  A new non-orthogonal multiple access scheme performing simultaneous
transmission to multiple users characterized by different signal-to-noise
ratios is proposed. Different users are multiplexed by storing their codewords
into a multiplexing matrix according to properly designed patterns and then
mapping the columns of the matrix onto the symbols of a higher-order
constellation. At the receiver, an interference cancellation algorithm is
employed in order to achieve a higher spectral efficiency than orthogonal user
multiplexing. Rate-Adaptive Constellation Expansion Multiple Access (RA-CEMA)
is an alternative to conventional superposition coding as a solution for
transmission on the degraded broadcast channel. It combines the benefits of an
increased spectral efficiency with the advantages of reusing the coding and
modulation schemes already used in contemporary communication systems, thereby
facilitating its adoption in standards.
