Non-orthogonal multiple access with sub-constellation alignment

A non-orthogonal multiple access (NOMA) technique is presented where the information bits of a user are carried by the least significant bits (LSBs) of other users' symbols. It is shown that the user performance can be improved by a receiver combining that relies on the alignment of sub-constellation corresponding to the LSBs at user receiver. The rate outage analysis shows that the proposed technique exploits the full diversity of the system. Therefore, a significant performance boost is achieved despite the LSBs being allocated a small fraction of the transmit power. Error performance investigation under perfect and imperfect channel estimation is performed to demonstrate the benefits of the proposed scheme.