雾霾天气下DAML空间相干光通信系统的研究

大气湍流引起的折射率起伏会导致光信号的振幅和相位发生随机波动，同时日益严重的城市雾霾现象带来的系统性能降低问题也不可忽视.基于湍流相位波动和激光器相位噪声的缓慢时变特性，本文提出了将DAML相位估计算法应用到空间相干光通信系统中来提高系统性能，并推导了在大气湍流、激光器相位噪声以及各种天气状况衰减效应影响下，针对DQPSK信号的DAML相位估计系统的误码率，最后重点分析了在雾、霾天气条件下该系统的传输距离限制和波长选择特性.仿真结果表明：湍流相位波动造成的信噪比损失低于0.1 dB；在雾和霾天气下，相较于传统的OOK IM／DD FSO通信系统，DAML相位估计系统在不同的大气湍流环境中至少有500 m的传输距离优势；在霾和薄雾天气，传输波长为1 550 nm的DAML系统比波长为850 nm的系统的性能更优，但在能见度低于200 m的浓雾天气，系统性能基本与波长选择无关.

Analysis of free space coherent optical communication system based on DAML phase estimation in fog/haze weather

Optical signal suffers from fluctuations in both intensity and phase due to the random refractive index variations induced by atmospheric turbulence.Meanwhile,system performance degradation due to urban fog/haze also should not be ignored.Given slowly time-varying characteristics of turbulent phase variations and laser phase noise,decision-aided,maximum likelihood (DAML) phase estimation algorithm is employed in free space coherent optical communication system to improve system performance.Then,BER performance of DAML system with DQPSK modulation is investigated considering the influences of atmospheric turbulence,laser phase noise and weather conditions.Transmission distance and wavelength selectivity is also analyzed in fog/haze weather.Simulation results show that turbulent phase noise can be ignored with less than 0.1 dB signal-to-noise ratio penalty compared with no turbulence.And in fog and haze weathers,free space coherent optical communication with DAML algorithm has longer transmission distance than OOK IM/DD free space optical communication,at least 500 meters.DAML system with 1 550 nm wavelength has better performance than that with 850 nm wavelength in haze and light fog weathers.But system performance is independent of wavelength in thick fog weather when visibility is less than 200 meters.