G Technical Revolution: Error Correction Coding Technology

The G optical transmission technology based on digital coherent receiving PM-QPSK modulation has a milestone in the long-distance optical transmission technology history. The error correction coding technology can be used to further improve the system performance when modulation, detection, balancing, and multiplexing cannot meet the system transmission performance requirements. To achieve the current transmission error rate of 10 Gb OOK on the existing line, the GB transceiver must increase the gain by at least 10 dB, the coherent detection PM-QPSK modulation combined with the current electric Balancing Compensation Technology can provide up to 7 dB gain, additional 3 dB gain needs to be provided by FEC. This requires that the FEC encoding Net Gain (NCG: Net Coding Gain) of the GB transmission system be above 11 dB.

1. Error Correction Coding Technology

Error Correction encoding is a channel code that manually adds redundant bit information according to certain encoding rules to facilitate error detection and correction at the receiving end. To be accurate, the original code word is transformed into a code word with a certain degree of surplus according to certain rules, and there is a certain correspondence between the code elements of each code word. The process of establishing a link is called encoding. After the code word arrives at the receiving end, it is verified by the encoding rules. If there is no error, the original rule must be satisfied, otherwise it will not be satisfied; this is enough to determine whether there is an error in the transmission or access process. When the error cannot be met, the error location should be determined and corrected according to certain rules within the error correction capability.

The performance evaluation indicators of Error Correction encoding include ① encoding gain (unit: dB). The minimum signal-to-noise ratio difference is generally described when the error correction algorithm is enabled or disabled, and is transmitted back to back without error code, the difference value is the improvement of the signal-to-noise ratio required by the error correction encoding processing gain. ② encoding Overhead: Also known as the redundancy ratio, that is, the ratio of the increased encoding length to the length of the pre-encoding code, usually expressed as a percentage. For example, the encoding overhead of RS (255,239) is (255-239)/239 = 6.69%; ③ implementation complexity: Generally, it is described by the number of physical logical doors or the number of software multiplication operations; ④ processing latency: it is related to the adopted algorithm and the structure implemented by the algorithm. The higher the parallel capability of the algorithm, the lower the processing latency.

The common error correction coding for optical transmission goes through three generations: the first generation is the basic encoding represented by RS and BCH. The coding gain is about 6 dB, and the coding overhead is about 7%; the second generation is the link Encoding Based on RS and BCH. The encoding gain is around 9 dB. For details, see G.975.1. The third generation is the soft decision iterative encoding represented by low-density inspection (BCH) and Turbo, the coding gain is about 12 dB, and the coding overhead is more than 15%.

Low Density Parity Check Code. dr. Gallager proposed a linear grouping code with a sparse validation matrix in 1963, which not only approaches the Shannon gain, but also has low decoding complexity, short processing latency, and flexible structure, it is a hot topic in the field of channel coding in recent years. It has been widely used in fields such as deep space communication, optical fiber communication, satellite digital video and audio broadcast. In the future, the fourth generation of error code correction will be the product of the fusion of modulation technology and low-density modulation technology.

The error correction encoding technology can jump out of the limitations of the transmission physical layer and compensate all physical transmission damages at the logic layer, especially the compensation for the influence of non-linear effects. The higher the encoding gain of the Error Correction Code, the lower the requirement on the incoming optical power and the lower the OSNR requirement of the receiver at the same transmission distance. On the other hand, the smaller the OSNR during optical signal transmission, the smaller the variation of the core optical power intensity, and the smaller the fluctuation of the core refractive index, the less obvious the influence of the nonlinear effect.

2. GB soft decision Correction Technology

In addition to the new encoding algorithm, soft decision can also improve the FEC encoding gain. Soft judgments are relative to hard judgments and are not necessarily associated with specific Error Correction Codes or subsequent algorithms. The difference between hardware and software judgments lies in the bit digits used for signal quantification. The hard decision uses the threshold value as the criterion to determine the input signal arbitrarily. The soft decision uses the threshold value as a reference to guess the input signal and declare the credibility of the guess. The soft decision does not make a decision. It only provides information on speculation and credibility, so that subsequent algorithms (such as Viterbi) can be further processed and comprehensively determined based on other information.

