On 9/27/12 7:23 AM, J. Forster wrote:
Jim,
What you are suggesting is essentially a spread spectrum system where the
chip pattern is time varient.
IMO, this is an incredible kludge. And, there is no gurantee that the
algorythm for generating the chip pattern will not change down the road.
I think I poorly explained what I was thinking.
Store the raw samples
Run the samples through a demodulator to recover the bits using whatever
technique works best: for instance, you can make your symbol transition
decisions based on many bits at once, as opposed to only those you have
already seen.
Then, take those decoded bits and use them in a second pass through the
data to remove the bits (sort of like the inphase arm in a Costas loop)
so you can get a "carrier only" version of the input signal (with some
noise at the symbol boundaries, most likely).
Excise the transitions where the SNR is lower.
Then, do your carrier frequency and phase recovery on what's left over.
There's probably some elegant approach to deciding what to excise and
what not to.
But, in any case, no a priori knowledge of the bits is needed.
(We did something like this at JPL to recover telemetry bits from
Phoenix coming out of the plasma on EDL. Recover the carrier and symbol
timing when you're farther down and then run the demodulator backwards
in time). It's always easier to track than to acquire, after all, so
why not acquire later when the signal is strong, and track backwards to
where the signal is weak.
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