With the development of optical communication systems with longer distance, larger capacity, and higher speed, especially when the single wave rate evolves from 40g to 100g or even super 100g, chromatic dispersion, nonlinear effects, polarization mode dispersion, and other transmission effects in optical fiber will seriously affect the further improvement of transmission rate and transmission distance. Therefore, industry experts continue to research and develop FEC code types with better performance to obtain higher net coding gain (NCG) and better error correction performance, to meet the needs of the rapid development of optical communication systems.
1、 Meaning and principle of FEC
FEC (forward error correction) is a method to increase the reliability of data communication. When the optical signal is disturbed during transmission, the receiving end may misjudge the “1″ signal as “0″ signal, or misjudge the “0″ signal as “1″ signal. Therefore, the FEC function forms the information code into a code with certain error correction capability on the channel encoder at the sending end, and the channel decoder at the receiving end decodes the received code. If the number of errors generated in the transmission is within the range of error correction capability (discontinuous errors), the decoder will locate and correct the errors to improve the quality of the signal.
2、 Two kinds of received signal processing methods of FEC
FEC can be divided into two categories: hard decision decoding and soft decision decoding. Hard decision decoding is a decoding method based on the traditional view of error-correcting code. The demodulator sends the decision result to the decoder, and the decoder uses the algebraic structure of the codeword to correct the error according to the decision result. Soft decision decoding contains more channel information than hard decision decoding. The decoder can make full use of this information through probability decoding so as to obtain a greater coding gain than hard decision decoding.
3、 Development history of FEC
FEC has experienced three generations in terms of time and performance. The first generation FEC adopts a hard decision block code. The typical representative is RS (255239), which has been written into ITU-T G.709 and ITU-T g.975 standards, and the codeword overhead is 6.69%. When the output ber=1e-13, its net coding gain is about 6dB. The second generation FEC adopts hard decision concatenated code, and comprehensively applies concatenation, interleaving, iterative decoding, and other technologies. The codeword overhead is still 6.69%. When the output ber=1e-15, its net coding gain is more than 8dB, which can support the long-distance transmission requirements of 10G and 40G systems. The third generation FEC adopts a soft decision, and the codeword overhead is 15%–20%. When the output ber=1e-15, the net coding gain reaches about 11db, which can support the long-distance transmission requirements of 100g or even super 100g systems.
4、Application of FEC and 100g optical module
The FEC function is used in high-speed optical modules such as 100g. Generally, when this function is turned on, the transmission distance of the high-speed optical module will be longer than when the FEC function is not turned on. For example, 100g optical modules can generally achieve transmission up to 80km. When the FEC function is turned on, the transmission distance through single-mode optical fibre can reach up to 90 km. However, due to the inevitable delay of some data packets in the process of error correction, not all high-speed optical modules are recommended to enable this function.
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