I have confused question regarding the possibility of combination between QAM modulation and PAM in order to improve the spectral efficiency. Is that possible?
My question, what's about using the amplitude with that? For example, in case of combination with PAM modulation, we can have eight status instead of four, which are: 1+j, 1+j, -1+j and -1-j at the same time the four added status will be 1/2(1+j), 1/2(1+j), 1/2(-1+j) and 1/2(-1-j), where the amplitude is used to represent additional bits to transmit too.
Is that reasonable and logical to be used ? Is there any articles or books which explain that ?