复合海水信道偏移四相相移键控调制系统性能研究

Performance of OQPSK Modulation System Based on Composite Seawater Channel

水下无线光通信受到海水信道中吸收、散射和湍流的影响而降低了系统性能。目前的研究多独立地考虑衰减信道和湍流信道对信号特性影响,或者从理论公式上分析系统性能。本文采用更加符合真实信道特性的指数-广义伽马分布湍流信道模型,研究海水湍流信道下无线光通信偏移四相相移键控调制系统的误码率理论,建立了海洋湍流和衰减信道协同作用下的复合信道模型,通过模拟信号波形,统计误码率,对系统性能进行分析。结果表明,σ_(I)^(2)=2的强湍流下,具有载波特性的模拟信号优于开关键控调制的数字信号,且与二进制移相键控相比,偏移四相相移键控可获得约3 dB信噪比增益。σ_(I)^(2)=0.2的弱湍流下,水质衰减系数为c=0.151 m^(-1)的偏移四相相移键控系统在接收信号信噪比为20 dB时,可实现误码率在10^(-3)以下的50 m可靠通信。相同湍流条件下,误码率随链路距离增加成线性劣化。同时,海水水质对系统BER影响很大,湍流强度为σ_(I)^(2)=0.2,湍流相干时间τ_(0)=10 ms的纯海水信道中,接收信号信噪比为10 dB左右时,可实现误码率在10^(-3)以下的40 m可靠通信,而在沿海海水无法正常通信。

Considering the competition for oceanic resources among different nations,Underwater Wireless Communication(UWC)technology has a lot of potential for development.As compared to its traditional counterparts,namely underwater acoustic communication and radio frequency communication,Underwater Wireless Optical Communication(UWOC)has many advantages,such as a strong information-carrying capacity,a faster communication rate,and good confidentiality,which can better suit the practical communication requirements of high-speed and large-capacity,lower implementation costs,and lower time latency in underwater wireless communication.The effects of the UWOC channel on the received laser pulse are typically categorized into the signal power attenuation caused by absorption,scattering,and the light intensity scintillation caused by oceanic turbulence,which leads to a decline in the transmission performance(bit error rate)of the UWOC system.The most widely used turbulence channel models are only suitable for a specific turbulence state.In order to further analyze the signal characteristics and system performance of the UWOC system of the Offset Quadrature Phase Shift Keying(OQPSK)modulation under the common action of turbulence channel and attenuation channel,this paper uses the Exponential Generalized Gamma(EGG)turbulence distribution model,which is more consistent with real oceanic channel characteristics.We obtain the turbulent random noises utilizing the acceptance-rejection sampling algorithm and further establish a composite channel model taking into account the attenuation channel,turbulence channel,and the Additive White Gaussian Noise(AWGN).In addition,according to the waveform of simulating signal,varying turbulence noise parameters,system noise parameters,and attenuation channel parameters,we analyze the average Bit Error Rate(BER)characteristics of the OQPSK modulation in the UWOC system.The simulation results show that the signal waveform does not change when it passes through the attenuation channel,but the amplitude is severely attenuated;the signal envelope passing through the turbulence channel changes with time,and the speed of signal amplitude change is negatively correlated with the turbulence coherence time;the signal waveform passing through the composite channel is distorted nonlinearly.For strong oceanic turbulence of the scintillation indexσ_(I)^(2)=2,the performance of analog signals with carrier characteristics is better than the performance of the digital signal,where as compared to the Binary Phase Shift Keying(BPSK),the SNR gain of OQPSK is rough by 3 dB.For weak oceanic turbulence of the scintillation indexσ_(I)^(2)=0.2 with a water quality attenuation coefficient of c=0.151 m^(-1),the OQPSK system can achieve reliable communication of 50 meters at an average BER of 10^(-3) when the SNR is 20 dB.Under the same parameter of oceanic turbulence channel,the BER decreases linearly with the increases of the communication distance.At the same time,the quality of seawater has a great influence on the average BER of the UWOC system.In the weak oceanic turbulence channel ofσ_(I)^(2)=0.2 and turbulence coherence timeτ_(0)=10 ms,the UWOC system with the OQPSK modulation can achieve reliable communication of 40 meters at the average BER below 10^(-3) by increasing the SNR in the case of pure ocean water or clear ocean water,but it is noticed that the system with the OQPSK modulation can hardly achieve effective communication in the coastal ocean water.