  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.
