不稳定分层海洋湍流对厄米-高斯光通信系统的影响
Underwater wireless optical communication (UWOC) has caught much attention due to its wide frequency band, high information capacity, and fast data transmission rate. However, ocean turbulence causes light intensity scintillation during beam propagation, which increases the difficulty of optical signal resolution in UWOC systems. During establishing underwater optical communication links, Hermite-Gaussian (HG) beams can help to improve system performance by reducing scintillation. Most of the performance studies on current HG UWOC systems employ stable stratification turbulent power spectra, whose computational accuracy cannot match unstable stratification cases. Additionally, current research on UWOC is at the level where the system pointing error must be considered, but this aspect is not addressed in the study of the HG UWOC. According to the literature findings, the study of average bit error rate (BER), average channel capacity, and outage probability of UWOC systems based on unstable stratified ocean turbulence and HG beam pointing error is not reported. Therefore, we investigate the performance of the HG beam wireless optical communication system under unstable stratified ocean turbulence.
We derive the theoretical formulae for the HG optical scintillation index under unstable stratified ocean turbulence and the probability distribution function of the channel coefficients of UWOC systems considering the combined effects of ocean turbulence and pointing error. Meanwhile, theoretical formulae for the main performance parameters of the system are derived, including BER, channel capacity, and outage probability. Additionally, we derive the closed-form expressions for these three performance parameters using the Gaussian-Hermite orthogonal integral approximation method, thus conducting an in-depth analysis of the system performance. To more comprehensively evaluate the system performance in different conditions, we perform simulations to analyze the effects of beam modulus, transmission distance, ocean turbulence, and pointing error on the average BER, average channel capacity, and outage probability of the system under unstable stratification and stable stratification turbulence.
We investigate the performance of HG beam wireless optical communication system under unstable stratified ocean turbulence. The results show that the system BER can be reduced by decreasing the beam width, the HG beam width is reduced from 0.05 m to 0.01 m at an SNR of 30 dB, and the average BER is reduced from 6.18×10-7 to 2.0×10-8. The increase in transmission distance results in the rising system BER. Additionally, we compare the differences in the effects of system performance on unstable stratification and stable stratification turbulence. It is found that the average BER for the stable stratification case in temperature-induced ocean turbulence is much lower than the average BER for the unstable stratification case. Since the eddy thermal diffusivity of seawater should be greater than the salt diffusivity in a temperature-induced ocean environment, the stable stratification assumption underestimates the ocean turbulence intensity. The system performance in the unstable stratification case is better when induced by salinity. Under the stable stratification assumption, the average channel capacity can be close to 0 in certain conditions to prevent reliable communication. In contrast, the average channel capacity in the unstable stratification case is greater than that in the stable stratification case. Meanwhile, the increase in the mean square temperature dissipation rate and the decrease in the turbulent kinetic energy dissipation rate per unit mass of fluid lead to decreased average channel capacity and increased outage probability. This indicates turbulence intensity increase, thus causing deteriorated system performance. Finally, as the variance of the pointing error increases, the light intensity at the receiver end becomes weaker and cannot meet the requirements of the resolved signals, with degraded system performance.
We investigate the performance of a wireless optical communication system using the HG beam under unstable stratified ocean turbulence. Meanwhile, The closed-form expressions for the scintillation index of the HG beams and the average BER, average channel capacity, and outage probability of the optical communication system which take into account the pointing error are derived from the power spectra of the unstable stratified ocean turbulence. The simulations analyze the effects of beam modulus, transmission distance, ocean turbulence, and pointing errors on the system performance. The results show that the unstable stratification theory can correct the bias of ocean turbulence intensity caused by the stable stratification assumption, and then reduce the calculation errors of BER, channel capacity, and outage probability of the UWOC system, and have better system performance in the salinity-induced cases. Additionally, the increasing mean square temperature dissipation rate leads to a decreasing turbulent kinetic energy dissipation rate per unit mass of fluid, rising pointing error, reducing system channel capacity, and increasing outage probability. The results have implications and significance for studying underwater optical communications.
