| |
VE2HAR > MT63 11.03.05 18:33l 267 Lines 10685 Bytes #-7345 (0) @ WW
BID : 30260SENTTO
Read: GUEST
Subj: Re: [MT63] 20 kHz wide Digital Proposal (Was: ARRL MUST GET
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0BI<DB0NOS<DB0EA<DB0ACC<DB0GOS<DB0EEO<
DB0RES<DK0WUE<7M3TJZ<ON0AR<VA2HAR<VE2HAR
Sent: 050311/1604z @:VE2HAR.#MTL.QC.CAN.NOAM Laval #:41355 $:30260sentto
Ok...here goes...
> > 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.
>
> > 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?
>
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.
> 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.
> 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.
>
> 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
>
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.
> 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.
There ain't no free lunch.
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?
>
> 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.
We can decode 200+ WPM at a -15 db SNR on a poor CCIR channel in a 6 KHz
channel...it a takeoff of the MT63 modem but a 64 tone modem runs nicely in a 6
KHz bandpass window. Its used by Russia all the time. I think there is also a
Chinese modem that does the same thing.
> 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.
> 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.
> > 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.
> > 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?
>
> > 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.
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.
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?
>
> 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.
And yes, we have several who paly the devil's advocate and some of them are also
on this list.
>
> Cheers,
> David n0ymv
>
>
73,
Walt DuBose/K5YFW
>
> << Try MT63 on 80m - great fun!>>
>
> - The MT63 Reflector -
> MT63@egroups.com
>
> (To unsubscribe. send email to
> MT63-unsubscribe@onelist.com)
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
------------------------ Yahoo! Groups Sponsor --------------------~-->
Music that listens to you.
LAUNCHcast. What's in your mix?
http://us.click.yahoo.com/8mKGzA/FARHAA/kkyPAA/CPMolB/TM
--------------------------------------------------------------------~->
<< Try MT63 on 80m - great fun!>>
- The MT63 Reflector -
MT63@egroups.com
(To unsubscribe. send email to
MT63-unsubscribe@onelist.com)
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/MT63/
<*> To unsubscribe from this group, send an email to:
MT63-unsubscribe@yahoogroups.com
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
Read previous mail | Read next mail
| |
