Low-Complexity Soft-Decision Detection for Combating DFE Burst Errors in IM/DD Links

• Published in: Journal of lightwave technology Vol. 42; no. 5; pp. 1395 - 1408
• Main Authors: Wu, Kaiquan, Liga, Gabriele, Riani, Jamal, Alvarado, Alex
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.03.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithms; Complexity; Complexity theory; Decision feedback equalizers; decision-feedback equalizer; Detection algorithms; Direct detection; Error correction; Error detection; Forward error correction; High speed; intensity modulation; Likelihood ratio; Links; MAP algorithm; Performance degradation; Precoding; Pulse amplitude modulation; soft-decision decoding; soft-output Viterbi algorithm; Symbols; Viterbi algorithm; Viterbi algorithm detectors
• ISSN: 0733-8724; 1558-2213; 1558-2213
• DOI: 10.1109/JLT.2023.3324602

The deployment of non-binary pulse amplitude modulation (PAM) and soft decision (SD)-forward error correction (FEC) in future intensity-modulation (IM)/direct-detection (DD) links is inevitable. However, high-speed IM/DD links suffer from inter-symbol interference (ISI) due to bandwidth-limited hardware. Traditional approaches to mitigate the effects of ISI are filters and trellis-based algorithms targeting symbol-wise maximum a posteriori (MAP) detection. The former approach includes decision-feedback equalizer (DFE), and the latter includes Max-Log-MAP (MLM) and soft-output Viterbi algorithm (SOVA). Although DFE is easy to implement, it introduces error propagation (EP). Such burst errors distort the log-likelihood ratios (LLRs) required by SD-FEC, causing performance degradation. On the other hand, MLM and SOVA provide near-optimum performance, but their complexity is very high for high-order PAM. In this article, we consider a one-tap partial response channel model, which is relevant for high-speed IM/DD links. We propose to combine DFE with either MLM or SOVA in a low-complexity architecture. The key idea is to allow MLM or SOVA to detect only 3 typical DFE symbol errors, and use the detected error information to generate LLRs in a modified demapper. The proposed structure enables a tradeoff between complexity and performance: i) the complexity of MLM or SOVA is reduced and ii) the decoding penalty due to EP is mitigated. Compared to SOVA detection, the proposed scheme can achieve a significant complexity reduction of up to 94% for PAM-8 transmission. Simulation and experimental results show that the resulting SNR loss is roughly <inline-formula><tex-math notation="LaTeX">0.3\!\sim\! 0.4</tex-math></inline-formula> dB for PAM-4, and becomes marginal 0.18 dB for PAM-8.