On Fri, Oct 25, 2013 at 4:56 PM, tom sutherland <alphatoz...@yahoo.com> wrote: > Tom, > I don't quite understand the use of the "Constellation Receiver" block. I > have transmitter and receiver and I was trying to use the Costas Loop to > replace the mixer section in the receiver to produce the I/Q signals needed > by the16QAM demodulator. Since the signal I am wanting to demodulate is a > real-signal and the Constellation Receiver accepts complex input, where does > that signal come from? I have attached a simple diagram without filters. > Tom
The image didn't really come through on your email. I'm confused how you can do 16QAM with a real signal? You need phase information for that. Tom > On Wednesday, October 23, 2013 10:37 AM, Tom Rondeau <t...@trondeau.com> > wrote: > > On Thu, Oct 10, 2013 at 9:30 PM, tom sutherland <alphatoz...@yahoo.com> > wrote: >> I am using a Costas loop for carrier recovery with QAM16 data. The carrier >> is only 2khz. The I/Q output of the Costas loop seems to track (the >> original >> sin/cos of the modulating carrier's Frequency & Phase) steadily for a long >> period (minutes) and then the Phase moves off, normally in +/- 45 or 90 >> degrees in one or both of the phases. Any thoughts on why this occurs or >> how I can fix this issue? >> Thanks...Tom > > > Tom, > > The Costas loop is not designed to handle QAM16. It only works for > BPSK, QPSK, and 8PSK. My guess is that you are using an order 4 loop? > What's probably happening is that the symbols at the four corners of > the constellation are dominating the error calculations because they > have the highest energy. But the loop can get biased by another symbol > in the constellation that turns it to another phase lock. The Costas > loop has no idea what the right constellation is; it's just minimizing > an error function and has probably gotten trapped in a local minimum > that your constellation presents to it. > > I would suggest turning instead to the constellation_receiver block. > This allows you to specify the constellation you want it to handle. > The constellation objects > (http://jenkins.gnuradio.org/manual/doxygen/page_digital.html) allow > you to specify a symbol mapping to a set of complex points. There are > specialized forms of the constellations for certain known types (BPSK, > QPSK, etc.) that have more computationally efficient decision making > functions. But for any given constellation, it will at least be able > to calculate the minimum Euclidean distance between each point. Slow > but reliable. > > > Tom > > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
