| With the introduction of the "Internet +" and "Digital China" strategies in recent years,a series of new services with high bandwidth and low latency have emerged,such as the Industrial Internet,driverless cars and short video,etc.Under the background of 5G interconnection of all things and large-scale commercial use of 5G networks,Internet traffic will continue to grow,which will inevitably promote the development of optical networks in the direction of increasing speed and increasing communication capacity.With the rapid development of informatization and the new service,Optical Fiber Communication Network is faced with increasing requirements such as transmission capacity,spectrum utilization and flexibility.The high-order modulation format is the mature technology to improve the spectrum efficiency of the system.For example,the16 QAM format of polarization multiplexing(PDM-16QAM)realizes 100Gbit/s transmission at Baud rate of 12.5G and theoretical loss of about 4d B relative to Binary Phase Shift Keying(BPSK).When the traditional IQ modulator generates QAM signal,the real and imaginary need multi-level modulation,and its modulation speed will be limited by the performance of electric amplifier and the bandwidth of Digital to Analog Converter(DAC).One of the methods to solve this problem is Photonic Vector Modulation.Due to the excessive device loss caused by the technology level,it is difficult to realize all-optical processing.Therefore,there are relatively few studies on vector generation and demodulation of data information directly in the optical domain in the past years.However,with the improvement of technology level and the demand for higher communication rate,Photonic Vector Modulation technology can simplify the complex vector optical signal modulation,and solve the problem of bandwidth limitation of electronic devices to a certain extent.Therefore,the research on QAM signal generation based on Photonic Vector Modulation and millimeter wave signal generation based on Vector Synthesis is of great significance to realize high-speed and high-capacity communication in the future.This thesis studies the principles of generating QAM signals in the optical domain,such as generating optical QAM signals based on single IQ modulator,generating optical QAM signals based on cascaded IQ modulator,generating high-order QAM signals based on Photonic Vector Synthesis.Secondly,the research of directly generating vector millimeter-wave signals based on the principle of Photonic Vector Synthesis under the structure of cascaded modulator is carried out.Finally,the compensation algorithm of digital signal processing in optical communication is studied.Through the improvement and optimization of the algorithm,the better compensation of phase noise is realized.The main work and innovations are as follows:(1)The scheme of generating Probabilistic Shaping(PS)-16 QAM signal without DAC device based on the architecture of dual-cascade IQ modulator is proposed and proved by numerical simulation.In this scheme,the non-uniform 4QAM signal located in the first quadrant of the complex plane is generated by using the distribution matcher(DM)and the first IQ modulator,then the non-uniform 4QAM signal is rotated by using the second IQ modulator,and finally the PS-16 QAM signal is generated.This scheme does not need a DAC device,and only needs to map two of the four input bit streams into the specific bit distribution through DM to realize the generation of arbitrary information entropy PS-16 QAM signal,which reduces the problem of bandwidth limitation of electronic devices at transmitter to a certain extent,and can realize higher rate modulation.(2)Based on the principle of vector synthesis,an optical-carrier terahertz vector Quadrature Phase Shift Keying(QPSK)signal generation scheme based on On-Off Keying(OOK)modulation is proposed.In this scheme,a pair of optical octave sidebands are generated by Double Parallel Mach Zehnder Modulator(DP-MZM),and then two independent OOK baseband signals are modulated to two polarization states of octave sideband by two intensity modulators.Two polarization signals are superimposed after phase and amplitude adjustment,the superimposed optical signal undergoes photoelectric conversion at the terminal after power amplification and optical fiber transmission,thereby the generation of terahertz QPSK signal is realized.The scheme has the characteristics of not needing precoding technology and digital-to-analog converter,thereby reducing signal processing complexity and system cost.(3)A self-adaptive phase recovery algorithm for probabilistic shaped signals based on K-means clustering is proposed.In this algorithm,K-means clustering is used to relocate the modulus radius of the enlarged constellation points after probabilistic shaped,and the enlarged constellation points are standardized according to the positioning results,then the feedforward carrier phase recovery algorithm is used to compensate the phase noise.The algorithm combines K-means clustering with feedforward carrier phase recovery algorithm to solve the problem of relative amplification of constellation points after normalization of probabilistic shaped signal,and the self-adaptive recovery of signal phase of probabilistic shaped is realized.Since the influence of noise is considered in the process of radius repositioning,the compensation effect of the feedforward carrier phase recovery algorithm is improved. |
