KHz BW). Do you expect to be able to decode that?
What I'm getting at is that adding bandwidth isn't a wonderful solution to your
problems.
I'll make a back of the envelope guess and say that olivia with 8 or 16 carriers
in a 2kHz BW is fairly close to ideal for amateur HF use. Any wider and you'll
need more power. Any faster and you break the 300 baud limit. And more carriers
and you cut throughput.
> How many CW QSOs running at 20 WPM will it take to produce 1200 WPM error free
> copy at a signal level that MT63 can copy? These are signal levels that even
> the best CW operator can't copy a signal at. So the answer is that NO number
> of CW QSOs can produce the throughput of 1200 BPS, near error free, at a -5 dB
> SNR on a poor CCIR channel.
This is a poorly constructed straw man. For one, your use of SNR is poorly
defined. -5db *in what bandwidth*? For CW, the necessary bandwidth is much
less than tha of the mt63 signal--2KHz vs <200Hz. That alone gives the CW
signal (or PSK31) a 10db head start--it's got 1/10th the noise to fight
against. Also, if the CW signals are generated by different stations, total
power for the CW operation increased with each staiton, so the CW stations
could fit 10 transmissions in the space of one MT63 transmission. Where the
single MT63 TX would have one unit of power, the CW operators get one unit
of power per station, so they get 10 units of power. Now they're up 20db vs
the MT63 signal. Are you saying that 10 CW signals at +15 db SNR (in the
appropriate BW) are going to copy less data than one MT63 transmission?
> All this IS possible if we are willing to step out of the box.
The 'box', yes, but you really do need to stay in this universe.
> AND the new modes that will develop will be FAR SUPERIOR to Pactor III or
> anything being done today on HF.
That is something we both agree on.
> We're also looking at doing much of this work using a computer sound card so
> separate controller hardware is not needed.
That is a good usage of existing COTS hardware, but never forget that it does
limit the flexability of such a system vs the CW competator. A simple CW rig
and a pad of paper and pencil are pretty easy to come by.
> Walt DuBose/K5YFW
> Asst Chairman
> ARRL HSMM WG
Out of curiousity, is there anyone on the HSMM WG who plays devil's advocate
at your meetings? From the stuff coming out of it, I get the impression that
the dreaming goes fairly unchecked.
Cheers,
David n0ymv
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