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 <t...@trondeau.com> wrote: > On Mon, Apr 2, 2012 at 11:45 AM, Anh Duc Nguyen <ducn...@gmail.com> 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 >
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