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Enabling Sphere Decoding for SCMA
In this paper, we propose a reduced-complexity optimal modified sphere
decoding (MSD) detection scheme for SCMA. As SCMA systems are characterized by
a number of resource elements (REs) that are less than the number of the
supported users, the channel matrix is rank-deficient, and sphere decoding (SD)
cannot be directly applied. Inspired by the Tikhonov regularization, we
formulate a new full-rank detection problem that it is equivalent to the
original rank-deficient detection problem for constellation points with
constant modulus and an important subset of non-constant modulus
constellations. By exploiting the SCMA structure, the computational complexity
of MSD is reduced compared with the conventional SD. We also employ list MSD to
facilitate channel coding. Simulation results demonstrate that in uncoded SCMA
systems the proposed MSD achieves the performance of the optimal maximum
likelihood (ML) detection. Additionally, the proposed MSD benefits from a lower
average complexity compared with MPA.Comment: Accepted for publication in IEEE Communications Letter
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