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 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
