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A Novel HARQ-CC Assisted SCMA Scheme

Man Wang, Zheng Shi, Yunfei Li, Xianda Wu, Weiqiang Tan, Xinrong Ye

TL;DR

The paper tackles reliability and latency challenges in 6G-style SCMA systems by proposing HARQ-CC-SCMA with both synchronous and asynchronous retransmission modes. It introduces two joint-detection strategies, Factor Graph Aggregation (FGA) and Log-Likelihood Ratio Combination (LLRC), to exploit information from erroneous packets across HARQ rounds. Simulation results show that FGA yields the best performance in synchronous mode, outperforming HARQ-IR-SCMA, while LLRC can outperform FGA at low SNR in asynchronous mode, revealing a complexity-reliability tradeoff. The approach enables joint detection before channel decoding (synchronous) or robust per-round LLR accumulation (asynchronous), offering flexible reliability enhancements for dense SCMA systems.

Abstract

This letter proposes a novel hybrid automatic repeat request with chase combining assisted sparse code multiple access (HARQ-CC-SCMA) scheme. Depending on whether the same superimposed packet are retransmitted, synchronous and asynchronous modes are considered for retransmissions. Moreover, factor graph aggregation (FGA) and Log-likelihood ratio combination (LLRC) are proposed for multi-user detection. Regarding FGA, a large-scale factor graph is constructed by combining all the received superimposed signals and message passing algorithm (MAP) is applied to calculate log-likelihood ratio (LLR). Whereas, owing to the same unsuccessful messages required to be retransmitted, LLRC adds up LLRs of erroneously received packets in previous HARQ rounds together with currently received packets for channel decoding and saves the LLRs for failed users. Finally, Monte Carlo simulations are preformed to show that FGA surpasses LLRC and HARQ with incremental redundancy (HARQ-IR) in synchronous mode. However, LLRC performs better than FGA at low signal-to-noise ratio (SNR) in asynchronous mode. This is because failed messages after the maximum allowable HARQ rounds in this mode can yield significant error propagation in low SNR regime.

A Novel HARQ-CC Assisted SCMA Scheme

TL;DR

The paper tackles reliability and latency challenges in 6G-style SCMA systems by proposing HARQ-CC-SCMA with both synchronous and asynchronous retransmission modes. It introduces two joint-detection strategies, Factor Graph Aggregation (FGA) and Log-Likelihood Ratio Combination (LLRC), to exploit information from erroneous packets across HARQ rounds. Simulation results show that FGA yields the best performance in synchronous mode, outperforming HARQ-IR-SCMA, while LLRC can outperform FGA at low SNR in asynchronous mode, revealing a complexity-reliability tradeoff. The approach enables joint detection before channel decoding (synchronous) or robust per-round LLR accumulation (asynchronous), offering flexible reliability enhancements for dense SCMA systems.

Abstract

This letter proposes a novel hybrid automatic repeat request with chase combining assisted sparse code multiple access (HARQ-CC-SCMA) scheme. Depending on whether the same superimposed packet are retransmitted, synchronous and asynchronous modes are considered for retransmissions. Moreover, factor graph aggregation (FGA) and Log-likelihood ratio combination (LLRC) are proposed for multi-user detection. Regarding FGA, a large-scale factor graph is constructed by combining all the received superimposed signals and message passing algorithm (MAP) is applied to calculate log-likelihood ratio (LLR). Whereas, owing to the same unsuccessful messages required to be retransmitted, LLRC adds up LLRs of erroneously received packets in previous HARQ rounds together with currently received packets for channel decoding and saves the LLRs for failed users. Finally, Monte Carlo simulations are preformed to show that FGA surpasses LLRC and HARQ with incremental redundancy (HARQ-IR) in synchronous mode. However, LLRC performs better than FGA at low signal-to-noise ratio (SNR) in asynchronous mode. This is because failed messages after the maximum allowable HARQ rounds in this mode can yield significant error propagation in low SNR regime.
Paper Structure (13 sections, 8 equations, 6 figures)

This paper contains 13 sections, 8 equations, 6 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Synchronous Transmission Mode
  4. Asynchronous Transmission Mode
  5. Proposed HARQ-CC-SCMA Scheme
  6. Synchronous Transmission Mode
  7. FGA
  8. LLRC
  9. Asynchronous Transmission Mode
  10. FGA
  11. LLRC
  12. Simulation Results
  13. Conclusion

Figures (6)

  • Figure 1: The system model of HARQ-CC-SCMA.
  • Figure 2: An example of factor graph for modeling HARQ-CC-SCMA in synchronous transmission mode.
  • Figure 3: An example of factor graph for modeling HARQ-CC-SCMA in asynchronous transmission mode.
  • Figure 4: BER of HARQ-assited SCMA schemes in synchronous mode.
  • Figure 5: BER of HARQ-assisted SCMA schemes in asynchronous mode.
  • ...and 1 more figures