# How to correct the phase offset for QPSK I-Q data

I have an I-Q data of QPSK modulation, the data is is smeared and I have to correct the phase offset that causes the smearing problem, how can I apply the Phased-locked loop or the Costas loop on this stage of I-Q data to correct the phase offset problem.

• This question is a bit too broad – you already know that you need a phase correction, so apply it. You don't tell us what your problem is, or at least in what framework you're working, etc. This is a bit like calling your uncle, who's a car mechanic and telling him "I want to build a car. I know I need to add a motor, but how do I do that?"; you will need to give us a lot more background. I'm pretty sure someone can help you - just give us more of a chance to do so :) – Marcus Müller Jun 13 '16 at 21:34
• Are you asking how to implement a Costas Loop as that would certainly track out an arbitrary phase offset? The other approach that may be easier depending on your implementation is to square your signal twice (^4), which will remove the modulation and leave you with a fixed carrier (at 4x your carrier frequency)- without the modulation present you can more simply measure your phase error as the error term into a PLL. – Dan Boschen Jun 14 '16 at 3:16