On 9/26/12 10:15 PM, Peter Monta wrote:
I'm not sure about residual carrier aiding the tracking process. A Costas
loop recovers the carrier pretty well, and a symbol aided loop (where the I
channel has a hard limiter, for instance) does even better.
Yes, these work (and a soft tanh() limiter improves on the hard
limiter a little bit), but I think they don't work as well as a PLL
with a pure carrier, where performance is measured as the variance of
the phase estimate at a given SNR.
After all, the energy is still the same.
True, but information has been lost as a result of introducing these
unknown phase transitions. Now if the phase transitions are known,
one can certainly wipe them off by multiplying by a noiseless replica
of the known phase modulation, and then you're back to pure carrier.
But if you don't know the transitions ahead of time, you need the
Costas loop to find them for you, and that costs SNR.
In WWVB's case, many of these phase transitions probably can be
predicted. But the point is not so much that good timing receivers
for the new signal are problematic. On the contrary, they're no
problem at all with a little DSP. But for the sake of backward
compatibility, putting 5 or 10 percent of the signal power into a
carrier seems a small price to pay.
Or, you can use an acausal processing scheme.. demodulate the bits, then
go back and remove them from the input signal. If what you're doing is
recovering timing to a gnat's eyelash, that would probably work.
Using a Costas-loop preprocessor to a legacy phase receiver is almost
to the point where you're better off tossing the legacy receiver and
just using the preprocessor.
I don't want to sound too negative here. I'm glad WWVB is getting
these improvements, and the clarification from John Lowe earlier today
about the openness of the signal is helpful. But backward
compatibility would have been so easy to put in.
Cheers,
Peter
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