面向带相对强度噪声（RIN）的下一代强度调制直接检测（IM-DD）系统的低复杂度软判决对数似然比（LLR）计算方法
Low-complexity Soft-decision LLR Calculations for Next-generation IM-DD Systems with RIN
AI总结：
针对RIN主导的IM-DD系统，提出PAM-4/8的低复杂度分段线性LLR近似，可避免类AWGN假设带来的性能损失，匹配精确LLR的BER性能，降低了FEC相关的BER和光调制幅度损失。
AI中文摘要：
数据中心内部互连对更高速率的需求最终将要求高阶脉冲幅度调制（PAM）结合软判决（SD）前向纠错（FEC）。激光器相对强度噪声（RIN）是限制高速强度调制直接检测（IM-DD）系统性能的重要损伤噪声，因为它会引入具有信号相关噪声的信道。本文表明，对数似然比（LLR）的准确计算对RIN主导的IM-DD系统中SD-FEC解码器的性能至关重要。首先，我们证明假设信号独立的加性高斯白噪声（AWGN）统计量来计算LLR会导致显著的性能损失。作为替代方案，我们针对PAM-4和PAM-8提出了精确LLR的低复杂度分段线性近似。我们利用广义互信息表明，与类AWGN假设不同，我们的近似与使用精确LLR相比无性能损失。此外，我们通过SD解码的误码率（BER）性能验证了该分析，结果表明我们的近似与使用精确LLR达到的BER匹配。因此，我们的近似避免了使用扩展汉明码时高达7.2倍的BER损失，以及在相同目标BER下使用低密度奇偶校验码时高达2.41 dB的光调制幅度损失。
英文摘要：
The demand for higher speeds in intra-data center interconnects will eventually require high-order pulse amplitude modulation (PAM) combined with soft-decision (SD) forward error correction (FEC). The laser relative intensity noise (RIN) is an important noise impairment that limits the performance of high-speed intensity-modulation (IM) and direct-detection (DD) systems, as it induces a channel with signal-dependent noise. In this paper, we show that an accurate calculation of the log-likelihood ratios (LLRs) is critical for the performance of the SD-FEC decoder in RIN-dominated IM-DD systems. First, we show that assuming signal-independent additive white Gaussian noise (AWGN) statistics to compute LLRs results in a significant penalty in performance. As an alternative, we propose a low-complexity piecewise linear approximation of the exact LLRs for PAM-4 and PAM-8. We show using the generalized mutual information that our approximation results in no performance loss versus using exact LLRs, unlike the AWGN-like assumption. Furthermore, we validate the analysis with bit error rate (BER) performance of SD decoding. We show that our approximation matches the BERs achieved using exact LLRs. Therefore, our approximation avoids a BER penalty of up to 7.2 times using an extended Hamming code, and up to 2.41 dB of optical modulation amplitude penalty using low-density parity-check codes for the same target BER.
