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

[Marcus Müller]

Hi Abhinav,

the problem is that you're not doing BPSK, really. It's an Amplitude shift keying, if you want so.
Two things:
* you might want to consider what AGC2 does while you're transmitting zeros -- it will increase amplification until noise scales up to signal power. What you're getting after that will more or less be useless, unless your AGC is really set to be slow (I don't think so, according to the attack rate of 6/100).
* you're throwing a FLL at your signal -- which would be fine, if there was definitely a carrier. I don't see how that should work for a signal that's zero most of the time.
* Assuming the polyphase clock synth did work with such a signal, the resulting signal would still contain a symbol that was '0', and hence had random phase (the phase of the additive noise at the sampling time). Hence, the costas loop can't do anything reasonable about that.

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.

So, I think it might be best to explain what kind of system you're trying to build, so that we can understand that better!

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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