 For Higher bit rates(M) the minimum Signal to Noise ratio (SNR) required by a QAM system with Error Correcting Codes is about 1.53 dB higher than minimum SNR required by a Gaussian source(>30% more transmitter power) as given in Shannon–Hartley theorem

where

This 1.53 dB difference is called the "shaping gap".Typically digital system will encode bits with uniform probability to maximize the entropy.Shaping code act as buffer between digital sources and modulator communication system.