[Discuss-gnuradio] Decoding constellation (0,1-1) using gnuradio

[abhinav narain]

Hi,

 I am trying to transmit -1,0,1 { [1]+ 100*[0]+[-1] }, basically BPSK lot of 0s  filled in between at certain frequency. I have few questions. "Constellation object" has 3 symbols ([1+0j,-1+0j,0+0j]) mapped to symbol map-> [0,1,2]  respectively. I agree that constellation with size 3 is weird but that is what I want. I want the decision boundaries to be at -0.5, 0.5 for bits and anything between that to be given symbol=2 which i can discard later on. I am currently doing this in baseband, but I will eventually trasmit at a some frequency X.

 I have the following baseband setup grc (http://postimg.org/image/8k7g1o375/  , grc file is attached).

 I have few problems which I am unable to fix since yesterday -

 1) The constellation after channel is stable (as expected) but it goes weird

 after the polyphase clock sync and costas loop ( http://postimg.org/image/pyg8h2vlh/  , attached image- constellation_before_after_costas.png)  and hence I don't get the expected output of equally spaced 1, 0 in the decoder  output ( http://postimg.org/image/a58cc35cj/  , attached image- custom_decoder_0_1_2_output.png)

 2) If I set the order of costas loop as 3 (size of constellation) my flowgraph doesn't work at all. I have used the parameters after reading some example code

 in gnuradio folders and web and i guess they are correct. Please correct me if  I am wrong.

Thanks,

Abhinav

grc-graph.png
Description: PNG image

constellation_before_after_costas_loop.png
Description: PNG image

custom_decoder_0_1_2_output.png
Description: PNG image