[RTTY] One more thought on emergency comm

[Kok Chen]

On Dec 12, 2013, at 10:03 AM, Joe Subich, W4TV wrote:

> They are
> are not likely to be sufficiently reliable - run the numbers for SNR,
> fade margin, etc. for non AWGN circuits with interference, selective
> fading and flutter and it will be clear that for reliability, over any
> significant distance, wideband data modes will require power levels in
> excess amateur limits and ERPs far greater than anything likely to be
> generated with basic transceivers and simple, field deployable
> antennas.

Joe,

I wouldn't generalize like this.  *We* have problems with running 2 tone FSK at higher bit rates.

But the modern modes have lots of technology that are built in to counter the ionosphere that steam RTTY can't even dream about.  

At the minimum, like Pactor-3, they would use Convolution codes, and either use long interleavers or Reed-Colomon type codes to get around slow QSB.

The military quality stuff are designed to be robust under 12 ms worth of multipath, and 50 Hz of Doppler spreading.   Try sending steam RTTY through *that.*

Notice that Pactor 3 uses 100 baud (10 ms data periods) and 120 Hz sub-carrier separations.  I would bet many Kopeks that they have both multipath and Doppler spreading in their minds.

The ones that use higher symbol rates apply pre-emphasis based on how known "preamble" bits are distorted by multipath.  So, that is yet another tool that is available if you want to go to higher symbol rates.  They are all synchronous modes, of course.

None of it is useful if you just want to send a 20 character message, of course.  The latency is high (but, that is true even with Amtor-FEC), and the overhead for such a short message is also very high.

However, if you want to pass lots of data in one long burst, you can do *much* better than steam RTTY in a Rayleigh channel.

Heck, even DominoEX wipes the floor with RTTY; DominoEX has many of the properties of the modern modes to countermeasure both multipath and Doppler spreading.  

73
Chen, W7AY