Sparse code multiple access (SCMA) is a nonorthogonal codebook based multiple access scheme, proposed to cope with the heterogeneous and challenging performance requirements for mission critical communication and massive machine-type communication in the fifth generation wireless system. In SCMA the codebooks are the main sources of performance improvement, which are generated based on the mother constellation. In this paper, a systematic sub-optimal approach of designing the mother constellation is proposed and a unified metric is derived to obtain the optimum codebooks using a specific mother constellation. It is demonstrated that the new mother constellation outperforms the existing mother constellations in terms of bit error rate (BER) performance. Moreover, it is shown that the optimum codebook obtained based on the unified metric can guarantee the optimum BER performance of the system.