中文摘要：

基于双伽马（Gamma—Gamma）大气湍流信道，研究了采用脉冲位置调制（PPM）技术的无线光通信系统的误码率（BER）性能。推导出平均EBR的闭合表达式，该表达式适用于任意湍流强度和任意调制阶数，与数值积分相比，可显著提高计算速度。对于给定的平均EBR，对比分析了在不同湍流条件下PPM调制阶数与平均发射功率需求间的关系。结果表明，在无湍流高斯信道和弱湍流信道下，增加调制阶数可以减小平均发射功率需求．然而随着湍流的增强这种作用逐渐减小，在强湍流区，甚至出现了调制阶数越高，平均发射功率需求越大的现象。因此，在实际系统设计时，应考虑可能的湍流环境，再选择合适的调制阶数。

英文摘要：

Based on Gamma-Gamma atmospheric turbulence channel, the bit error rate （BER） periormance of wirdess optical communication system with pulse position modulation （PPM） is analyzed. The closed-form expression of average BER is derived theoretically. The formula is suitable for a wide range of turbulence conditions and any modulation level. Compared with the numerical integral, it can increase the speed of calculation. For a given average BER, the comparative analysis of the relationship between the modulation level and the required average transmitted optical power is done over different turbulence conditions. The results show that under the non-turbulence and weak turbulence channel,PPM yields an average power requirement that decreases with modulation level increasing. However, with the enhancement of turbulence,the effect of saving transmitted optical power becomes weak with the increasing of modulation level. In the strong turbulent regime, increasing modulation level even leads to increasing average transmitted optical power requirements. In actual system design, possible turbulent environment should be considered,and then the right modulation level is selected.