Shortened Codewords for PON Decoding Reliability
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Solution Overview
Problem
In passive optical networks (PONs), high bit error rates (BER) can render existing FEC codes unreliable, leading to communication failures and difficulties in dynamically switching to more suitable codes due to the need for two-way communication, which may not be possible when BER exceeds the correction capabilities of the current code.
Innovation Solution
The OLT instructs ONUs to transmit bursts with shortened codewords, which have a full set of parity bits and a partial set of information bits, allowing for more reliable decoding at high BERs and enabling the estimation of current BER to switch to a more suitable FEC code, effectively using a lower code rate for error correction without explicitly changing the FEC code.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If full-size codewords are used for transmission, then throughput is maximized, but reliability deteriorates when bit error rate exceeds the correction capability of the FEC code
Solution Approach 1:
The patent implements dynamic adjustment of codeword length based on channel conditions. The OLT monitors BER and dynamically switches between full-size codewords (for good channels) and shortened codewords (for poor channels), making the system adaptive to changing conditions rather than static
Solution Approach 2:
The patent changes the parameter of codeword length (specifically the number of information bits) to adapt to channel conditions. By reducing the information bit length while maintaining parity bit length, the code rate decreases, improving error correction capability when the channel quality deteriorates
2Reliability
If FEC code switching is implemented to adapt to channel conditions, then reliability improves, but device complexity increases due to the need for two-way communication protocols
Solution Approach 1:
The patent uses shortened codewords as an intermediary mechanism to maintain communication when full-size codeword decoding fails. Instead of requiring complex two-way negotiation to switch FEC codes, the system uses shortened codewords as a fallback that can be activated unilaterally by the OLT when BER is too high
Solution Approach 2:
The patent prepares shortened codeword templates in advance that contain only parity bits and a reduced information portion. These pre-configured shortened codewords can be immediately transmitted when channel conditions deteriorate, avoiding the need for complex real-time negotiation protocols
3Reliability
If shortened codewords with reduced information bits are used, then reliability improves for high BER channels, but throughput decreases
Solution Approach 1:
The system dynamically adjusts codeword length based on monitored BER. When BER is low, full-size codewords are used for maximum throughput. When BER exceeds the correction threshold, the system switches to shortened codewords to maintain reliability, and switches back when conditions improve
Data Source
AI summary
An upstream (US) optical line terminal (OLT) for a passive optical network having at least one downstream (DS) optical network unit (ONU). The OLT generates a trigger signal indicating a need to receive at least one US burst having a shortened codeword for a first forward error-correction (FEC) code. Based on the trigger signal, the OLT transmits a DS message instructing the ONU to transmit an US burst having a shortened codeword. The OLT receives and decodes the US burst having the shortened codeword using the first FEC code. During periods of high bit-error rate, the shortened codewords increase the ability of the OLT to decode the US bursts and keep communications from the ONU alive. The OLT can use the decoded US bursts to measure BER and, if appropriate, instruct the ONU to use a different FEC code.