Re: [Discuss-gnuradio] qam constellation problem

[Brook Lin]

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 <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
> 
> 
> 
>>
> 
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> 
> 

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