Why change QAM constellation to π 2 rotation invariant constellation?

A: In digital TV signal transmission, the coherent carrier at the receiving end is extracted from the transmission signal. Due to the different layout of signal sets, different degrees of phase ambiguity will occur. The degree of phase confusion is related to the constellation. When the extracted coherent carrier has a phase shift of 90,180,270, it will inevitably lead to subsequent decoding errors. The main method to solve this problem is to apply the concept of difference to QAM modulation, so that the angle of constellation signal points depends on relative difference, not directly linked to the absolute value of angle. This QAM constellation which is not confused by coherent carrier phase is called π/2 rotation invariant QAM constellation.