  The 5G air interface, namely, dynamic multiple access (MA) based on multiuser
superposition transmission (MUST) and orthogonal multiple access (OMA), may
require complicated scheduling and heavy signaling overhead. To address these
challenges, we propose a a unified MA scheme for future cellular networks,
which we refer to as structured multiuser superposition transmission (S-MUST).
In S-MUST, we apply complex power allocation coefficients (CPACs) over
multiuser legacy constellations to generate a composite constellation. In
particular, the in-phase (I) and quadrature (Q) components of the legacy
constellation of each user are separately multiplied by those of the CPACs. As
such, the CPACs offer an extra degree of freedom for multiplexing users and
guarantee fairness in symmetric broadcast channels. This new paradigm of
superposition coding allows us to design IQ separation at the user side, which
significantly reduces the decoding complexity without degrading performance.
Hence, it supports low-complexity frequency-selective scheduling that does not
entail dynamically switching between MUST and OMA. We further propose to
quantize the CPACs into complex numbers where I and Q components of each
quantized coefficient are primes, facilitating parallel interference
cancellation at each user via modulo operations, last but not least, we
generalize the design of S-MUST to exploit the capabilities of multiantenna
base stations. The proposed S-MUST exhibits an improved user fairness with
respect to conventional MUST (134% spectral efficiency enhancement) and a lower
system complexity compared with dynamically alternating MUST and OMA.
