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From: | Marcus Müller |
Subject: | Re: [Discuss-gnuradio] Chunks to symbols with 16 QAM modulation |
Date: | Mon, 17 Jul 2017 14:25:47 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 |
Hi Rafik, I should have been a little more constructive in my last email:
The GNU Radio Costas Loop supports none of that. It's really
meant for lower-order PSK constellations, and doesn't have the
flexibility to address inequal phase decision boundaries etc.
Really, Costas Loops are usually not used to recover phase
of non-PSK modulations, as far as I (limited experience with QAM
receivers) can tell. So, generally, if you can do timing recovery (and that might be
possible, e.g. with a Polyphase Filterbank Clock Recovery
approach), you should be able to see the constellation diagram.
However, that will be rotated, and possibly also still slowly
continue to rotate, depending on how good your timing recovery
works. As I tried to hint at above, it's not trivial to design a
phase recovery for true QAM modulations – in fact, the need to do
that is, in practice, often avoided by having sufficient channel
state info from somewhere else – e.g. by having a known preamble
that can be used to derotate. Phase sync, like any sync, is always pretty application-specific, and it's not inherently easy to say, "yeah, that's the optimal algorithm" for any given case. You'll often find something like a receiver getting the initial phase estimate by correlating the post-timing-recovery stream to a known sequence, and then either using a phase error loop with a small bandwidth, assuming that phase jumps will never be large enough that you end up jumping the minimum phase distance between constellation points, or looking for periodically inserted pilot symbols and updating their internal phase tracking from these. Personally, I kind of still consider the Costas Loop a concept
rooted in analog signal processing (though it definitely works
good in digital domain, too) with the error signals etc actually
being analog voltages being fed back to analog summing amplifiers
and so on. That makes it easy (ok, easier) for me to visualize how
phase stabilization for PSK-type modulations works. If I have to
imagine I'd have to synchronize a quantized QAM with such a thing
optimally, I'd have a hard time describing what the analog error
estimator and analog loop filter should look like. In digital
domain, we get all the "logic" coming cheap, so, yeah, why not
have a few % of symbols being used for phase recovery and skip the
phase error loop – we can double-use them to estimate more of the
channel than just the phase, so that's not such a bad deal,
usually. Again, this is all very application-specific! Marcus On 07/17/2017 12:56 PM, Rafik ZITOUNI
wrote:
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