PAM4 Downstream Transmission Interleaving for Burst-Error 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
An apparatus and method for an Optical Line Terminal (OLT) and Optical Network Unit (ONU) that interleaves FEC codewords according to a predetermined scheme, mapping them into 4-level pulse amplitude modulation (PAM4) symbols, with specific assignment of bits to least significant bits (LSBs) and most significant bits (MSBs, ensuring maximal mitigation of error bursts and maintaining good FEC code performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multi-level PAM modulation is used to increase data rate to 100G, then productivity is improved, but reliability deteriorates due to error bursts from correlated noise
Solution Approach 1:
The patent applies segmentation by dividing the interleaving process into two independent stages: first interleaving FEC codewords across multiple symbols, then interleaving bits within each codeword across PAM4 labels. This two-stage segmentation effectively breaks up error bursts caused by correlated noise in PAM4 modulation, preventing consecutive errors from concentrating in single FEC codewords and thereby maintaining reliability at 100G data rates
Solution Approach 2:
The patent introduces a new dimension of interleaving by separating the treatment of codeword-level interleaving from bit-level interleaving within PAM4 labels. This dimensional separation allows independent optimization of each interleaving stage, with codeword interleaving handling symbol-level correlation and bit interleaving handling label-level correlation, thus resolving the reliability issue without sacrificing the high data rate capability
2Reliability
If bit-wise interleaving of FEC codewords is applied to mitigate error bursts, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent reduces device complexity through segmentation by dividing the interleaving function into two independent modules: a codeword interleaver that operates on FEC codewords and a bit interleaver that operates on bits within PAM4 labels. This modular segmentation allows each module to be optimized independently, simplifying the overall implementation while maintaining effective error burst mitigation
Solution Approach 2:
The patent applies preliminary action by performing codeword-level interleaving before PAM4 label mapping and bit-level interleaving. This preliminary arrangement of codewords and bits in an optimized sequence before modulation reduces the complexity of subsequent processing stages, as the interleaving structure is pre-established to counteract expected correlation effects in the channel
Data Source
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AI summary
The present invention discloses an apparatus for use by an OLT, communicatively connected to ONUs, comprising means for performing: interleaving a first number N of codewords according to a predetermined interleaving scheme, obtaining an interleaved bit stream, wherein, N is an even number; mapping the interleaved bit stream into a sequence of 4-level pulse amplitude modulation, PAM4, labels according to a predetermined mapping scheme, with a respective PAM4 label comprising 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; 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 predetermined interleaving scheme and the predetermined mapping scheme and respectively comprising labels located N labels apart in the sequence of PAM4 labels.