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VE2HAR > MT63 11.03.05 22:05l 417 Lines 18413 Bytes #-7343 (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.
> >
> > Well I'm not sure either but that can be a goal.
> >
> > I suppose we could use a GPS clock. I do know that some hams are working on GPS
> > sync. and I think that perhaps the SDR-1000 radio by FlexRadio uses this.
>
> So, to TDMA a channel that propogates around the planet--even with everyone in
> perfect GPS sync--you still have signal propogation time between people being
> able to transmit. So, every 'turnaround' of the channel costs you as long as
> it takes for a signal to get all of the way around the planet--bouncing up and
> down as it does so. Even then, you're left with the problem of who arbitrates
> timeslots and how do you ask for one?
The first station on becomes the master station. I think that there are already
marine modes that do this.
>
> What seems to make more sense is a bunch of smaller channels feeding into a
> set of BBS like systems which could coordinate message flow. The BBSs would
> need to have a different set of channels to pass traffic amongst themselves.
Cool...great idea.
>
> TX/RX could be on different bands so that it could be simultanious, even.
Yep.
>
> Ad hoc networks are a nice idea, but they just don't scale. A well thought
> out system will beat them every time.
You are so right...that's why there's so much emphasis on the NTS and formal nets.
>
> > > But, until there is a *need*, why do we 'need' to do this?
I think I answered this earlier on the MT63 list. If not, someone forward my
response on who needs our "services" to David.
> >
> > Well different strokes for different folks...I want to be able to receive a
> > message from stations in the middle of a hurricane who are running HF on battery
> > power and a NVIS antenna laying on the ground or in close proximity to the
> > ground. Even CW can't do this...you need a QRO signal of more than 500 watts to
> > do this. Or, a mode that will give you 10 db of gain with no increase in noise.
> > So this is where modes like MSFK16 and MT63/Olivia come into play.
>
> I'm curious now, you say NVIS doesn't work with less than 500W? Could you
> explain why CW doesn't work here? NVIS should be a good propogation channel
> as there is only once bounce off of the ioniosphere to cause doppler.
>
> > > What we have here is a proposal by a small group of people for a large
chunk of
> > > an internationally shared (and contested) resource.
> >
> > I keep hearing this but to tell you the truth, I talk to the EComms hams in the
> > UK and other parts of the EU and they are thinking the same way...more/faster
> > throughput and more robust signals and realize that you cna't do this with a 3
> > KHz bandpass.
>
> EComms hams in Europe? The last time we had this 'need an international standard
> for HF EComms' discussion, the european hams looked at us funny because in many
> countries in europe, hams are expressly prohibited from EComms. Has that changed?
Yes, in some countries and I expect it to change more in other countries.
> > > Also, your throughput vs signal level vs bandwidth thinking needs to be
clarified.
> >
> > Yes, we need a standard.
> >
> > I have proposed that the minimum throughput of 200 WPM, achieved at a SNR of -5
> > dB on a poor CCIR channel and within a 2.7 KHz bandpass. The first GOAL should
> > be 400-800 WPM at a -5 dB SNR on a poor CCIR channel within a 6 KHz bandpass.
> > The second goal should be 800-1200 WPM at a -5 dB SNR on a poor CCIR channel
> > within an 8 KHz bandpass.
>
> Okay, we're back to the problem that I was mentioning. Those channels aren't
> similar. You have a signal at the same level VS noise in changing bandwidth.
> Either you're cranking up TX power or you're looking at a path that is shorter.
>
Yes...and that's why all this "testing" needs to be done on a simulator but with
defined parameters.
Perhaps the only standard we should have is throughput, bandwidth and the
transmitted power it takes to get the signal through...Oh wait, that's
on-the-air testing. So maybe we have to define the standards we are to test under.
> > Currently with the types of signal detection used, a ~45 baud modulated signal
> > is about all you can deal with due to intersymbol interference...remembering
> > that with each additional "hop" and propagating through more than one
> > ionospheric layer, your intersymbol interference increases. Part of the
> > solution is controlling you radiation pattern. If you want to talk out to 600
> > or so miles, make sure that you antenna's take-off angle is between 45 and 89
> > deg. I see lots of success on MT63 betweem the Central U.S. and Europe from
> > stations using beams aimed correctly...they are creating the fewest hops between
> > stations. Stations using other antennas may find that they don't do as well as
> > neighboring stations using beams.
