PAM4 Downstream Transmission Mapping for FEC Burst Mitigation
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Solution Overview
Problem
Higher-rate PON channels, such as 50G and 100G, suffer from inter-symbol interference (ISI) due to chromatic dispersion and bandwidth limitations, leading to correlated noise and error bursts that degrade forward error correction (FEC) code performance, especially when using multi-level PAM modulation.
Innovation Solution
Implementing a specific interleaving and mapping scheme for FEC codewords in optical line terminals (OLTs) and network units (ONUs), where bits from the same codewords are assigned to both least significant bits (LSBs) and most significant bits (MSBs of PAM4 labels, with these labels being strategically positioned in the sequence to maximize error burst mitigation and maintain good FEC code performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multi-level PAM modulation is used to increase data rate, then productivity is improved, but reliability deteriorates due to error bursts from correlated noise
Solution Approach 1:
The patent applies bit-wise block interleaving that segments the transmission by distributing bits from different FEC codewords across different PAM4 symbols. Specifically, it interleaves bits from multiple codewords and maps them to different symbol positions, ensuring that error bursts affecting contiguous symbols do not concentrate errors in a single codeword, thus maintaining FEC performance at higher data rates
Solution Approach 2:
The patent introduces a new dimension to the interleaving process by differentiating between LSB and MSB mapping to separate subsets of PAM4 labels. This dimensional approach assigns bits from the same codeword to both LSBs and MSBs of different labels, creating additional separation in the symbol sequence and enhancing protection against error bursts while maintaining high data rate transmission
2Reliability
If bit-wise block interleaving is applied to mitigate error bursts in OOK-NRZ, then reliability is improved, but it becomes insufficient for PAM4 modulation
Solution Approach 1:
The patent modifies the interleaving parameters specifically for PAM4 modulation by changing the mapping strategy from simple sequential mapping to a scheme that separates LSB and MSB assignments. It uses different subsets of PAM4 labels for LSB and MSB mapping, with labels positioned N labels apart, adapting the interleaving parameters to the specific characteristics of PAM4 modulation to achieve effective error burst mitigation
3Reliability
If equalization is applied to mitigate ISI, then reliability is improved, but noise becomes coloured leading to correlated errors
Solution Approach 1:
The patent applies preliminary bit-wise block interleaving and strategic LSB/MSB mapping to PAM4 labels before transmission. This preliminary arrangement distributes bits from the same codeword across non-contiguous symbol positions, proactively preventing the concentration of errors that would result from correlated noise, thus maintaining FEC performance despite the presence of coloured noise from equalization
Data Source
AI summary
In one embodiment, an apparatus is configured to perform mapping an interleaved bit stream into a sequence of 4-level pulse amplitude modulation (PAM4) labels according to a mapping scheme, with a respective PAM4 label including a least significant bit (LSB) and a most significant bit (MSB) respectively corresponding to a bit in the interleaved bit stream; transmitting a sequence of PAM4 symbols generated based on the sequence of PAM4 labels to the ONUs; and wherein, bits from respective one of the N codewords are assigned to LSBs of a first subset of labels, and to MSBs of a second subset of labels, with the first and the second subset of labels being determined based on the interleaving scheme and the mapping scheme and respectively comprising labels located N labels apart in the sequence of PAM4 labels.