The testing that was done on DRM showed that the worst multipath spread is on 
NVIS paths and is about 8 ms. They originally could accomodate only about 5 ms 
of multipath but had to create a new mode with a longer guard interval to 
support South American stations using the 41, 49, 60, 75, 90 and 120 meter 
broadcast bands.

73,

John
KD6OZH

  ----- Original Message ----- 
  From: Rick 
  To: digitalradio@yahoogroups.com 
  Sent: Wednesday, October 10, 2007 14:05 UTC
  Subject: Re: [digitalradio] Dev: Multipath


  SCS decided on 100 and 200 baud rates when they developed Pactor 2 
  around 1995. They also used constellations as large as 16PSK. At the 
  time, Dr. Rink explained why they selected certain design parameters and 
  it was his view that with strong FEC, the high baud rates would be 
  workable and 100 baud would be a good compromise lower speed. After 
  many years of experience they must have proven to themselves that 100 
  baud was the sweet spot. There was no shifting back and forth between 
  baud rates which can really slow down a link. G-TOR had 100/200/300 baud 
  rates and from what I have heard had a lot of problems with taking too 
  much time trying to hunt to find the right baud rate for conditions at 
  that moment.

  They also must have found that the complex constellations greater than 
  4PSK were not very productive compared with a much wider bandwidth. The 
  tones are spaced slightly wider than normal orthogonal spacing and, if 
  my understanding is correct, normal spacing would have been one Hz for 
  each baud. Perhaps this reduces the tones from overlapping if there is 
  increasing ISI?

  The MIL-STD/FED-STD/STANAG modems use the 2400 baud 8PSK waveform which 
  is many, many times higher in baud rate than anything we normall use on 
  HF here in the ham bands. They supposedly use very strong DSP to cope 
  with ISI, but when you look at the actual specifications of these modes, 
  they consider 2 ms and particularly 5 ms of ISI as worst case. This 
  might be possible close to the MUF, but on the lower bands in the 
  evening, this seems unrealistic.

  We have only heard a few comments from hams outside the U.S. who have 
  used these modes and while the throughput can be quite good under good 
  conditions, as you would expect, they don't do well when conditions are 
  more difficult.

  I am betting that even if 8PSK2400 modes were permitted here in the 
  U.S., these modes would not work as well as some think they will. People 
  who design, install, and maintain military ALE equipment have told me 
  that the modes are not that great in many conditions and can be 
  frustrating to use. Especially when you consider the astronomical cost 
  of this equipment.

  The one problem that SCS noted with too slow a baud rate is that short 
  term ISI can be a problem, such as fast doppler. It was his position 
  that we will not normally expect anything worse than a 5 ms ISI and a 
  few Hz dispersion. Of course we do run into much worse ISI at times and 
  I suspect that Pactor modes will not work at all under those conditions. 
  Then a mode like Domino EX might perform better. But from what I have 
  been able to gather, PSK modes are the best route to go at this time.

  One of these days, some enterprising ham is going to build a multi-tone 
  design modem similar to Pactor 3 and we will find some very impressive 
  throughputs. This fresh start would be easier to program than to make it 
  backward compatible to previous modes such as had to be done with Pactor 
  2 and 3. Perhaps it could follow the FAE asynchronous ARQ design? It 
  could be manually adjustable at first so that you can switch between 
  2PSK and 4PSK, but forget higher constellations. And it could be 
  manually adjustable so that it could drop tones to get fewer tones with 
  wider spacing as needed for more robustness. Eventually, further 
  progression of the design could lead to automatically changing numbers 
  of tones and/or constellations, but at least it would be usable for 
  manual control.

  In other words, Pactor 3 has proven what works and tells you where to 
  focus for higher throughput modes.

  73,

  Rick, KV9U

  Rud Merriam wrote:
  >
  > Okay, the MFSK decoding via FFT gives me some things to work on the 
  > next couple days.
  >
  > We know from Shannon-Hartley the limit for throughput based on 
  > background Gaussian noise. Another major problem with HF 
  > communications is multipath. This is where a signal takes multiple 
  > paths through the atmosphere. Some of the paths may delay the signal 
  > by many milliseconds, even 10s of milliseconds. This cause 
  > self-interference where a symbol is distorted because of the late 
  > arrival of a previous signal. Also known as Inter-symbol Interference 
  > or ISI.
  >
  > It seems the best way to handle this is by a slow symbol or baud rate. 
  > I find it interesting that Pactor III maintains a constant 100 baud 
  > while adjusting the waveform in other ways to adjust for adverse 
  > conditions. At a 100 baud the timing is 10 ms per symbol. Does anyone 
  > know if they arrived at this because 100 is a round number? Or is 
  > their research behind this choice?
  >
  > 
  >


   

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