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

[Anh Duc Nguyen]

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,

On Tue, Apr 3, 2012 at 1:51 AM, Tom Rondeau <address@hidden> wrote:

On Mon, Apr 2, 2012 at 2:00 PM, Anh Duc Nguyen <address@hidden> wrote:
> 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,

The amplitude has nothing to do with the phase. A small amplitude
might make the phase harder to determine because you are closer to
zero on the unit circle, so that noise could push you over the edge of
a decision boundary, but affecting the amplitude does _not_ change the
phase at all. The only way to do that would be to change I and Q
separately, which won't happen (there will be an IQ imbalance in the
hardware, but a) it's minor and b) it will not change fast over time).
So what you're suggesting isn't really a problem for real systems.

Tom

> 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
>
>