How to perform a phase recovery for higher order modulation schemes My objective is to demodulate the signals with different modulation orders and schemes. In particular, I want to recover the phase first.
Let us say, my signal looks like in the constellation below
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The idea is to position the constellation in such a way that the largest excursions are at 45 degrees angle. In other words, IQ diagram must be shifted by some angle. This angle is determined by the average shift of all points from the reference axis (real or imaginary). The idea itself is coming from this post, where BPSK phase recovery is described.
My problem is that I still do not understand, how do we perform the phase recovery for the higher order modulation schemes. The idea of simply mirroring the constellation to one quadrant (as it works in the case of BPSK) does not seem correct to me and also does not work well. So how we can do it?
Thanks!
This post provides many details on implementing carrier recovery for a higher order QAM system (or even QPSK).
High modulation index PSK - carrier recovery
Using a PI loop filter where in addition to a simple accumulator, a direct (proportional) path is also summed in results in a 2nd order Type 2 tracking loop as required to track phase offsets to zero error under static frequency offset conditions. Symbol-aided synchronization (where a known pilot is used) is recommended for higher order QAM to minimize acquisition time. The coefficients of the loop filter are set to stabilize the carrier recovery loop and also set the appropriate loop BW.
See this post for further considerations on setting the loop BW properly:
Loop bandwidth for symbol timing recovery