强海洋湍流水下光通信系统误码率研究

水下无线光通信（underwater wireless optical communication，UWOC）系统具有宽带宽、低衰减、低延时时间、高安全、高速率等优点，可满足水下数据、图像、传感器网络、视频通信的高速需求，受到了广泛研究。然而，除了水下的吸收、散射效应外，湍流效应也会使光信号衰减和衰落，限制了UWOC系统的通信质量和传输距离。探讨了外差式差分移相键控（differential phase shift keying，DPSK）调制对误码率（bit error rate，BER）的提升能力。假定强海洋湍流信道为Gamma-Gamma分布，利用改进的Rytov方法，得出了UWOC系统采用平面波和球面波传输时的闪烁系数表达式。借助于Whittaker M函数推导了DPSK调制的UWOC系统采用这两种光波传输时BER的解析式，并利用数值结果验证了解析结果的正确性。应用解析公式，仿真分析了DPSK调制的UWOC系统采用两种光波在三种重要的海洋湍流因素和通信距离下的BER，并对比分析了DPSK调制与OOK调制的BER。仿真结果表明，UWOC系统采用DPSK调制和球面波传输，在较小的均方温度耗散率、较小的温度和盐度波动对海洋湍流贡献的比值、较大的湍流动能耗散率的海洋中及通过较短的通信距离传输可以得到更优的BER。 

Bit error rate analysis for underwater optical communication system through strong oceanic turbulence

Underwater wireless optical communication (UWOC) systems have higher bandwidth, lower attention, lower time latency, lower power loss, better security and higher communication rate compared to acoustic communication as well as underwater radio frequency communication. UWOC systems receive growing attention as an alternative technology to meet the high-speed and large-data requirement in various underwater applications such as underwater broadcast data, imaging, high-throughput sensor networks, even the real-time video transmission. Despite all these advantages, expect absorption and scattering effects, the turbulence effect under water, will also cause loss and fading on the received optical signal, and limit the viable communication range of UWOC systems. The improvement of differential phase shift keying modulation (DPSK) to the bit error rate (BER) of UWOC system is presented. Formulations of plane and spherical waves propagating through strong oceanic turbulence, which described as Gamma-Gamma model, are obtained by using the modified Rytov method. Based on these formulations and the characteristics of Whittaker M function, the analytical expressions for BER of a DPSK modulated UWOC system are derived. The analytical expressions are verified by comparing with the numerical calculations of the definition. The system performance is simulated under three important oceanic parameters and the propagation distance L. Also the system performance of the DPSK UWOC system with the on-off keying (OOK) UWOC system is compared. The simulated results show that the performance improves when the UWOC system adopting spherical wave with DPSK modulation at a lower value of the rate of dissipation of mean-squared temperature, the ratio of temperature to salinity contributions to the refractive index spectrum, the propagation distance, or a higher value of the rate of dissipation of kinetic energy per unit mass of fluid.