| Optical communication has the advantages of high transmission rate and good security performance.It will be the main form of future space communication.With the popularization of satellite-to-ground optical communication networks,improving the demodulation technology of optical communication systems has become a key research topic.First,coherent detection technology is commonly used in long-distance communication,which requires strict matching between the signal light field and the local oscillator light field.However,laser phase noise and long-distance space transmission will cause the phase and direction mismatch of the coherent optical field,resulting in loss of communication performance and reduced communication sensitivity.Secondly,in the battlefield environment,the modulation format needs to be changed according to the channel environment,so it is of strategic significance for communication terminals to adaptively identify the modulation format.This dissertation starts from the coherent optical field mismatch problem and modulation format identification of satellite-to-ground optical communication.Specific studies include:Research on carrier phase estimation(CPE)algorithm based on dichotomy(DBPS).Coherent detection technology is often used in optical communication systems.The phase noise of the laser obeys the Wiener distribution.When linewidth is wide,the constellation diagram will be significantly deflected,which seriously reduces the performance of the optical communication system.In high-speed optical communication,the blind phase search(BPS)algorithm has better performance for carrier phase estimation,but its computational complexity(CC)is high,which brings a large computational burden to the digital signal processing unit.This dissertation proposes a new low-complexity CPE algorithm.It uses the BPS algorithm to estimate the compensation phase interval,and then uses the dichotomy to determine the compensation phase value quickly and accurately.The simulation results show that the CC(multiplication/addition)of DBPS is 2.79/2.84(16-QAM),5.35/5.45(64-QAM)and 2.98/3.01(128-QAM)lower than BPS.Research on tracking error compensation technology based on signal laser nutation.Transmission loss includes tracking and alignment errors between communication terminals and optical wavefront distortion caused by atmospheric turbulence.In the inter-satellite free optical communication system,factors such as platform vibration cause the signal laser to generate a large off-axis error at the coherent receiving end,which reduces the mixing efficiency and sensitivity of the communication system.This dissertation proposes a method to compensate for off-axis tracking errors by providing a small amount of additional jitter to the signal laser and repeating iterations.The simulation results show that when the additional jitter angle is in the range of 0.2-1.2 μrad,the sensitivity attenuation of the optical communication system is lower than 0.1d B,and the mixing efficiency and sensitivity of the coherent optical communication system are greatly improved after compensation.Research on wavefront distortion compensation technology based on signal laser nutation.In the downlink of satellite-to-ground communication,atmospheric turbulence leads to distortion of the signal laser wavefront,which reduces the sensitivity of optical communication.The Shack-Hartmann wavefront sensor(SHWFS)measures wavefront distortion using a microlens array(MLA)and a charge coupled device(CCD).Due to the limitation of MLA sub-aperture size,the dynamic range and detection accuracy of SHWFS are reduced.This dissertation proposes a scheme to detect wavefront distortion using signal laser nutation,which can achieve high spatial resolution and high dynamic detection range.Simulations show that the root mean square error(RMS)and mixing efficiency are effectively improved after wavefront compensation.In the case of strong turbulence,the mixing efficiency is increased by a factor of nearly 105.Wavefront distortion compensation technology based on detector array.The detector array is a two-dimensional detection device,which can filter out the interference of stray laser while measuring the wavefront information of the signal laser.In this dissertation,the SPGD algorithm and dual detector arrays are used for wavefront compensation,and the area laser intensity ratio is used as the beam quality evaluation standard to optimize the wavefront quality by blind search.An optical communication integrating signal laser wavefront compensation and demodulation is built.Since the recovery process of SPGD is unstable,this dissertation combines the detector array with local oscillator laser nutation: by providing a small amplitude nutation angle to the local oscillator laser,it is estimated according to the output mixing signal and the position change law of the local oscillator laser wavefront slope and reconstruct the signal laser wavefront using multi-order Zernike modes.Simulation studies show that the estimated compensation error of the wavefront slope can be reduced to0.008 under direct sunlight.Under strong turbulence,the mixing efficiency is increased by nearly 97.5 times,and the RMS is reduced to the original 0.048.Research on modulation format identification based on search boundary clustering algorithm.The battlefield environment is changeable.In order to ensure high-speed and effective data transmission,the communication format needs to be changed according to the channel environment,which brings great pressure to the modulation format identification technology.This dissertation designs a modulation format identification scheme based on parallel clustering algorithm to identify the modulation format of space optical communication of non-cooperative targets.This dissertation first proposes a parallel clustering algorithm that groups all symbols according to the principle of dense neighbors,and then clusters the groups.Then the algorithm is applied to modulation format identification.The experimental results show that the parallel clustering algorithm has achieved good results for clusters of different shapes,and the designed modulation format identification technology can accurately identify modulation modes of different orders with fewer symbols. |
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