Study On Dynamic Range Widening Mechanism Of Multi-Carrier Microwave Photon Link

With the rapid development of information technologies such as Internet of things,Internet,big data,cloud computing,artificial intelligence and so on,as well as people's favor for wireless access,the data flow of wireless access network increases sharply.The generation,processing and transmission system of microwave/millimeterwave signal based on traditional electronic technology is facing great challenges.Assisted by photonic technology,microwave photonic technology generates,transmits and processes microwave/millimeterwave signals in the optical field,which can realize functions that are difficult or even impossible to realize in the electrical field.Microwave photonic link is a photon-assisted microwave/millimeter-wave signal transmission system.The microwave/millimeterwave signal needs to be modulated to the light wave,processed by device,and transmitted through the optical fiber to achieve low loss,long distance transmission,and recovered by photodetector at the receiving end.Microwave photonic link is often referred to as RoF link.As the noise and the nonlinearity of the electro-optical/photoelectric conversion devices and the fiber link in the microwave photonic link,microwave/millimeterwave signal with multiple frequency components is subject to various interferences during optical modulation,transmission and detection,such as Harmonic(HD)and Intermodulation Distortion(IMD).These interferences are key factors that limit the capacity and performance of RoF link.In this paper,the formation mechanism and elimination technology of IMD caused by the nonlinearity of multi-carrier RoF link will be studied in depth,covering two main points:the elimination method of the Third-order Intermodulation Distortion(IMD3)and the elimination method of the Signal-to-Signal Beat Interference(SSBI)of Optical Orthogonal Frequency Division Multiplexing(OOFDM)signal,so as to expand the Spurious Free Dynamic Range(SFDR)of the link:1.Based on the in-depth analysis of IMD3 generation mechanism in microwave photonic links,a scheme of IMD3 elimination based on DPMZM is proposed.Through the electric phase control technology,the optical nonlinear frequency components in the output optical field of two submodulators in DPMZM have the same amplitude and opposite phase,so they cancel each other at the output end of DPMZM,so as to eliminate the nonlinear source of IMD3 in the optical domain.The simulation results show that the IMD3 is restrained about 30 dB,the SFDR is improved about 18 dB·Hz2/3.2.A scheme of IMD3 elimination based on carrier replaced single side band(CR-SSB)modulation is proposed to further filter out the nonlinear frequency components in the optical domain resulting in residual IMD3 and fifth order intermodulation distortion(IMD5)in DPMZM output optical field.The simulation results show that the proposed scheme can completely suppress IMD3 and IMD5 at the same time even if the modulation depth is increased to a high level,so that the linear performance of the microwave photonic link is limited by IMD7.Compared with the traditional microwave photonic link based on a single orthogonal biased MZM,the signal interference ratio(SIR)is increased by more than 60 dB,and SFDR reaches 133 dB·Hz6/7.3.A predistortion elimination scheme for SSBI of single side band optical orthogonal frequency division multiplexing(SSB-OOFDM)signal is proposed.The SSBI is pre-generated at the transmitter based on digital signal processing(DSP)and modulated to an optical carrier orthogonal to the polarization state of the SSB-OOFDM signal.At the receiving end,the SSBI generated by beat frequency and the pre-distortion SSBI component can be offset each other by direct detection only by a photoelectric detector(PD).The simulation results show that when the guard band(GB)of 40 Gbit/s 16-QAM SSB-OOFDM signal is reduced to 2.5 GHz,the error vector amplitude(EVM)can meet the upper limit of forward error correction(FEC)of 16.3%.4.A post-processing elimination scheme for SSBI of polarization multiplexing(PDM)SSB-OOFDM signal is proposed.At the transmitter,two SSB-OOFDM signals are polarized multiplexed,and their optical carriers are located on both sides of the sideband signals.At the receiving end,the electrical signal containing only SSBI component is obtained through direct detection after filtering out the optical carriers on both sides.Then,the post-processing technology based on DSP is used to subtract the SSBI from the electrical signals directly detected by two SSB-OOFDM signals.The simulation results show that when the GB of 56 Gbit/s 16-QAM PDM SSB-OOFDM signal is reduced to 2 GHz,SSBI is effectively eliminated by using the proposed post-processing scheme,and EVM is kept below 16.3%under two conditions of back-to-back(BTB)and 40 km transmission via standard single-mode fiber(SSMF).