Re: [Discuss-gnuradio] PSK/QAM Demodulation Issue

[Cinaed Simson]

m-psk for m=2, has 2 samples/symbol and log2(m)=1 - or 1 bit per symbol.

Also, the polyphase clock has an input of 4 samples and output of 1 sps.
It should have input of 2 sps and output of 1 sps.

Just change the sps everywhere to 2.

Also, note there are only 2 symbols in the symbol map of the
Constellation object so for sps equal to 2 the symmetry is 2 - or there
will be 2 clusters on a horizontal straight line.

Except when you use a tap in the Channel Model of 1+1j which will
probably rotate the line by 45 degrees to the horizontal.

In any case, this is known as bpsk - or your case dbpsk - differential
bpsk.

I would add the CMA Equalizer between the Polyphase clock and Costa Loop.

-- Cinaed



On 05/17/2017 08:59 AM, Mojtaba Mansour Abadi wrote:
> Hi Everyone,
> 
> The flowgraph I am implementing is simple and straight forward. I am
> modulating a set of data plus preamble. The modulations I am looking at
> are either PSK or QAM. After a block of "Channel Model" in the
> flowgraph, I want to demodulate the signal and obviously, for fair SNR
> values I should receive the same preamble and data values.
> 
> Now the problem is that when I use the deprecated "PSK/QAM Mod" and
> "PSK/QAM Demod", everything works fine. However, based on the examples
> in "/digital/demod" and "/digital/packet" shiped with
> gnuradio companion, when I try to use the "FLL Band-Edge" followed by
> "Correlation Estimator", "Polyphase Clock Sync", and "Costas Loop" to
> compensate the channel for PSK modulation, the results I am getting are
> not correct (see the attached picture and file).
> 
> It would be your kind if you could answer my questions please:
> 
> 1- What is the correct way of demodulating the received signal?
> 2- In the provided examples, no equalizer is used after "Polyphase Clock
> Sync", whereas in a tutorial available at Gnuradio website a "CMA
> Equaliser" is needed there. So which one is the correct one?
> 3- If I move to QAM modulation, the only thing I need to do would be
> replacing "Costas Loop" with a "Constellation Receiver", right?
> 
> Cheers.
> 
> Regards,
> Mansour.
> 
> https://www.linkedin.com/in/mojtaba-mansour-abadi-4311b451
> 
> 
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