Performance Improvement of Polar-Coded Offload Method for SD-FEC Complexity Reduction in Ultra-High-Bitrate Optical Fiber Transmission

As an offload method to reduce the complexity of soft-decision (SD) FEC decoders, we consider polar coding with a CRC-aided successive cancellation (SC) list decoder to enhance decoding performance. While applying CRC error detection to candidate paths generated by multiple SC list decoders can improve performance, two critical issues arise. First, because a single CRC frame must cover the outputs of multiple SC list decoders, the number of path combinations grows exponentially, making exhaustive CRC error detection infeasible. Second, in optical fiber transmission systems without CRC-based error detection, the additional overhead introduced by CRC cannot be neglected. To mitigate the exponential growth of path combinations, we propose a method that limits the number of candidate path combinations by preferentially performing CRC error detection on less-reliable ones. Furthermore, we investigate the dependence on CRC parity length to identify suitable CRC codes for the polar-coded offload method. Numerical evaluations show that the proposed method suppresses the error floor of offloaded bit sequences not protected by SD-FEC to approximately one-tenth of that observed with a conventional SC decoder. This enables the application of various hard-decision FEC and SD-FEC codes with different overheads, thereby enhancing flexibility. We also evaluate the assignment of CRC parity bits in the optical transport network layer, showing that physical-layer overhead can be avoided.