# Distorted QPSK Constellation **URL:** \ **Category:** GNU Radio\ **Created:** [January 14, 2015, 4:58am UTC](https://www.ruby-forum.com/t/distorted-qpsk-constellation/233804 "2015-01-14T04:58:12Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [January 14, 2015, 4:58am UTC](https://www.ruby-forum.com/t/distorted-qpsk-constellation/233804/1 "2015-01-14T04:58:12Z")
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. Regards Salman D. ---
**Author:** ![Salman\_D](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c0e974/32.png) [@Salman\_D](https://www.ruby-forum.com/u/Salman_D)\ **Post date:** [January 14, 2015, 8:55am UTC](https://www.ruby-forum.com/t/distorted-qpsk-constellation/233804/2 "2015-01-14T08:55:27Z")
On 01/14/2015 04:56 AM, Salman D. 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 ---
**Author:** ![Salman\_D](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c0e974/32.png) [@Salman\_D](https://www.ruby-forum.com/u/Salman_D)\ **Post date:** [January 14, 2015, 2:52pm UTC](https://www.ruby-forum.com/t/distorted-qpsk-constellation/233804/3 "2015-01-14T14:52:04Z")
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 ---
**Author:** ![Salman\_D](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/c0e974/32.png) [@Salman\_D](https://www.ruby-forum.com/u/Salman_D)\ **Post date:** [January 14, 2015, 3:24pm UTC](https://www.ruby-forum.com/t/distorted-qpsk-constellation/233804/4 "2015-01-14T15:24:19Z")
On Wed, Jan 14, 2015 at 8:50 AM, Brian P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 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