This paper studies the affine frequency division multiplexing (AFDM)-empowered sparse code multiple access (SCMA) system, referred to as AFDM-SCMA, for supporting massive connectivity in high-mobility environments. First, by placing the sparse codewords on the AFDM chirp subcarriers, the input-output (I/O) relation of AFDM-SCMA systems is presented. Next, we delve into the generalized receiver design, chirp rate selection, and error rate performance of the proposed AFDM-SCMA. The proposed AFDM-SCMA is shown to provide a general framework and subsume the existing OFDM-SCMA as a special case. Third, for efficient transceiver design, we further propose a class of sparse codebooks for simplifying the I/O relation, referred to as I/O relation-inspired codebook design in this paper. Building upon these codebooks, we propose a novel iterative detection and decoding scheme with linear minimum mean square error (LMMSE) estimator for both downlink and uplink channels based on orthogonal approximate message passing principles. Our numerical results demonstrate the superiority of the proposed AFDM-SCMA systems over OFDM-SCMA systems in terms of the error rate performance. We show that the proposed receiver can significantly enhance the error rate performance while reducing the detection complexity.

翻译：本文研究了用于支持高移动性环境下大规模连接的仿射频分复用（AFDM）赋能的稀疏码多址（SCMA）系统，称为AFDM-SCMA。首先，通过将稀疏码字放置在AFDM啁啾子载波上，给出了AFDM-SCMA系统的输入输出（I/O）关系。接着，我们深入探讨了所提AFDM-SCMA的通用接收机设计、啁啾速率选择以及误码率性能。研究表明，所提AFDM-SCMA提供了一个通用框架，并将现有的OFDM-SCMA作为特例包含在内。第三，为了设计高效的收发机，我们进一步提出了一类用于简化I/O关系的稀疏码本，本文称之为I/O关系启发的码本设计。基于这些码本，我们提出了一种基于正交近似消息传递原理、采用线性最小均方误差（LMMSE）估计器的、适用于下行和上行信道的新型迭代检测与解码方案。我们的数值结果表明，所提出的AFDM-SCMA系统在误码率性能方面优于OFDM-SCMA系统。我们证明，所提出的接收机能够在降低检测复杂度的同时，显著提升误码率性能。