Hybrid Rate-Splitting and Sparse Code Multiple Access (RS-SCMA): Design and Performance

TL;DR

This paper introduces RS-SCMA, a SISO downlink framework that unifies rate-splitting with sparse code multiple access by superposing an $M$-QAM common stream with SCMA-encoded private streams, controlled by a tunable split factor $α$. It develops two receivers featuring soft SIC and MPA-based detection, and derives a unified sum-rate expression $R_{RS-SCMA}(α)$ that accounts for perfect/imperfect SIC and CSIR, along with finite-alphabet constraints. The authors provide a comprehensive rate analysis (exact and Jensen-based lower bounds) for joint and dominant-phase transmissions, and quantify the overloading and spectral efficiency through phase-specific factors. Simulations show RS-SCMA outperforms conventional SCMA and MC-RSMA in BER and sum-rate, with $α$ enabling a practical trade-off between throughput and reliability, and demonstrate robustness to channel estimation errors; results suggest promising applicability to future 6G-like systems, with future work targeting MIMO extensions and joint optimization of $α$ and power under imperfect CSIT.

Abstract

This paper proposes, for the first time, a hybrid multiple access framework that integrates the principles of rate-splitting (RS) and sparse code multiple access (SCMA) in an SISO downlink scenario. The proposed scheme, termed RS-SCMA, unifies the powerful interference management capability of rate-splitting multiple access (RSMA) with the near-optimal multiuser detection of SCMA. A key feature of RS-SCMA is a tunable splitting factor $α$, which governs the allocation between the generic $M$-ary modulated common messages and SCMA-encoded private messages. This enables dynamic control over the fundamental trade-off between system sum-rate, bit error rate (BER), and the overloading factor. We develop novel transmitter and receiver architectures based on soft successive interference cancellation (SIC), incorporating message passing algorithm (MPA) detection and soft-symbol reconstruction. Furthermore, a unified analytical expression for the achievable sum-rate is derived as a function of the splitting factor $α$. The performance of the proposed RS-SCMA system is evaluated in terms of both BER and sum-rate. Simulation results confirm the superiority of RS-SCMA over conventional SCMA and multi-carrier RSMA, demonstrating its scalability and robustness even in the presence of channel estimation errors.

Figures (16)

• Figure 1: Factor graph of $J=6$ users and $K=4$ subcarriers with $d_v=2$ and $d_f=3$.
• Figure 2: Structure of an $6\times 4$ SCMA codebook set.
• Figure 3: Downlink MC-RSMA system.
• Figure 4: A typical user-subcarrier matching for a MC-RSMA downlink system.
• Figure 5: Proposed downlink RS-SCMA system architecture for $J$ users over $K$ subcarriers. The common messages are transmitted using $M$-QAM modulation, while private messages are SCMA-encoded. The receiver shown corresponds to Rx-1, which performs soft SIC using LLRs obtained directly from the QAM demodulator.

Theorems & Definitions (1)

• Example 1