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As the line rate of WDM ultra-long-haul backbone networks is upgraded from 2.5G to 10G and from 10G to 40G, WDM transmission technology has been facing a series of physical constraints, and in the process of upgrading the line rate from 40G to 100G again, these physical constraints still exist, and the resultant transmission impairments are even more serious. Huawei's use of coherent transmission technology can effectively overcome the hazards of various physical effects on the transmission system and achieve reliable transmission of high-speed signals. This section describes the key technical challenges in coherent transmission, as well as the key modulation code types and reception techniques used for coherent transmission to overcome these challenges.
Coherent Transmission Fundamentals and Key Technologies
OptiX OSN 9800 coherent transmission adopts key advanced technologies such as ePDM-QPSK, ePDM-BPSK modulation code types and coherent reception to overcome the challenges of high-speed transmission system in terms of OSNR requirements, CD tolerance, PMD tolerance and nonlinearities and other transmission physical effects, and provides a solution of high-capacity and ultra-large bandwidth 100G and 40G coherent transmission system for transmission networks. This section addresses the challenges of transmission physical effects in terms of CD tolerance, PMD tolerance, and nonlinearity. This section introduces the basic principles of coherent transmission and key technologies.
The basic principle and signal processing process of Huawei's 100G coherent communication are shown in Figure 1. The transmitter uses ePDM-QPSK modulation, and the receiver uses coherent reception technology.
- Using a polarization beam splitter, the laser is divided into two perpendicular polarization directions, x and y.
- The optical signals in the x and y polarization directions are QPSK modulated. 112Gbit/s signal streams are converted into four 28Gbit/s signals through "serial-parallel" conversion.
- The polarization combiner combines the modulated optical signals in the x and y polarization directions into a single optical fiber for transmission.
- The receiving end separates the received signals into two perpendicular polarization directions, X and Y. The receiving end is then connected to the optical fiber.
- Coherent reception converts the optical signals in the X and Y polarization directions into current/voltage signals.
- ADC (Analog to Digital Converter) High-precision analog-to-digital conversion that turns current/voltage signals into 0101... digital code stream.
- DSP (Digital Signal Processing) High-speed digital processing removes dispersion, noise, non-linearity and other interfering factors and restores the 100G signal from the transmitter.
The basic principle of 40G and 100G signal coherent communication is basically the same, the difference is that 40G signal coherent communication transmitter uses ePDM-BPSK modulation.
The coherent transmission system involves several key technologies as follows:
ePDM-QPSK modulation technique
ePDM-QPSK (enhanced Polarization Division Multiplexing-Quadrature Phase Shift Keying) modulation, i.e., Polarization Multiplexing Quadrature Phase Shift Keying (PDM-QPSK) modulation, which firstly makes use of the orthogonal polarization characteristics of light to divide a beam of light into polarization components in the X/Y directions, and then in the QPSK modulation of the X/Y polarization components of the light.
The 112G bit/s service signal from the customer side is divided into four parallel 28G bit/s rate signals through serial-parallel conversion. Two of the signals are loaded on the two beams divided by the X component for modulation, and the other two signals are loaded on the two beams divided by the Y component for modulation.
The "0" and "1" information codewords of the two 28G bit/s binary signals are modulated so that the phase of the first beam has only two values of 0 and π, and the phase of the second beam has only two values of π/2 and 3π/2, and then the two beams are combined together. Combined together, the phase of this beam at each moment in time has only four possible values, π/4, 3π/4, 5π/4, and 7π/4, which correspond to 00, 01, 11, and 10 of the binary information code elements, respectively. ultimately, two subchannels are actually used inside the beam of the X-component, and the baud rate of each channel is 28G bit/s, while the total line rate is 56G bit/s.
It is customary to refer to the first light as the "inphase" component, or I channel, and the second light as the "quadrature" component, or Q channel.
- Through PDM, an optical signal is separated into two polarization directions, and then the signal is modulated into these two polarization directions. This is equivalent to a "1 divided into 2" processing of the data, the rate is reduced by half;
- Through the QPSK, a phase represents 2 digital bits, also equivalent to the data to do "1 into 2" processing, the rate is reduced by half;
- Above, 112G signal, in fact, the baud rate of data processing is only 100 ÷ 2 ÷ 2 = 28G Baud.
ePDM-QPSK is the best solution for 100G WDM transmission.
ePDM-BPSK Modulation Technology
ePDM-BPSK (enhanced Polarization Division Multiplexing-Binary Phase Shift Keying) modulation, i.e., polarization multiplexing two-phase shift keying modulation, is based on ePDM-QPSK technology, which reduces the four phases of QPSK, namely, 0, π/2, π, and 3π/2, to two phases, 0 and π, and then reduces them to two. are reduced to two phases 0 and π, as shown in Fig. 3.