Figure 1: Soft Decision of PSK Modulation

For a single-bit decision, the difference in hardware and software judgments is represented by the number of BITs used for signal quantization in physical implementation. The number of bits quantified by hard decision is one bit. The decision result is not "0" or "1", and there is no room for maneuver. In Soft Decision, multiple bits are used to quantify the signal. One bit is the information of speculation, and the other bit provides the credibility information of the prediction.

The following example uses the symbol decision of the PSK modulation as shown in 1: Since the PSK modulation contains four phase states (a Constellation Map with four vertices), each carrier symbol carries 2bit information, the I (In-phase) and Q (Quadrature-phase) Dimensions carry 1 bit, in each dimension, the hard decision only determines "0" or "1" based on the "Decision threshold; soft judgments also provide a set of "Confidence threshold" based on the "Decision threshold" of hard judgments ", the three reference values in the "confidence level threshold" in the figure divide the space after the "Decision threshold", "0", and "1" into four regions based on the Decision reliability probability, 2bit is required to distinguish the region. Therefore, the information provided by the soft decision to the subsequent error correction algorithm is the 1-bit decision value + 2-bit confidence level information.

The Reliability Information provided by the soft decision can further improve the FEC encoding gain. Generally, when the same overhead and encoding algorithm are used, compared with hard decision, soft decision can improve the encoding gain by more than 1.1dB. For G optical transmission systems that are subject to nonlinear effects, the 1dB error correction coding gain improves the system transmission performance much higher than the attenuation or dispersion limited optical transmission systems. According to the 60% G Test Results of China Mobile and China Telecom and the G benchmark, the transmission distance of G.655 optical fiber is 6 more than that of hard decision, and the transmission distance is increased.

3. Gbit/s Error Correction Coding Technology

The 13% G code for Error Correction of beacon fire uses 7% soft-decision Low-Density Parity Check Code (Low-Density Parity Check Code), supplemented by EFEC code, and is placed in ASIC and framer respectively (figure 2 ). Among them, 7% of the hard decision error codes are the second-level chaincodes defined by G.975.1. This combination actually forms a third-level chaincodes.

Figure 2: Gbit/s Error Correction Code

The third-level chain code encoding is used because of the extremely powerful burst error correction capability of the low-frequency error codes of e-2 to less than 1e-5, however, due to loops and deadlocks in the decoding process, low-level error codes cannot be reduced to less than 1e-12. The third-level link hard-judgment Error Correction Code defined by G.975.1 is used externally to eliminate the influence of the "error code flat layer" of the low-level checksum codes. In layman's terms, the internal 13% soft-decision-encoding function is a large-scale artillery-type "Surface Attack", and the external 7% hard-decision-correction code is a sniper rifle-type "Point clearing ".

The Gbit/s Error Correction encoding method of beacon fire uses the error correction capability of the Low-Density bits for large-sized codes and the external hard-coded Error Correction Code to eliminate the impact of "code leveling layer, on the other hand, the high gain of mature commercial hard decision Correction Code 7% reduces the complexity, power consumption, and latency of the low-cost decoding algorithm as much as possible.

4.g features of beacon fire

The 100g otn product of beacon fire supports the following features: 96 * 2600Km GB ultra-large system capacity, no-power relay ultra-long transmission distance, NCG> 11.5dB super-strong Error Correction Code, ultra-high dispersion margin, less than 50 ms ultra-fast protection recovery. To further reduce the transmission cost per bit and continuously improve the network capacity, beacon is developing a 400G wavelength division transmission system, which provides two solutions for long-distance trunk transmission and Metro applications, meet the urgent requirements of mobile in terms of network resizing and investment income.