# Questions with the blocks inside PSK Mod/Demod blocks

**URL:** <https://www.ruby-forum.com/t/questions-with-the-blocks-inside-psk-mod-demod-blocks/233300>\
**Category:** GNU Radio\
**Created:** [December 2, 2014, 5:05pm UTC](https://www.ruby-forum.com/t/questions-with-the-blocks-inside-psk-mod-demod-blocks/233300 "2014-12-02T17:05:49Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**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:** [December 2, 2014, 5:05pm UTC](https://www.ruby-forum.com/t/questions-with-the-blocks-inside-psk-mod-demod-blocks/233300/1 "2014-12-02T17:05:49Z")

</div>

* * *

Discuss-gnuradio mailing list  
removed\_email\_address@domain.invalid  
[https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

---

<div class="post-metadata">

**Author:** ![Yi\_Chu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/919ad9/32.png) [@Yi\_Chu](https://www.ruby-forum.com/u/Yi_Chu)\
**Post date:** [December 5, 2014, 3:38pm UTC](https://www.ruby-forum.com/t/questions-with-the-blocks-inside-psk-mod-demod-blocks/233300/2 "2014-12-05T15:38:25Z")

</div>

On Tue, Dec 2, 2014 at 11:03 AM, Yi Chu [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Hello everyone,
> 
> I am working on the GNURadio simulation of a 3-hop system using  
> non-differential QPSK: source generates QPSK-modulated signal and sends  
> it to relay, relay performs certain operation on the received complex  
> samples and sends them to the receiver, and the receiver decodes the data.  
> The flow graph is as below and I attached the .grc file.

​

> same), then the signal can be correctly decoded…
> 
> Does anyone have any ideas on this problem? Any advice are appreciated!
> 
> Many thanks and best regards  
> Yi

The ISI is from the transmit pulse shaping filter, a root raised cosine.  
The RRC filter introduces ISI, which is what you see there. You need to  
pass it through a second RRC filter to remove the ISI.

The problem with non-differential coding is that QPSK has a 4-way phase  
ambiguity and nothing in your receiver path is able to line up the  
received  
symbols in phase to how they were transmitted. It simply locks the  
points  
to the constellation without regard for which symbol is where. You might  
transmit a QPSK with ‘11’ in the first quadrant, but when you receive  
it,  
where it locks might have that ‘11’ in any one of the four quadrants of  
the  
complex plane. Differential encoding doesn’t care about that absolute  
placement in the plane, just the change in phase, which is invariant  
with  
the rotation.

Tom