On 08/02/2017 01:20 AM, jimlux wrote:
On 8/1/17 12:11 PM, Graham / KE9H wrote:
Dider:
This is a CDMA signal. (With a 'chip' rate that far exceeds the
information
rate.)
If you put a different correlator on every multipath signal, which are
each
differently delayed in time, then they can be independently demodulated.
(Or time shifted and added back together with some quality indicator for
weighting.)
So, in CDMA, multipath is used as a form of (time) diversity reception
and
will improve the signal to noise of the combined signal.
By definition, the signal with the least time delay either is, or is
closest to, the most direct path.
You don't have to necessarily fully demodulate this early signal by
itself,
just know what its timing is. (And remember where it was, if fading in
and
out.)
Since multi-path is a destructive mechanism in most narrow band radio
systems, the above may not be intuitive to people not familiar with CDMA
and "rake receiver" based systems.
Multipath helps, not hurts, these systems, as long as the multipath
delays
are most of one chip apart, or more.
For GPS raw (off the air), when they post process at JPL, they use a
fairly sophisticated correlation process, incorporating an estimator of
the underlying time delay trajectory: you can use a later "big peak" to
help recover the early "first peak", for instance.
Rake receivers (under the name "adaptive equalizers") are also why
digital TV works fairly well in high multipath environments - with
modern receivers that do this.
The real increase in precision comes from the higher sampling rate, i.e.
sample per chip, which allows for more elaborate schemes to be used.
Narrow correlation, RAKE etc. is all just different approaches to that,
but the core comes from the oversampling and increased bandwidth.
Just doing increased bandwidth with traditional correlator is know to
improve precision as narrow-range multi-path can be better suppressed.
Novatel patented an extension to that using a par of narrow correlators
to further improve on that aspect.
Regardless how you do it, improved bandwidth and related oversampling
factor is the enabler, then the weapon of choice to make use of it is
the next thing to look at.
BTW, I saw that the JPL receiver at ISS got upgraded to support Galileo.
Cool stuff.
Cheers,
Magnus
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