Abstract
Sparse code multiple access (SCMA) is a promising
non-orthogonal multiple access scheme for enabling massive connectivity in next generation wireless networks. However, current
SCMA codebooks are designed with the same size, leading to inflexibility of user grouping and supporting diverse data rates. To
address this issue, we propose a variable modulation SCMA (VMSCMA) that allows users to employ codebooks with different
modulation orders. To guide the VM-SCMA design, a VM matrix
(VMM) that assigns modulation orders based on the SCMA
factor graph is first introduced. We formulate the VM-SCMA
design using the proposed average inverse product distance and
the asymptotic upper bound of sum-rate, and jointly optimize
the VMM, VM codebooks, power and codebook allocations. The
proposed VM-SCMA not only enables diverse date rates but
also supports different modulation order combinations for each
rate. Leveraging these distinct advantages, we further propose
an adaptive VM-SCMA (AVM-SCMA) scheme which adaptively
selects the rate and the corresponding VM codebooks to adapt
to the users’ channel conditions by maximizing the proposed
effective throughput. Simulation results show that the overall
designs are able to simultaneously achieve a high-level system
flexibility, enhanced error rate results, and significantly improved
throughput performance, when compared to conventional SCMA
schemes.