256-QAM encodes 4 bits on the "phase" component and 4 on the quadrature one. They can be regarded as independent signals. Think about a sin() and a cos() being

256-QAM encodes 4 bits on the "phase" component and 4 on the quadrature one. They can be regarded as independent signals. Think about a sin() and a cos() being orthogonal to each other even for the same frequency.