The ePDM-BPSK technology uses two phases for data transmission, which has strong anti-interference capability and good OSNR tolerance, and can realize longer transmission distance.
ePDM-BPSK is the best solution for 40G wavelength division transmission.
Coherent reception technology
- ① The same direction of vibration;
- ② vibration frequency is the same;
- ③ the same phase or phase difference remains constant.
Coherent systems use coherent reception technology, which uses a laser of the same frequency as the laser at the transmitting end for coherence with the received optical signal, and then processes it through a synchronization circuit so that the phase at the receiving end remains the same as that at the transmitting end (in-phase), thus forming a coherent condition that recovers the amplitude, phase, and polarization state information from the received signal. In addition, the benefit of coherent reception is that it can provide higher OSNR than non-coherent systems, which is very advantageous for enhancing 40G/100G transmission distance.
High-speed analog-to-digital conversion (ADC) and digital signal processing (DSP) technologies
The traditional DCM compensation is to use and fiber dispersion coefficient of the opposite negative dispersion fiber to compensate for the essentially optical method; and coherent technology through the DSP to eliminate CD/PMD, is the use of electrical signal technology to solve the dispersion brought about by the signal aberration and time delay problems.
Compared with the direct demodulation and differential demodulation methods, the power of the local laser used in coherent detection is much larger than the optical power of the input optical signal, so the optical signal-to-noise ratio can be greatly improved. Coupled with the coherent detection technology to take full advantage of the powerful DSP to process the polarization mode multiplexed signals, can be compensated by subsequent digital signal processing and signal reconstruction, can be restored to the characteristics of the transmitted signal (polarization mode, amplitude, phase), substantially eliminating the transmission of fiber-optic transmission impairments brought about by the fiber, such as without the need to compensate for the dispersion of the line can be tolerated for several tens of thousands of ps / nm of CD.
High-Performance FEC Algorithm
Huawei's coherent transmission systems use high-performance hard verdict FEC (e.g., HFEC), second-generation hard verdict FEC (e.g., HFEC2), soft verdict FEC (e.g., SDFEC), and second-generation soft verdict FEC (e.g., SDFEC2) technologies to improve system OSNR tolerance.
- Huawei's hard verdict FEC and second-generation hard verdict FEC technologies have a net coding gain that is close to the error correction limit of current HFEC technologies.
- Huawei's soft-judgment FEC technology further improves the net coding gain compared to the traditional hard-judgment FEC solution. In addition to the same extremely low latency characteristics, it also achieves higher integration and lower power consumption. It can better support the application requirements of 100G high-speed optical transmission systems.
- Huawei's second-generation soft-judgment FEC technology is based on advanced LDPC coding/decoding technology with convolutional codes and a unique cascaded flow architecture, which greatly reduces implementation complexity. It improves the net coding gain relative to the soft-judgment FEC scheme, and offers higher integration and lower latency.
Single boards using different rates, code types, and FEC types cannot be docked.
The FEC overhead ratios are different for different rates, code types, and FEC types, as shown in Table 1 for the corresponding FEC overhead ratios for common rate, code type, and FEC type combinations.
|
Rate + Code Type |
FEC Type |
FEC Overhead Ratio |
|---|---|---|
|
100G QPSK |
SDFEC2 |
25% of the total FEC overhead |
|
100G QPSK (wDCM)a |
SDFEC2 |
25% SDFEC2 |
|
100G QPSK |
SDFEC |
20% SDFEC |
|
200G QPSK |
SDFEC |
11% SDFEC |
|
200G 16QAM |
SDFEC |
25% SDFEC |
| a: 100G QPSK (wDCM) and 100G QPSK are two different code types, e.g., Optical Module Extended C-band-Tunable Wavelength-ePDM-QPSK (SDFEC2, LH, T5G) -100G CFP and Extended C-band-Tunable Wavelength-ePDM-QPSK (SDFEC2, wDCM, LH, T62) -100G CFP. The single boards that use these two different code types are also not dockable with each other. | ||
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