An article to understand the principle and composition of DSP and its advantages in optical communication

The advent of coherent transmission changed the evolution of optical transmission networks, and the introduction of Electronic digital signal processors (DSPs) became a key enabler for increasing the capacity of metro and long-haul WDM networks. In the past, while increases in wavelength capacity have relied on the speed evolution of light sources, modulators, and detectors, DSPs, and the associated complex modulation codes they implement, have been the main drivers for increasing network capacity. As optical transmission speeds reach over 400Gbit/s per wave, the increasing importance of coherent DSPs opens up the possibility of significant change for optics suppliers and the industry landscape.
  

What is DSP? DSP principle and composition
DSP is digital signal processing technology, and DSP chip refers to the chip that can realize digital signal processing technology. It is a fast and powerful microprocessor. The unique feature is that it can process data in real time. The interior of the DSP chip adopts a Harvard structure in which the program and data are separated, and has a special hardware multiplier, which can be used to quickly implement various digital signal processing algorithms. In the context of today’s digital age, DSP has become a basic device in the fields of communications, computers, and consumer electronics.
  

Principle of DSP Module
The DSP module processes the two polarized electrical signals output from the coherent receiver, and completes the restoration of the original signal after processing by the functional modules as shown in the figure below. The main task of DSP is to sample and quantize analog signals, convert analog signals into digital signals, remove chromatic dispersion and polarization mode dispersion in optical fiber links, complete carrier frequency offset estimation, carrier phase recovery and other functions.

DSP module functional block diagram
  

DSP module composition
The clock synchronization and ADC modules generally use interpolation filters to recover the digital clock. Since the symbol clock (T) and the ADC sampling clock (Ts) are independent of each other, in order to make the transmitted symbol clock (T) and the adjusted receiver sampling The clock (Ti) is synchronized, so the receiver’s symbol sampling instant must be modulated.
  

Using the interpolation filter as the main algorithm is a relatively mature digital clock recovery technology. In order to make the digital receiver output the correct model (synchronized with the symbol clock), that is, to adjust the sampling time of the receiver, the open-loop structure symbol is usually used. Clock synchronization algorithm.
  

Equalization and polarization demultiplexing module In order to deal with the interference between polarization signals and the non-ideality of the channel, polarization demultiplexing and equalization technology must be used for signal processing. First of all, the function of polarization demultiplexing is realized by using a filter with a specific structure, which is to cancel the interference between polarization signals, which is caused by a certain degree of deflection generated by each polarization signal during transmission. In addition, the adaptive equalization technology is to deal with the damage caused by the non-ideal channel characteristics in the transmission process of the optical fiber link. This linear damage is mainly caused by the first-order polarization mode dispersion and the optical fiber.

In order to demodulate the received signal correctly, the frequency offset estimation and phase recovery module needs to complete the frequency offset estimation of the carrier. The main reason is: because there is no feedback control for the local oscillator signal, the received signal will have a frequency deviation from the local oscillator source in the optical coherent receiver, so the method of frequency deviation estimation must be implemented in the receiver.
  

Why does coherent optical communication use DSP technology and what are the advantages?
The combination of coherent detection and DSP technology can perform carrier phase synchronization and polarization tracking in the electrical domain, which removes the two major obstacles of traditional coherent reception; the DSP-based coherent receiver has a simple structure, has hardware transparency, and can compensate for each other in the electrical domain. It eliminates all kinds of transmission damage, simplifies the transmission link, and reduces the transmission cost; supports multi-ary modulation format and polarization multiplexing, and realizes transmission with high spectral efficiency.
  

What are the disadvantages of using DSP technology and how to solve them?
Because DSP introduces DAC/ADC and algorithm, its power consumption must be higher than the traditional CDR chip based on analog technology. Both the module itself and the Panel thermal design of future switches are huge challenges. Therefore, its power management and low-power design technology have also become the focus of current research. In actual operation, the system is in idling or low-load state for a considerable part of the operating time, and the extra energy consumed by the system during these time periods can be avoided by low-power design measures.
  

The main entry point of low-power design is to reduce the relevant performance of the system reasonably to achieve low-power operation of the system according to the actual load of the system operation, on the premise of ensuring that the processing tasks are completed as required. In order to achieve this goal, it is necessary to implement a reliable low-performance operation mechanism in the system, to effectively monitor each component of the system and to adopt a reasonable strategy to manage the system power consumption.
  

Coherent optical communication has always been the commanding heights of optical communication technology. Adhering to the concept of an innovator in optical interconnection design, Yifeiyang formally invested in the development of coherent optical modules in early 2018, carried out strategic cooperation with the upstream supply chain openly, and optimized and innovated in low-power design and signal modulation models, and achieved significant results.
  

In order to start commercial use smoothly, Yifeiyang invited relevant domestic and foreign manufacturers to conduct joint tests on OTN transmission equipment, and achieved excellent results in terms of compatibility, service opening and transmission performance. Test experiments have also fully verified the excellent performance of the currently adopted silicon-based phase modulator chips and DSP chips. After the joint test, Yifeiyang has obtained formal orders for coherent optical modules from overseas customers.

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