Study On Multiple-beam Transmission And Reception Techniques For Atmospheric Laser Communication

The multiple-beam transmission and reception techniques for atmospheric laser communication are studied in this dissertation. The thesis consists of six chapters. The main contents and contribution are as follows:Firstly, the concept and research status of multiple-beam-transmission-and-reception for atmosphenc laser communication are expounded.Secondly, the traditional Rytov method and Markov approximation method are summarized in the weakly turbulent atmosphere and the strong or satuated one, respectively. The observed intensity fluctuation caused by atmospheric turbulence is studied, and the effect of the intensity fluctuation on the performance of atmospheric laser communication is analyzed. Based on the method of two-dimensional time-correlated random phase screens, the propagation of multiple beams through atmosphere id simulated. The statistical distribution of the observed intensity of the multiple beams is investigated. Based on the parameters of multiple-beam atmospheric laser communication system, a channel model of multiple-beam laser propagation is put forward for a typical structure of multiple-beam transmission.Thirdly, the techniques of space-time coding and diversity under the atmospheric turbulence channel are studied. The feasibility of applying space-time coding to atmospheric laser communication with intensity modulation and direct detection is analyzed. A new diversity coding scheme using four transmit apertures, one receive aperture and two detectors is proposed for the atmospheric laser communication. Numerical simulations and bit error probability analysis show that the new scheme can yield a signal-to-noise ratio gain of about 6dB at 10^-4 error probability. Based on the concept of outage probability, the performance of diversity system with different combing methods is given by statistical analysis. The capacity of the multiple-beam atmospheric laser communication is investigated. The expressions for the ergodic capacity of the turbulent channel in the high and low signal-to-background ratio are developed.Fourthly, the temporal-domain detection techniques for the multiple-beam system are studied. The feasibility of the detecting techniques, including Maximum Likelihood Symbol-by-Symbol Detection, Maximum Likeli-hood Sequence Detection and Pilot Symbol Assisted Detection, is analyzed for atmospheric laser communication. A single-step Markov chain is introduced to approximate the intensity fluctuation. Per-survivor processing (PSP) is applied to develop a sub-optimal MLSD to reduce the computational complexity. It's validated that Pilot Symbol Assisted-Maximum Likelihood Detection and Pilot Symbol Assisted Detection-Variable Threshold are effective to mitigate turbulence-induced intensity fluctuations. Wavelet transform is applied to detect the observed signals of multiple-beam. A soft threshold is used to process the empirical wavelet coefficients. The feasibility of the APD signals detecting method based on the wavelet analysis is confirmed.Fifthly, several problems in the design of multiple-beam transmission and receptionsystem are investigated. The transmission rates of atmospheric absorption and dispersion are computed for various wavelengths. The parameters design method of the optical antenna in the multiple-beam transmission and absorption system is given. A numerical simulation, using the thermal analysis software FLUENT, is performed to gain the distribution of the temperature field of the laser diodes in the laser communication terminal. The way to select the photoelectronic detectors is presented. The way to calculate the receiving field of view is given. The problems about the structrure of acquisition, tracking and pointing sub-system and the pointing error are also discussed for the multiple-beam transmission and absorption system.A set of theory of multiple-beam transmission and absorption is set up comparative completely. It is very beneficial and valuable for the design of multiple-beam atmospheric laser communication system.