Research On DSP Algorithms For Free Space Coherent Optical Communication Systems

With the rapid development of satellite communication systems and space networking technologies,the number of various high-precision observation and communication satellites launched worldwide has increased dramatically in recent years.The demand for satellite communication capacity has increased exponentially due to the real-time transmission of space exploration data in the order of Tbit from a single satellite on a single day.Free space optical communication technology has become an important solution to the demand for ultra-high-speed and largecapacity satellite communication because of its abundant spectrum resources,large available bandwidth,and good confidentiality.For the future development needs of free space optical communication technology with high speed,high bandwidth and long distance,the free space coherent optical communication system with advantages such as high spectral efficiency and high receiver sensitivity combined with higher order modulation technology becomes one of the most feasible technical solutions.However,the undesirable characteristics of the devices in the coherent receiver system and the atmospheric turbulence effect in the spatial channel can cause damage to the transmitted signal and make the system performance deteriorate.Therefore,how to compensate the transmitted signal in the atmospheric channel efficiently and flexibly using digital signal processing algorithms is a key technical challenge to be faced today.In this paper,the core digital signal processing algorithm and the atmospheric turbulence suppression scheme for free space coherent optical communication system are studied in depth,and the main research work and innovations are as follows.1.This paper designs and builds a high-speed free space coherent optical communication system simulation platform(including a spatial diversity receiver system simulation platform)for binary phase shift keying and quadrature phase shift keying modulation formats and conducts a comprehensive simulation analysis of the system performance.The simulation results show that the receiver sensitivity of the 4-branch spatial diversity receiver system is improved by about 3.5 dB compared with that of the 1-branch spatial diversity receiver system under strong turbulence channel conditions with equal gain combining.2.Aiming at the problems of low frequency offset estimation accuracy and small frequency offset estimation range of existing time-domain carrier frequency offset estimation algorithms in atmospheric channels,this paper proposes a carrier frequency offset estimation algorithm based on symmetric sequences in the time domain.The algorithm uses the optimized symmetric training sequence structure to accurately synchronize the timing and remove the modulation phase information of the estimated offset symbols in different atmospheric turbulence states,which improves the frequency offset estimation accuracy,avoids the traditional quadratic operation,achieves the theoretical frequency offset estimation range±Rs/2(R,is the symbol rate)and reduces the computational complexity of the algorithm.The simulation results of the 1OGbps PM-QPSK free space coherent optical communication system show that the frequency offset estimation algorithm based on symmetric training sequence has about 1.5 orders of magnitude higher frequency offset estimation accuracy and 4 times higher frequency offset estimation range than the quadratic phase difference frequency offset estimation algorithm,and the receiver sensitivity at BER=3.8E-3 is improved by 1.92 dB and 1.2 dB under strong and weak turbulence conditions,respectively.Meanwhile,the experimental platform of high-speed free space coherent optical communication system built indoors is used to further verify the improvement of receiver sensitivity of the proposed algorithm in strong and weak turbulence channels.3.Aiming at the existing time domain feed-forward carrier recovery scheme in which there are repetitive quadratic operations between the carrier frequency offset estimation algorithm and the phase offset estimation algorithm leading to high system complexity and poor noise immunity,this paper proposes a joint compensation scheme for feedforward based carrier recovery in the time domain.The scheme is designed to remove the signal modulation phase information in advance of the received signal pre-processing operation using real absolute value summing and subtracting in the signal pre-processing stage,which avoids the massive repetitive operation of the traditional time-domain prefeedback carrier recovery scheme while eliminating the impact of the additional noise generated by the cross term of the quadratic operation on the system performance.Finally,the scheme uses a joint compensation of frequency offset estimator and phase offset estimator,which further reduces the overall scheme complexity.The complexity analysis shows that the computational complexity of the proposed scheme is reduced to 32%of that of the conventional time-domain feed-forward carrier recovery scheme.In the simulation platform of free space coherent optical communication system,the accuracy of carrier frequency offset estimation of the proposed scheme is improved by about one order of magnitude compared with the conventional scheme,and the receiver sensitivity is improved by 3.6 dB,2.2 dB and 1.2 dB at BER=3.8E-3 in strong,medium and weak turbulence channels,respectively.The improved receiver sensitivity of the proposed scheme is also further verified in the experimental platform of a 10 Gbps QPSK high-speed free space coherent optical communication system built in the laboratory.4.Aiming at the problem of low combining gain caused by the constant weight coefficients of the equal gain combining scheme of the spatial diversity reception system that cannot track the atmospheric channel changes in real time,this paper proposes an adaptive gain combining scheme.The scheme adopts an adaptive equalizer to dynamically adjust the weight coefficients of each branch,which effectively improves the combining gain of the system;to further improve the adaptive combining performance of the system,the gain factor of this scheme adopts a realtime feedback adjustment mechanism to improve the convergence speed and convergence accuracy of the adaptive equalizer of the combining scheme.Simulation results of the 20Gbps PM-BPSK/PM-QPSK spatial diversity reception system show that the adaptive gain combining scheme with dynamic gain factor adjustment significantly improves the convergence accuracy of the combining scheme and increases the convergence speed by 50%in both modulation formats.The simulation results further show that the receiver sensitivity of the free space coherent optical communication system with 4-branch spatial diversity reception is improved by 6.2 dB and 3.1 dB in BPSK data modulation format and 6.5 dB and 2.7 dB in QPSK modulation format when the BER reaches 3.8E-3 in the strong and weak turbulence state compared with the equal gain merging scheme.The simulation results further show that the receiver sensitivity of the free space coherent optical communication system with 4-branch spatial diversity reception BER reaches 3.8E-3 with 6.2dB and 3.1dB improvement in BPSK data modulation format and 6.5dB and 2.7dB improvement in QPSK modulation format with the adaptive gain combining scheme compared to the equal gain combining scheme.