>
> Quite true. The less of the 'sky' you illuminate means you're taking a smaller
> 'sample' of the ioniosphere and that can be expected to be more uniform than
> a larger chunk.
>
> So, that leads to the question: are you looking for a mode that will require
> staitons to be using directional beams?
Not at all...but you will find that most modes work differently under different
propagation conditions. A good strong signal from Europe does not mean that
there isn't a lot of intersymbol interferrence or doppler on it...its just what
you hear and see on the S Meter but if you look at it on s specturm display and
know what you are looking at, you might find a very different signal. Ya think?
>
> > > 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
> >
> > I recently attended a end of project briefing at Southwest Research Institute
> > which indicated that current data detection methods of HF propagated PSK signals
> > limit the transmitted baud rate fo no more than 30 Baud to detect a clean bit
> > constallation. However, the techniques which they have developed allow for
> > error free detection of sifnals and a good constallations with transmitted baud
> > rates of over 100 baud. The "kicker" here is that for each transmit time
> > period, there is a detection time of 4 times the transmit time before received
> > data signals become usable.
>
> The limitations for baud rate are for 'dumb' receivers. Several times, it has
> been proposed (TAPR HFSIG mailing list) that some kind of adaptive channel
> equalization be applied to the systems in use--or to a newly designed systems.
> The problem always falls back to the methods being pattent encoumbered.
>
Yes, and they addressed that very problem in their solution.
> > There ain't no free lunch.
>
> No, indeed, it probably requires a hefty bit of DSP to acheive, as well, right?
A HEAFTY bit...well that's the understatement of the year. Hi Hi.
So do we off-load the DSP to an external box like the TAPR EVM boards they were
pushing or something like the SCS controller or try to use a computer's internal
sound card.
If we choose to use a computers internal sound card, or maybe even two or three
or four in one computer, we will be limited to what kind of modem we will
create. And that should probably become part of the spec.
>
> > Develop a goal and/or standard and then develop you modem, whatever it turns out
> > to be, to meet the goal or standard.
> >
> > What do we want?
>
> Start there. Define the problem you want solved and the constraints on the
> solution--your hurricane scenario, for instance. Then publish that and let
> hams chew on it for a bit.
Absolutely and I have done that here on this list and the LinLink list as well
as on the TAPR HFSIG.
>
> > > 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?
> >
> > Sure, you can right now with software you can decode a signal at -15 dB SNR on a
> > poor CCIR channel at ~50 WPM. We call than mode MFSK16.
>
> Yes, that's because it's carrying *less* information, not more as your
hypothetical
> 6KHz signal is. MFSK16 can do that because it encodes that little information
with
> a lot of FEC.
>
> This is why I hate SNR for these kinds of discussions, it can be misleading. The
> proper term is Eb/N0. The energy given to a bit of information divided by the
> spectral power density. Eb/N0 describes the payload data, not the modulated
> data. You can do anything you want to the payload data and the Eb/N0 of the
> payload stays the same. See the SS discussion in the email to Karl for more info
> on that.
Your right of course but its har enough to get the general ham population to
understand SNRs on a standard channel much less the Eb/NO.
The bottom line is that hams will try it on the air and if one mode performs
better than an other for the purposed used (chatting or sending files), then it
will win...survival of the fittest.
>
> You can then transmit log2 (6Khz/1KHz) = 2.6 times more bits in that bandwidth.
> So, we're looking at 520 WPM if we can keep all other things equal.
>
> > > What I'm getting at is that adding bandwidth isn't a wonderful solution to
your
> > > problems.
> >
> > I don't know...do we have a problem? We can pump out 1 KW of signal at 200 baud
> > in a 2 KHz bandwidth and overcome lots of propagation problems...its called
> > Pactor III and QRO. Or, we can run 400 WPM at 100 watts PEP (10 watts average)
> > in a 6 KHz channel with a mode that allows several (Ok 2 or 3) signals to work
> > in the same 6 KHz channel at the same time.
> >
> > Your choice.
>
> This gets me back to 'what problem are we trying to solve'? Are we talking about
> a guy in a bunker in the middle of a hurricane? How much traffic does he need to
> pass? He's head down in a bunker, it's not like he can go walking around and
> collect H&W info. :) For 'after the storm' H&W traffic you'll be in quite a
> different boat.
