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synchronization of quadrature signals?

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Junior Member level 1
Jun 12, 2010
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I have a question concerning phase synchronization for QPSK signals. I am having a hard time finding information specifically on how you synchronize a quadrature signal. Meaning I have found a ton of resources on how you can use a PLL to lock onto a sinwave but if you have both a sin and cosine wave how can you perform the synchronization? I understand that if you were able to obtain synchronization to either the sine or cosine wave then it would be easy to generate a local oscillator that is 90 degree out of phase then demodulate the signal that way, but how would you lock on to either the sin or cosine wave in the first place if there is another signal 90 degree out of phase that is at the same frequency?
I have tried looking for information on this in a few books such as “Synchronization Techniques for Digital Receivers” by Mengali and “digital clokc for synchronization and communication” by masami kihara. But they don’t see to answer my question. If anyone could help me out or at least point me to a resource that would clear this issue up for me I would appreciate it.

Hi dionysian,

There is no spectral line at the carrier freuency in QPSK. For QPSK, the more common methods are:
- 4th power loop: the idea is to obtain a spectral line at the 4th harmonic using a 4th order ninlinearity. This can be accomplished squaring the signal, filtering and squaring again. This component can be used for locking a PLL.
- Costas loop for QPSK (the more quoted is for BPSK, but there is a variant specifically for QPSK)
There is another method (remodulator) without advantages over these.
These are described in Floyd Gardner's book "Phaselock Techniques" (there are al least two editions).
The sublect of carrier recovery (carrier synchronization) is well treated in Proakis and Salehi's "Digital Communications (5th Ed.)" an in other communications books as well.


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