On Mon, May 16, 2011 at 9:23 PM, Brook Lin
<address@hidden> 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