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,

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

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,

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,