Dual-mode codeword position index based SCMA with transmit diversity

Codeword position index based sparse code multiple access (CPI-SCMA) is an effective scheme that expands codeword positions in the time domain and carries extra information by their index. This paper proposes a dual-mode CPI-SCMA (DCPI-SCMA) scheme to improve the error propagation caused by the mismatch between SCMA codewords and information bits. Furthermore, a DCPI-SCMA with transmit diversity scheme is proposed where index bits are repeatedly transmitted to obtain a transmit diversity gain; hence, a higher reliability can be achieved with a slight loss of spectral efficiency. An optimal approach for the codebook index pattern is derived where the decision of index bits is involved in the demodulation of SCMA. It can be seen from simulation results and system analysis that the proposed schemes achieve better bit error rate performance with a decrease in complexity and obtain better robustness and flexibility compared with the existing CPI-based SCMA schemes under the same spectral efficiency.