  Non-orthogonal multiple access (NOMA) is a promising technology which meets
the demands of massive connectivity in future wireless networks. Sparse code
multiple access (SCMA) is a popular code-domain NOMA technique. The
effectiveness of SCMA comes from: (1) the multi-dimensional sparse codebooks
offering high shaping gain and (2) sophisticated multi-user detection based on
message passing algorithm (MPA). The codebooks of the users play the main role
in determining the performance of SCMA system. This paper presents a framework
to design the codebooks by taking into account the entire system including the
SCMA encoder and the MPA-based detector. The symbol-error rate (SER) is
considered as the design criterion which needs to be minimized. Differential
evolution (DE) is used to carry out the minimization of the SER over the
codebooks. The simulation results are presented for various channel models.
