| Optical fiber communication is one of the three pillars of modern communication networks and is a very important means of communication.Compared with cable communication,optical fiber communication has the advantages of wide band,low loss,anti-electromagnetic interference,small size,light weight,low price,and so on.In optical links,signal transmission is closely related to the type of communication network where optical fiber communication is located.Selecting the appropriate optical link technology according to the traffic demand and transmission distance of the communication network where the optical link is located is an effective method to realize the large-scale application of optical links at a low cost to ensure communication quality.At present,backbone networks and metropolitan access networks have the characteristics of long transmission distances and great communication capacity,in which coherent optical transmission technology plays a major role.However,the network within and between data centers and some edge networks have short transmission distances,relatively low-capacity demand,but high communication port density.So,the intensity modulation direct detection(IMDD)structure with low cost is adopted.With the rapid growth of demand for multimedia and broadband services,the next-generation communication network requires more and more capacity.Especially for medium and short-distance access networks with high port density,more and more attention has been paid to the communication capacity bottleneck.In the future,there is a great potential demand for low cost and high capacity communication network technology,and the transceiver architecture,modulation and demodulation technology are closely related to the link capacity and cost,so it gradually becomes the focus of research.Larger optical fiber communication capacity can be achieved by increasing the number of optical cables,increasing the number of wavelength division multiplexing in a single fiber,using polarization multiplexing,improving the bandwidth of a single carrier signal,using high order modulation format,and so on.Among them,improving the capacity of single-carrier communication is the most fundamental way to improve capacity.The bandwidth of a single carrier signal is limited by the transceiver and other systems,while the existing IMDD structure is limited by the square-law detection characteristics of the photodetector(PD).Only the amplitude information of the optical link communication signal is retained,while the phase information is erased.Coherent optical communication can simultaneously retain signal intensity information and phase information,but its transceiver structure is more complex and the cost is higher than IMDD,and the coherent receiving structure has a higher requirement for the consistency of the optical wavelength of the receiving and sending terminals,which usually requires the devices to be used in pairs,which is not conducive to the standardization and universality of the equipment.The new transceiver architecture modulation and demodulation technology is between IMDD and coherent optical communication technology,which hopes to have the simple structure of IMDD and to use the intensity and phase dimension simultaneously.Based on the new type of transceiver architecture modulation and demodulation technology,this paper mainly studies the direct detection technology based on Kramers Kronig(KK)algorithm,which mainly considers sideband(SSB)signal,and also studies the signal-signal beat interference(SSBI)cancellation technique based on constructed signal cancellation and based on digital signal processing(DSP).The specific work of the paper is as follows:1.The system structure of direct detection technology and coherent optical communication technology in the existing communication network is briefly introduced,and their characteristics are described.On the basis of the two,the modem technology of the new transceiver architecture has the potential to discard the disadvantages and retain the advantages.This paper introduces different new transceiver architecture modulation and demodulation technologies.This paper mainly introduces the direct detection technology based on KK algorithm and compares it with traditional direct detection technology to verify the superiority of the new transceiver architecture modulation and demodulation technology in the same rate system performance.The results show that it has the potential to complement the existing system structure.2.In KK algorithm,the Signal must meet the Minimum Phase(MP)condition,and the Carrier Signal Power Ratio(CSPR)will affect this condition.This paper takes 16QAM as an example to study the effects of CSPR and Optical Signal Noise Ratio(OSNR),and proposes to use probabilistic shaping technology to change the energy distribution of constellation image to improve the performance of BER(Bit Error Ratio).The results show that probabilistic shaping can be used as an effective means to improve system performance in direct detection system based on KK receiver.3.In this paper,a data-aided DC recovery method is proposed to solve the problem that the KK algorithm requires a large number of complete DC components to participate in the calculation,which conflicts with the practical application of electrical amplifiers and other devices.Through the design of the overall amplitude of the auxiliary data series,the erased DC component can be obtained through the operation of the auxiliary data series,so that the complete photocurrent can participate in the calculation of the KK algorithm.The proposed DC recovery method is verified by simulation and experiment,and can show good accuracy in a wide range of CSPR and OSNR in back-to-back environment.Compared with the traditional DC scanning method,the BER performance of the system greatly reduces the computational complexity with very limited performance loss.4.In this paper,a new type of modulation and demodulation technique for constructing signal to eliminate SSBI is proposed,which can simultaneously depolarization multiplexing and eliminate SSBI.By using optical filters to construct branches that can obtain copies of SSBI,SSBI suppression can be eliminated.The optimal CSPR of the scheme falls near 0dB,which is near the theoretical optimum.Thanks to the cancellation structure,the performance of the two polarizations of the system can be consistently maintained at a good level.In addition,in this paper,the SSBI suppression elimination technology using DSP is improved on the basis of the traditional iterative calculation,which not only reduces the number of iterations required for system performance convergence,but also improves the BER performance. |
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