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

[abhinav narain]

Hi Marcus, others,

If I understand you correctly, however, you *actually* want to do BPSK, and the 0 symbol is just a "filler" in between symbols, right? That might actually work, if you just increase the rolloff duration/samples per symbol of your pulse shaping filter.

I think my system sort of works, with just one problem :

Lets say I transmit two 1s with 50 0 apart (correspondingly 0 IQ values given to ADC) in my transmission file. I see random number 0s decoded at the receiver(which means there can be any number of them) between the two 1s. This is a problem as I cannot decode my symbols.

Is this a problem with clock drift and cannot be fixed with gnuradio ?

Do you recommend any other way to deal with it ?

Thanks,

Abhinav

 

Best regards,
Marcus

On 10/12/2015 06:00 PM, abhinav narain wrote:

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

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