A question on the order of the processing stages for the psk receiver

GNU Radio
1

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,

2

On Mon, Apr 2, 2012 at 11:45 AM, Anh Duc N. [email protected]
wrote:

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 thedifferential 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

3

Dear Tom,

Thank you for your kind help, I appreciate it much. I am at the starting
point of studying GnuRadio and radio communication in general, so your
support is valuable to me.

With best regards,

4

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( jd_omega(n) - jd_omega(n-1) ) =
exp( jd_omega(n) ) * exp( -jd_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,