针对大气湍流引起的系统频带利用率下降的问题，研究了一种离散速率条件下的大气激光通信自适应调制编码系统。从信号层角度建立了湍流信道模型，给出了湍流信道下的瞬时信噪比概率密度函数，并采用外场实验验证了该信道模型。理论推导了系统频带利用率和误码率表达式，仿真结果表明大气激光通信自适应调制编码系统较传统单一传输模式系统具有优越性。研究了其频带利用率和误码率曲线特性，分析了误码率要求和湍流强度对系统性能的影响。结果表明降低误码率要求可大幅提高系统的频带利用率，且误码率要求越低，湍流强度越弱，频带利用率越高。

For the decrease of system spectral efficiency caused by atmospheric turbulence, an adaptive modulation and coding system for atmospheric laser communication under discrete rate condition is studied. A turbulent channel model is established from the viewpoint of signal layer, and the probability density function of instantaneous signal to noise ratio in turbulent channel is presented and verified by outfield experiments. The system spectral efficiency and bit error rate formulas are derived theoretically. Simulation results indicate that the adaptive modulation and coding system for atmospheric laser communication is superior to the traditional single transmission mode system. In addition, the characteristics of spectral efficiency curves and error rate curves are studied. Influences of bit error rate requirement and turbulence intensity on system performance are analyzed. Results show that the spectral efficiency is greatly improved when we decrease the bit error rate requirement. The lower the bit error rate requirement is, the weaker the turbulence intensity is, and the higher the spectral efficiency is.