MATLAB: Does the CMA equalizer not restore a 4-ary QAM signal after a Multipath Rayleigh Fading Channel

Question

I have a Simulink model in which I generate random integers and modulate them in a 4-ary QAM fashion. Next, I pass the modulated signal through a multipath Rayleigh fading channel.

This channel introduces multipath effects, which an equalizer is often very good at repairing, and fading effects that affect the phase of the signal. However, when I pass the signal into the equalizer and examine the signal constellation, it does not look like the constellation of the original signal.

Answer 1

The CMA equalizer operates in a blind fashion without training data and attempts to "calm" the system by bringing the signal constellation to a circle. This technique works best with PSK modulations but will work for a 4-ary QAM modulation.

The problem of using the CMA equalizer in this set up is that it cannot resolve phase ambiguities introduced by the channel. It needs to be used in conjunction with differential modulation or another form of trained equalizer.

Therefore, adding either one of the above suggestions should solve the issue when designing a communications link utilizing the CMA equalizer.