Re: [Discuss-gnuradio] A question on the order of the processing stages for the psk receiver

[Anh Duc Nguyen]

Dear Tom,

Thank you very much for your prompt reply. It is pretty new and makes sense to me. But I still have a little doubt, let me present it briefly as follows:

Denote omega(n) and d_omega(n) the phases of  psk and dpsk modulated signals, we have

omega(n) =   d_omega(n)   -  d_omega(n-1)

Then

exp(j* omega(n) ) = exp( j*d_omega(n)   -  j*d_omega(n-1) ) =  exp( j*d_omega(n) ) *  exp( -j*d_omega(n-1)) 

In the case of (I^2+Q^2) is constant over time, we have the psk modulated signal is determined through the dpsk modulated signal:

y(n) = d_y(n) * d_y**(n-1);    y(n), d_y(n) are complex numbers, d_y**(n) is the complex-conjugate of d_y(n)

And here is the point to do the differential decoding before symbol demapping, right?

But the question is what happens if the amplitude (I^2+Q^2) changes over time due to some transmission conditions, and in that case, do we have to normalize signal amplitudes, just consider the phases?

I am sorry if I disturb you with my lack of digital communications background ,

Thank you in advance,

On Mon, Apr 2, 2012 at 10:59 PM, Tom Rondeau <address@hidden> wrote:

On Mon, Apr 2, 2012 at 11:45 AM, Anh Duc Nguyen <address@hidden> wrote:
> Dear all,
>
> I have a question on the order of the processing stages for the psk
> receiver.
> As I have seen from some source code (e.g., dqpsk.py in earlier version of
> GnuRadio 3.4.2 backwards), the sequence of blocks for a psk receiver is:
>
> AGC -> RRC filter -> mpsk receiver (mpsk_receiver_cc) -> differential
> decoding -> symbol demapping (find closest constellation point) -> gray code
> decoding (optional) -> and so on.
>
> My confusion here is why the differential decoding is placed before the
> symbol demapping block, while as my usual thought the demapping should goes
> ahead of  the differential decoding stage.
>
> Can anyone explain that to me or just show me the related documents, i would
> be greatly grateful for it,
>
> With best regards,

We use the phasor of the symbol to do the differential decoding.
Instead of looking at the bits, the phasor looks at the phase change
between symbols. The major benefit of this is you don't have to care
at all about the starting point or any rotation in the constellation;
you just see a phase change. It's a much easier method to implement
and is equivalent to looking at the bits, which is what's always
taught in the text books.

Tom