Re: [Discuss-gnuradio] Distorted QPSK Constellation

[Tom Rondeau]

On Wed, Jan 14, 2015 at 8:50 AM, Brian Padalino <address@hidden> wrote:

It's a constellation plot for sure - says it right on the block in the
PNG, but it's all internally generated anyway.  There should be 1
perfect phase, then 3 out of phase points.  No noise - no
phase/frequency offset.  Timing shouldn't be an issue here even if it
were a normal plot.

I think the issue is no matched filter on the other side.  RRC
filtering won't look very appealing to the eye, but RC filtering
should match what you expect.

Try adding a matched RRC filter before you do the constellation.  You
should see the 4 samples/symbol and their trajectories as they
converge onto the 4 nice points.

Brian

Yes, indeed, Brian is correct. The modulator block uses an RRC pulse-shaping filter that also interpolate to some number of samples/symbol. So what you are seeing is due to the ISI (inter-symbol interference) as a result of the RRC filter. Applying a matched RRC filter will remove (most of) the ISI.

Tom

 

On Wed, Jan 14, 2015 at 2:51 AM, Martin Braun <address@hidden> wrote:
> On 01/14/2015 04:56 AM, Salman Dinani wrote:
>> Hi to all,
>>
>> I am a beginner in using GNU radio(GRC). I have made a DQPSK transmitter
>> using the blocks of GNU radio (attached Figure QPSK_1). when I connected
>> the constellation plot with it, It gave me the a very scattered
>> Constellation Plot Instead of four Clean Constellation Dots (attached
>> Figure QPSK_2). I have checked with all forms of signals but the
>> constellation remains distorted.(i.e. vector source, signal source etc.)
>>
>> Is it normal plot or am I missing some thing? Kindly help.
>
> My guess is yes, it's a normal plot, and you have no time sync in there.
>
> M
>
>
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