Actually its after the storm but H&W traffic is low proprity, its the disaster
relief traffic that has priority. Actually before last year, there wasn't
sufficient disaster relief communications which caused government agencies to
use assets and send assets to the wrong place and slow relief and caused feeding
units to have to serve the same menu for three days...you can just eat so much
spaggetti and meat sauce for three meals a day for 3 days.
But actually we do want to know what is happening while the storm is
passing...it gives the planners an idea of what kind of and how much disaster
relief support is needed.
>
> > > 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.
> >
> > Ok, I haven't studied the specs on Olivia so don't know what it's theoritical
> > limits are.
>
> It's just a flexable multi carrier/flexable bandwidth experamental digital mode.
>
> > > > 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?
> >
> > Ok...-5 dB SNR on a poor CCIR channel which I believe is a 3 KHz channel with
> > defined fading and doppler.
> >
> > Wail...all the CW signals must come from the same operator and the data must all
> > be in context. Otherwise you are mixing apples and oranges.
>
> You said 'how many CW QSOs' which was somewhat misleading. :) I don't think this
> comparison is meaningful, anyway, so it's pretty acedemic.
>
Its the best I could do at the moment.
> > > > 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.
> >
> > The commercial folks ARE doing this. Are they out of the universe?
>
> They have resources that we as hams just don't have--large budgets, access
> to propriatary techniques, no/little limiation on modulation scheme, more
> bandwidth, etc. They live with a completely different set of constraints.
Yes and the agencies I mentioned want that technolgy but aren't the recipient of
the technology. We're of course talking about the military as the ones having
all the nice toys as well as a few other corporations who are not a majority of
corporations by any means.
However, disaster relief agencies have a huge lobby...I wonder who'se loggy in
D.C. is larger, the ARRL's or the American Red Cross? Did I say that? The
squeaky wheel gets the grease.
>
> > > > 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.
> >
> > Certainly...but my computer and filtering can receive 30-40 WPM CW ar signal
> > levels that no CW operator can hear....but there are also errors.
>
> Sure, and the message passing protocol that goes with message traffic handling
> is meant to deal with that--just like the FEC and ARQ of the digital modes.
> The only reason I brought up this point was to make it clear that you're requiring
> a good deal of equipment be operational affter a disaster. A littl CW radio
> and enough wire to make an antenna can be stuffed into a padded ammo can and
> chained to a cement block. That'll survive anything that doesn't suck it up
> and take it with. Doing that with a PC+monitor+generator is harder.
>
Actually we were thinging of a Laptop, Small HF rig like one of the HF semi-qrp
rigs (SGC's SG-2020) and gellcell battery.
> > The idea is error free data at high speeds. How many messages can an NTS CW net
> > pass in one hour? How many characters do they send?
>
> You can pack in many more of them in 8KHz than you can pack in this large digital
> signal. They can each be passing different infrmation to different people.
Generally during a hurricane you have one agency talking to it upward
counterpart...EOC in the hurricane area talking to another EOC in a non-affected
area or the state EOC. But after the hurricane passes, you have hundreds of
disaster relief users wanting to talk all over the place.
The critical time is usually 3-4 days after the hurricane has passed...sometimes
its a week or two. But generally disaster relief events last 21 days...the you
start going home.
>
> The goals for EComms seem to be:
> 1) survive the event with an operable station
> 2) some traffic must get through
> 3) the more that gets through, the better
Yes...but then there is disaster relief communications where you may have 50
agencies all wanting to talk to several "suppliers" and assistaing agencies and
lots of logistics coordination communications.
>
> > > 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.
> >
> > Well, we don't have meetings, we have an internal group mail list and a publis
> > maillist which you can join by going to URL
> > http://listserv.tamu.edu/archives/arrl-80211b.html
> > Select join the list. Or go directly to URL:
> > http://listserv.tamu.edu/cgi/wa?SUBED1=arrl-80211b&A=1
> > and fill in the blanks.
>
> Hmmm, no wonder it wasn't obvious where this discussion was coming from. :)
> No mention of HF nor EComms in the name there. 802.11b? A mode designed
> for the multipath/LOS microwave environment? This starts to put things in
> perspective. :)
>
> > And yes, we have several who play the devil's advocate and some of them are also
> > on this list.
>
> Good. A bad idea squashed early can save a lot of pain later.
>
> Cheers,
> David n0ymv
>
Ok David...CUL,
Walt/K5YFW
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