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 <t...@trondeau.com> wrote: > On Mon, Apr 2, 2012 at 2:00 PM, Anh Duc Nguyen <ducn...@gmail.com> 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 <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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