Slow CW vs. BPSK etc.]
Fri, 02 Jun 2000 16:54:06 -0400
IMHO, a well written message,
> >There's a lot of discussion going on at the moment on the US "LowFER"
> >mailing list about the relative merits of slow CW versus BPSK
> Indeed there has been. Some of it has gotten rather silly and occasionally a
> bit abusive, in fact. I'm glad to see the discussion has been more civilized
> here, although there are still evident tendencies to compare apples with
> oranges and to draw too many conclusions from accepted premises.
> Some of the rather extreme arguments in favor of BPSK and similar modes tend
> to count the same chickens twice; perhaps more often, for those claiming 23db
> advantages <grin>.
> The most credible proponents say that, for a given bit rate, coherent methods
> have a 9db advantage. Andy Talbot's post yesterday argues this case well.
> Six decibels are the result of the effective doubling of detected signal
> output by one state of the received signal adding in-phase and the other
> adding out of phase in the detector. Integration of voltage over the symbol
> period accounts for the other three. The latter effect is a direct
> consequence of coherence (one might call it the definition of coherent
> detection), and the former requires coherence to exist before it can be
> true...or for any detection to exist at all. With coherence, a BPSK-based
> mode has a 9db advantage over on-off keying detected non-coherently, or 6db
> over coherent CW.
> Does this mean, therefore, that BPSK is inherently "better" than Slow CW?
> Andy was careful to preface his comments with the qualification, "To compare
> theoretically, firstly we need to make the assumption that data rates are
> similar for the two modes." However, there is an additional qualifier that
> follows from all this: we must also assume that coherence _can_ in fact be
> achieved and maintained over the duration of enough symbols for the message
> to be reconstructed.
> If this latter condition cannot be met, then PSK-based modes have no
> advantage. They simply don't work at all.
> The traditional technique of reducing data rates and narrowing communication
> bandwidth only work for coherent methods to the extent that the path between
> the transmitter and receiver is stable enough to accomodate the slower bit
> rate. It doesn't matter if the sender and receiver are using perfectly
> synchronized atomic clock frequency standards: if propagation effects distort
> the all-crucial phase information enough during each individual symbol, the
> symbol will not be recovered.
> Hence, for a given propagation mode--and, where the ionosphere is involved,
> this is also a function of carrier frequency, path length, launch angle,
> geomagnetic conditions, and so on--there will be some _minimum_ data rate
> below which a given coherent method will not work. Weak-signal reception
> techniques based on power integration over extremely narrow bandwidths (i.e.,
> spectral analysis techniques) do not suffer nearly as strongly from the same
> Let's re-evaluate the numbers in this light. Suppose, over a given long
> path, we find that BPSK will let us receive an acceptable percentage of a
> transmission at 10 bits per second, but faster rates are limited by noise and
> slower rates are limited by phase errors in the propagating medium. To
> achieve the same result, we would have to slow down our on-off keying by 9db,
> to roughly one dot per second.
> Now, however, let's suppose that either the noise levels increase, or we
> attempt to receive the same signal over a greater distance. We can't slow
> the speed of the BPSK transmission any further without losing the ability to
> recover it coherently; which is to say, at all. Yet, we can slow down the CW
> transmission and tighten the resolution of our receiving software and
> continue the exchange of information.
> (This can't be done without limit, of course. At some point, propagation
> phase shifts will spread the spectrum of the on-off carrier enough to
> disperse it across multiple channels, considering how narrow the detected
> channels have become at that point, and the analog signal-to-noise ratio will
> then rapidly collapse too.)
> I don't follow the BPSK e-mail group, so this may not be the most current
> information, but as far as I am aware, the best LF DX results with BPSK over
> here have been at 100 and 200 milliseconds per bit. Some have tried 500 and
> 1000 milliseconds, but have not done as well; whereas on HF, rates of 50
> milliseconds per bit or faster seem most effective.
> If 200 milliseconds per bit turns out to be the lowest practical speed for
> longwave DX, then we already know that Slow CW at 3 seconds per bit should be
> able to match its data recovery performance if the time penalty is
> acceptable. We also know that 10 seconds per dot, or longer, is entirely
> feasible with the available receiving software. The only thing we don't know
> is how Slow CW performs under conditions of very long paths at LF, on the
> order of 1600km or more at 5mw radiated power, for which we do have some
> experience with BPSK.
> The essential point is that there are conditions below which coherent methods
> cannot work at all, but in which other (slow) weak-signal methods may provide
> at least a chance of getting signal through. There's room for a lot of
> experimentation with these "different horses!"
> John KD4IDY