How Does Transceiver Forward Error Correction (FEC) Work?

With the development of longer distance, larger capacity and higher speed of optical communication system, especially when the single wave rate evolves from 40G to 100G or even super 100G, the transmission effects such as chromatic dispersion, nonlinear effect and polarization mode dispersion in optical fiber will seriously affect the further improvement of transmission rate and transmission distance. For this reason, 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 high-speed development of optical communication systems.

1. Meaning and Principle of FEC

FEC (Forward Error Correction), also known as forward error correction, is a method to increase the confidence of data communication. When the optical signal is scrambled during transmission, the receiver may misjudge the “1” signal as a “0” signal or the “0” signal as a “1” signal. " signal. Therefore, the FEC function improves the quality of the signal by forming the information code into a code with certain error correction capability on the channel coder at the transmitter, and decoding the received code by the channel decoder at the receiver, so that if the number of errors generated in transmission is within the error correction capability (discontinuous errors), the decoder will locate and correct the errors.

2. Two Reception Signal Processing Methods of FEC

FEC can be divided into two categories: hard-judgment decoding and soft-judgment decoding, depending on how the received signal is processed. Hard-judgment decoding is a decoding method based on the traditional view of error correction codes, where the demodulator sends the judgment result to the decoder, which uses the algebraic structure of the code word to correct the errors according to the judgment result. Soft-judgment decoding contains more channel information than hard-judgment decoding, and the decoder can make full use of this information through probabilistic decoding to obtain greater coding gain than hard-judgment decoding.

3. Development History of FEC

FEC has gone through three generations in terms of time and performance. The first generation of FEC adopts hard-judgment cascade codes, typically represented by RS(255, 239), which has been written into ITU-T G.709 and ITU-T G.975 standards, with a code word overhead of 6.69% and a net coding gain of about 6dB when the output BER=1E-13. The second generation FEC adopts hard judgment cascade code, which integrates cascade, interleaving and iterative decoding techniques, and the code word overhead is still 6.69%, and the net coding gain is more than 8dB when the output BER=1E-15, which can support the long-distance transmission requirements of 10G and 40G systems. The third generation FEC adopts soft judgment, the code word overhead is 15%~20%, and when the output BER=1E-15, the net coding gain reaches about 11dB, which can support the long-distance transmission demand of 100G and even super 100G systems.

4. Application of FEC and 100G Optical Transceiver

The FEC function is used for high-rate optical transceivers, such as 100 Gbit/s optical transceivers. After this function is enabled, the transmission distance of the high-rate optical transceivers is longer than that when the FEC function is not enabled. For example, the 100G QSFP28 ZR4 optical transceiver can transmit up to 80KM in normal cases. When the FEC function is enabled, the transmission distance through single-mode optical fibers can reach up to 90KM. However, it is not recommended to enable this function for all high-rate optical transceivers because some data packets may be delayed during bit error correction. For example, if the 100G QSFP28 LR4 optical transceiver is used, you are not advised to enable the FEC function. In addition, the switch must support the FEC function. If the FEC function is enabled on optical transceiver A, the FEC function must also be enabled on optical transceiver B. Otherwise, the interface is not Up.