[Discuss-gnuradio] costas ambiguity and correlate-and-sync block in qpsk

[Landsman, Arik]

Hello folks, 

I am trying to resolve the 90* ambiguity of costas for a QPSK receiver, and was 
hoping folks could weigh-in in case anyone had success with this in the past. 

yes, diff encoding works.. :) trying to make it work without though. 

Also, I had seen Tom R's example of his cor-and-sync block implementation in 
BPSK (but not qpsk). maybe the block could be "hacked" to support qpsk, such as 
by passing the preamble as bpsk but, say, upsampling the block to make the 
generated complex reference align with the incoming qpsk stream. going to try 
this when I get home tonight. 

Since Trx will be bursty and will use a preamble anyways, another thought was 
to correlate the stream with the 4 possible versions of the preamble (i.e. 
constellation rotations), and pass the best candidate downstream to select the 
proper constell object for demod (as opposed to adjusting costas, as in 
cor-and-sync block), or use the result for post-processing the incorrectly 
demodulated data. But both seem a bit indirect or wastefull..

Any thoughts?  

Thank you in advance, 
Arik