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Normally, in the downlink Network-Coded Multiple Access (NCMA) system, the same power is allocated to different users. However, equal power allocation is unsuitable for some scenarios, such as when user devices have different Quality of Service (QoS) requirements. Hence, we study the power allocation in the downlink NCMA system in this paper, and propose a downlink Network-Coded Multiple Access with Diverse Power (NCMA-DP), wherein different amounts of power are allocated to different users. In terms of the Bit Error Rate (BER) of the multi-user decoder, and the number of packets required to correctly decode the message, the performance of the user with more allocated power is greatly improved compared to the Conventional NCMA (NCMA-C). Meanwhile, the performance of the user with less allocated power is still much better than NCMA-C. Furthermore, the overall throughput of NCMA-DP is greatly improved compared to that of NCMA-C. The simulation results demonstrate the remarkable performance of the proposed NCMA-DP.

Sparse code multiple access (SCMA) is one of the competitive non-orthogonal multiple access (NOMA) techniques for the next generation communication systems. In this paper, we put forward a simple and efficient design method named divided extended mother codebook (DEMC) to construct SCMA codebooks based on golden angle modulation (GAM) constellation points. First, we generate a vector defined by the GAM. Second, we design the extended mother codebook ( $\mathbf {EMC}$ ) by introducing the power and phase dependent constraints of symbols in multi-dimension codewords. The power constraints can ensure the power of each codeword is the same which leads to the optimal peak to average power ratio (PAPR) especially for the uplink channel. Third, we divide the $\mathbf {EMC}$ into several mother codebooks ( $\mathbf {MC}\text{s}$ ) to generate the constant bit rate (CBR) or variable bit rate (VBR) SCMA codebooks. The structure of the VBR SCMA codebooks is compatible with that of CBR. The VBR codebooks and CBR codebooks use the same factor graph, therefore, the users employed the VBR codebooks can also utilize the efficient message passing algorithm (MPA) for multi-user detection. Simulations reveal that the bit error rate (BER) performance of the proposed CBR DEMC-SCMA codebooks is outstanding with low complexity. The VBR DEMC-SCMA codebooks can flexibly satisfy the different service requirements, and the BER performance of the VBR DEMC-SCMA codebooks is close to each other though their modulation orders are various. This feature shows that the users can apply for a high order codebook to get a faster information transfer rate without increasing transmission power and bandwidth.