SNR, P1dB relationship

Can anyone explain this relation or point me to some books or papers discussing this. I always hear these explained separately, but my question is how does SNR change as an amplifier becomes compressed. As we saturate an amp does how does SNR respond? Thanks.

The P1dB is when your amplifier is starts to compress the output signal. Think about the information that you have in the amplitude of your signal (AM or OOK, or QAM modulations), once this signal is compressed you start to loos information and thus the SNR is degrading. It is happening also because your noise is being amplified, but the signal is already in compression (S/N is going down).