From: bcoleman@hayes.com (Bill Coleman)
Newsgroups: rec.radio.amateur.packet,rec.radio.amateur.misc
Subject: Re: New packeteer frustration
Date: 25 Aug 92 11:42:43 EDT
Organization: Hayes Microcomputer Products, Norcross, GA

In article <1992Aug20.122336.7175@ke4zv.uucp>, gary@ke4zv.uucp (Gary Coffman) writes:
> In article <5888.2a92524b@hayes.com> bcoleman@hayes.com (Bill Coleman) writes:
>>In article <1992Aug17.125034.22941@ke4zv.uucp>, gary@ke4zv.uucp (Gary Coffman) writes:
>>> PSK methods available today can push 9600 bps over good HF channels. 
>>
>>There's a couple of problems using PSK over normal HF channels. First of
>>all, most crystal filters in HF radios have really gross phase characteristics.
>>This causes some signal degradation with FSK, but more with PSK.
> 
> Well if you insist on using Japanese voice radios, you shouldn't expect
> good performance. Military radios with mechanical filters don't seem to
> have a problem.

No problem, Gary. We'll just have all amateurs dispose of their rice boxes
and replace them with nice, inexpensive military gear. <grin>


>>Second, (and most important) is that multipathing of HF signals introduces
>>tremendeous phase distortion. The only way to combat this problem is to
>>lower the symbol rate so that the distortions don't last longer than the
>>symbols. (Which is also why 45 baud RTTY and even 100 baud AMTOR work
>>on 80m, but 300 baud packet does not)
>>
>>Another way to combat the multipath distortion problem is to always operate
>>near the MUF, which results in clean, single path signals. Tom Clark had
>>some success running 1200 baud PSK on 10m, for example. 
> 
> Multipath selective fading can be a serious problem. Operating near the
> MUF is the best dodge around this, but diversity reception will also 
> help a lot. 9600 bps on HF is not going to be a weak signal mode.
> Systems like Piccolo and Clover that use multiple carriers to transmit
> bits in parallel with long symbol durations are likely a more robust
> way to go at HF.

Yes. And note that Piccolo and Clover are *NOT* PSK. 

>>> But the rules prevent it.
>>
>>Not really. The rules dictate a maximum symbol rate (300 baud below 28 MHz)
>>and a maximum shift (1000 Hz below 30 MHz). Other than that, the rules are
>>remarkably open.
>>
>>Of course, you'd have a hard time stuffing 9600 bps data down a channel
>>with the characteristics of most HF radios....
> 
> You can encode 9600 bps into 600 baud, but cramming it into 300 baud
> is a remarkable achievement even on excellent circuits. With the quality
> of typical HF paths, QPSK is about all you could do today. I'd love to
> be proven wrong of course.

Don't use PSK, Use something else. Use multiple carriers. Use amplitude
variation. PSK doesn't survive well unless you are close to the MUF.

So far, I've heard good things about PacTor, which lowers the symbol rate
to 200 bps, but improves throughput.

I've also heard good things about Clover, which apparently changes encoding
rates to suit conditions (which is exactly what we need for HF).

-- 
Bill Coleman, AA4LR                ! CIS: 76067,2327    AppleLink: D1958
Principal Software Engineer        ! Packet Radio: AA4LR @ W4QO
Hayes Microcomputer Products, Inc. ! UUCP: uunet!hayes!bcoleman
POB 105203 Atlanta, GA 30348 USA   ! Internet: bcoleman%hayes@uunet.uu.net
Disclaimer: "My employer doesn't pay me to have opinions."
Quote: "The same light shines on vineyards that makes deserts." -Steve Hackett.

Xref: sparky rec.radio.amateur.packet:2270 rec.radio.amateur.misc:14508