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Re: [RTTY] High speed RTTY

To: RTTY Reflector <rtty@contesting.com>
Subject: Re: [RTTY] High speed RTTY
From: Kok Chen <chen@mac.com>
Date: Wed, 27 Jan 2010 18:32:12 -0800
List-post: <rtty@contesting.com">mailto:rtty@contesting.com>
On Jan 27, 2010, at 3:57 PM, Bill, W6WRT wrote:

> I rarely use PSK myself, so I have a question:  Since PSK (any speed)
> has a reputation of not working well for multi-hop DX due to phase
> shift, wouldn't PSK125 be even worse there than the traditional PSK31?

Yes, PSK31 is heavily influenced by multipath distortion, to the point that 
given enough multipath, you cannot punch through no matter how much practical 
power you use (HAAPR type power not included :-).  With RTTY, you can work 
through pretty bad flutter if you apply enough juice.

However (there is always a "however" :-), counterintuitive as it might sound, 
shortening the symbol period can actually reduce the effect of multipath 
distortion when it comes to phase shift keyed signals.

PSK31 is a form of *differential* phase shift keying.  The value of a bit is 
not the absolute phase of the signal itself, but the phase difference between 
the signal at time t and the signal from one symbol period ago, at time t-T.  A 
symbol is equal to one bit in BPSK31 and one dibit in QPSK31, both have the 
duration of T = 32 ms.

If the change of relative attenuations of signals from two different paths 
causes a change of phase by a constant rate, n-degrees per second, then 
reducing the time between symbols actually reduces the component of 
demodulation errors that are caused by the multipath phase error.  (The Eb part 
of Eb/No will still be smaller, of course.  But your question is about phase 
error when baud rate is increased.)

The symbol period of PSK63 is half (16 ms) that of PSK31 (32 ms), and the 
symbol period of PSK125 is a quarter (8 ms) of the period of a PSK31 symbol.  

73
Chen, W7AY

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