On Tue, May 17, 2011 at 5:36 PM, Brook Lin <[email protected]> wrote:
> > Hi Tom, > > Thanks for your reply. Yes, you are right. Currently, I am seeing the > transmitter side. > > Now, I understand that the constellation after the root raised cosine > filter > is a kind of "fuzzy" due to the ISI. But if I observe the constellation > from > the data saved by chunks2symbols.dat, which is before the rrc filter, > shouldn't I see clear 8 dots without ISI? Because when I observe the > constellation for dqpsk before the rrc filter, there are four clear points, > which is attached along with this message. Do I understand right that there > is no ISI at the stage self.chunks2symbols = > gr.chunks_to_symbols_bc(rotated_const) but there is ISI at the stage > self.rrc_filter = gr.interp_fir_filter_ccf(self._samples_per_symbol, > self.rrc_taps)? So could you please help me explain that why I got four > clear points for dqpsk but "fuzzy" 8 points for qam8 before rrc filter and > after self.chunks2symbols? > > Thanks so much, > Brook > > http://old.nabble.com/file/p31639677/dqpsk_before_filter.png Oh, I see. It's because you are using the constellation plot widget. This tries to do some phase/freq lock on the constellation, but I'm not sure if it can properly lock to a QAM signal (almost certainly not; especially a non-square one). Try using a time scope and set it to use dots instead of lines. Tom > > Tom Rondeau wrote: > > > > On Mon, May 16, 2011 at 9:23 PM, Brook Lin <[email protected]> wrote: > > > >> > >> Hi All, > >> > >> I am plotting the constellation plot for qam8 before and after the root > >> raised cosine filter. Why I didn't get clear 8 dots constellation before > >> the > >> root raised cosine filter (plotted from the data saved by > >> chunks2symbols.dat)? For both constellation before (data saved by > >> chunks2symbols.dat) and after (data saved by rrc_filter.dat) the filter, > >> I > >> got 8 fuzzy dots. Can anyone tell me why is that? However, for dbpsk, I > >> got > >> clear two points before the filter, but fuzzy two points after the > >> filter. > >> > >> Thanks in advance, > >> Brook > >> > >> http://old.nabble.com/file/p31632653/qam8_before_filter.png > >> http://old.nabble.com/file/p31632653/qam8_after_filter.png > > > > > > > > I'm assuming you are just talking about in the transmitter? I so, then > > what > > you are seeing is ISI. A root raised cosine filter is NOT a Nyquist > > filter, > > but a raised cosine filter is (or approximately so, at least). We > > basically > > split the filtering responsibilities between the transmitter and > receiver. > > When you pass the signal through two root raised cosines, you end up with > > a > > raised cosine. So the filter in the receiver removes the ISI introduced > by > > the first filtering operation introduced in the transmitter. > > > > So the transmitter filters to reduce out of band transmission while the > > receiver uses a matched filter to remove the ISI and detect the symbol. > > > > If you pass the "fuzzy" signal through another RRC filter, it should > clear > > things up. > > > > Tom > > > > > > > >> > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > [email protected] > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > > -- > View this message in context: > http://old.nabble.com/qam-constellation-problem-tp31632653p31639677.html > Sent from the GnuRadio mailing list archive at Nabble.com. > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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