Codeword position index based sparse code multiple access (CPI-SCMA) is a novel variant of SCMA that employs the idea of index modulation (IM), which has been proposed recently as an effective alternative to the conventional SCMA system. In contrast to the conventional SCMA that directly maps the information bits of each user to a predefined $M$-point codebook, the data in CPI-SCMA is not only conveyed by the $M$-point codebook as in SCMA, but also by the indexes of the codeword positions. In this paper, we first propose a simplified detector termed as message passing algorithm aided index reliability detection (MPA-IRD) that is more suitable for practical implementation. Moreover, we investigate the spectral efficiency (SE) in CPI-SCMA, and two important features are identified to facilitate the design of the system. A tight approximation of the average block error probability (ABLEP) under the maximum likelihood detector (MLD) is also derived from studying the error rate performance of CPI-SCMA. Finally, to further improve the SE of CPI-SCMA, an enhanced version of CPI-SCMA termed as hybrid CPI-SCMA (HCPI-SCMA) is proposed. It is shown via both simulation, and analytical results that the proposed scheme not only achieves better error rate performance in the high signal-to-noise ratio (SNR) region but also improves the SE, and the level of robustness under channel estimation error compares to SCMA.

中文翻译：

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稀疏码多址系统码字位置索引调制设计

基于码字位置索引的稀疏码多址 (CPI-SCMA) 是 SCMA 的一种新变体，它采用了索引调制 (IM) 的思想，最近已提出作为传统 SCMA 系统的有效替代方案。与将每个用户的信息位直接映射到预定义的 $M$-point 码本的传统 SCMA 相比，CPI-SCMA 中的数据不仅像 SCMA 一样由 $M$-point 码本传送，而且通过码字位置的索引。在本文中，我们首先提出了一种更适合实际实现的简化检测器，称为消息传递算法辅助索引可靠性检测（MPA-IRD）。此外，我们研究了 CPI-SCMA 中的频谱效率 (SE)，并确定了两个重要特征以促进系统设计。最大似然检测器 (MLD) 下的平均块错误概率 (ABLEP) 的严格近似也源自研究 CPI-SCMA 的错误率性能。最后，为了进一步提高 CPI-SCMA 的 SE，提出了一种称为混合 CPI-SCMA (HCPI-SCMA) 的 CPI-SCMA 增强版本。通过仿真和分析结果表明，所提出的方案不仅在高信噪比（SNR）区域实现了更好的误码率性能，而且提高了 SE，并且在信道估计误差下的鲁棒性水平与到 SCMA。提出了称为混合 CPI-SCMA (HCPI-SCMA) 的 CPI-SCMA 的增强版本。通过仿真和分析结果表明，所提出的方案不仅在高信噪比（SNR）区域实现了更好的误码率性能，而且提高了 SE，并且在信道估计误差下的鲁棒性水平与到 SCMA。提出了称为混合 CPI-SCMA (HCPI-SCMA) 的 CPI-SCMA 的增强版本。通过仿真和分析结果表明，所提出的方案不仅在高信噪比（SNR）区域实现了更好的误码率性能，而且提高了 SE，并且在信道估计误差下的鲁棒性水平与到 SCMA。