In this paper, we propose an uplink system which combines power-domain non-orthogonal multiple access (PD-NOMA) and sparse code multiple access (SCMA) together, in order to enable massive user connectivity. The receiver adopts successive interference cancellation (SIC) and iterative SCMA detection algorithm for decoding the superimposed signals. An optimization method of the code rate for the error-correcting channel codes is proposed by analyzing the extrinsic information transfer (EXIT) charts, so as to reach a trade-off between system convergence behavior and spectral efficiency. To evaluate the performance, a conventional SCMA system is used as comparison baseline. Numerical results show that the proposed system along with the optimization method achieves good bit error rate (BER) performance and significantly improves the spectral efficiency.