Tailored LDPC Code Design for Energy-Efficient Uplink SCMA Systems

Abstract

Sparse Code Multiple Access (SCMA), a code-domain non-orthogonal multiple access (NOMA) scheme, has gained significant attention for enhancing system robustness and supporting more users, making it ideal for uplink transmission. This paper proposes an energy-efficient uplink SCMA system that reduces overall energy consumption by dynamically adapting the code rate for individual users. LDPC codes, known for their near-capacity performance, are widely used in SCMA systems. However, when transmit power is reduced to improve energy efficiency, LDPC decoding becomes the bottleneck, which requires a higher SNR threshold for successful decoding. To address this, the paper extends the single-user LDPC design methodology to multi-user scenarios by incorporating the CSI of other users. A two-stage LDPC design method is proposed that integrates joint detection and decoding (JDD) with protograph extrinsic information transfer (P-EXIT) chart analysis, enhancing energy efficiency in the uplink SCMA system. A simplified method focusing on SCMA codebook characteristics is introduced to reduce design complexity due to large CSI combinations. Simulation results demonstrate that the proposed design improves overall energy efficiency by more than 20% compared with existing SCMA systems.