1 引言
随着海洋探索和研究活动日渐深入,对水下无线通信的需求持续增加。现阶段水下无线通信技术包括声波通信、电磁波通信和无线光通信[1]。与前两者相比,水下无线光通信(UWOC)具有频带宽、信息容量大、数据传输速率快、延迟小、能耗低等优点[2-4]。文献[5]中进行的实地演示证明了UWOC的发展潜力。
海洋湍流导致光束的传播出现光强波动现象,该现象被称为光强闪烁,这会增大UWOC系统解析光信号的难度。研究人员分析了不同类型光束在海洋湍流中的传播效果,其中包括椭圆涡旋光束[6]、部分相干环状偏心光束[7]和高斯阵列光束[8]等。此外,厄米-高斯(HG)光束模型也受到了关注[9-12]。陈斐楠等[10]研究了HG光束的M2因子在海洋湍流中的变化情况。杜星等[11]建立了海洋湍流中HG光束的强度分析模型,仿真分析了湍流参数对均方束腰宽、瑞利区间和湍流距离的影响。Baykal[12]指出,在设计水平水下光学无线通信链路时,HG光束可以通过减弱闪烁来提高系统性能。近年来,海洋湍流信道中光通信系统的性能也得到了广泛研究。Zou等[13]研究了部分相干高斯波束在弱海洋湍流无线光通信系统中的平均信道容量。傅玉青等[14]分析了强海洋湍流效应和瞄准误差对UWOC系统的平均误码率和中断概率的影响。Lin等[15]推导了考虑Málaga海洋湍流和瞄准误差的UWOC系统中平均信道容量的闭合表达式,证明了湍流衰落参数对系统性能有显著影响。目前对于瞄准误差层面的研究以高斯光束模型为主,而针对HG光束的UWOC模型精度有显著偏差。Kiasaleh[16]通过泰勒级数近似研究了较小的瞄准误差对HG光束在自由空间平均辐照度分布的影响。Liu等[17]使用统计平均方法推导了任意瞄准误差下HG光束的平均辐照度表达式,并研究了HG光束的传输特性、平均接收功率和信噪比损耗。
理论分析UWOC性能需要依据海洋湍流的空间功率谱,目前大量的研究使用Nikishov等[18]提出的湍流功率谱,它基于海水涡流热扩散率
本文推导了不稳定分层海洋湍流情况下HG光束的闪烁指数表达式,分析了使用HG光束传输时UWOC系统的性能;仿真研究了UWOC的平均误码率、信道容量和中断概率与海洋湍流参数、HG光束参数以及瞄准误差的关系,指出它们在不稳定分层和稳定分层情况下的差别,并在不稳定分层理论中通过改变涡流扩散率修正了稳定分层假设导致的理论计算误差。
2 理论分析
2.1 HG光束的闪烁指数
计算HG光束在海洋湍流中的闪烁指数,需要获得其在源平面的光场分布[26],即
式中:
在弱湍流环境中,闪烁指数
式中:Re表示对积分式取实部;
为了消除稳定分层假设带来的误差,采用Elamassie等[21]提出的不稳定分层海洋湍流功率谱模型:
式中:
式中:
2.2 HG光UWOC系统的性能
衡量UWOC系统性能的重要指标包括平均误码率、信道容量和中断概率。水下环境复杂,UWOC系统受到海洋湍流效应和瞄准误差的影响,信道系数
式中:
海洋湍流导致的信号衰落影响通信性能。采用对数正态的分布统计模型来表征光信号衰落
在UWOC系统中,湍流和海水流动引起HG光束的中心在接收平面上随机移动,将导致瞄准误差。瞄准误差的随机变化不可避免地影响HG光束的光强分布,HG光束的光强会随着瞄准误差的增大而减小,进而降低UWOC系统的信噪比。在本研究中,瞄准误差导致的信道衰减
式中:
式中:
综合考虑海洋湍流和瞄准误差的影响,可得到信道系数
下面,将基于
2.2.1 平均误码率
UWOC系统在信道
式中:
HG光信号经过对数正态分布的海洋湍流传播,在接收端的平均误码率(BER)可以表示为
应用
2.2.2 平均信道容量
利用一定容量的通信信道传输信息时,所能达到的最高信息传输速率[30]为
式中:
式中:
2.2.3 中断概率
中断概率是通信系统在传输过程中发生中断的概率。具体来说,在发送数据的过程中,由信道噪声、干扰、衰落等导致数据无法被接收而中断的概率。中断概率的表达式[31]为
式中:
3 数值仿真
本节将对第2节中推导的不稳定分层海洋湍流中HG光UWOC系统性能的理论公式进行仿真验证。假定沿水平方向传播的HG UWOC链路的仿真计算参数为:激光波长为532 nm、束宽
3.1 平均误码率
图 1. 平均误码率与平均信噪比的关系。(a)不同束宽和光束模数;(b)不同传输距离
Fig. 1. Relationship of average SNR and average BER. (a) Different beam widths and beam modulus; (b) different transmission distances
图 2. 两种情况下 对平均BER的影响。(a)不稳定分层情况;(b)稳定分层情况
Fig. 2. Effect of on average BER under two conditions. (a) Unstable stratification; (b) stable stratification
3.2 平均信道容量
图 3. 平均信道容量与海洋湍流参数的关系。(a)均方温度耗散率;(b)湍流动能耗散率
Fig. 3. Relationship between average channel capacity and ocean turbulence parameters. (a) Mean square temperature dissipation rate; (b) turbulent kinetic energy dissipation rate
图 4. 平均信道容量与瞄准误差的关系
Fig. 4. Relationship between average channel capacity and pointing error
3.3 中断概率
图 5. 归一化判决阈值 与中断概率的关系。(a)不稳定分层;(b)稳定分层
Fig. 5. Relationship between normalized decision threshold and outage probability. (a) Unstable stratification; (b) stable stratification
图 6. 海洋湍流参数和瞄准误差对系统中断概率的影响。(a)均方温度耗散率;(b)湍流动能耗散率
Fig. 6. Effect of ocean turbulence parameters and pointing error on outage probability. (a) Mean square temperature dissipation rate; (b) turbulent kinetic energy dissipation rate
4 结论
研究了不稳定分层海洋湍流下HG光束无线光通信系统的性能。根据不稳定分层海洋湍流功率谱推导出HG光束的闪烁指数表达式与考虑瞄准误差的光通信系统的平均误码率、平均信道容量和中断概率的闭合表达式。仿真分析了光束模数、传输距离、海洋湍流和瞄准误差对系统性能的影响。结果表明,不稳定分层理论可以修正稳定分层假设导致的海洋湍流强度偏差(低估以温度波动为主的海洋湍流强度,高估以盐度波动为主的海洋湍流强度),进而减小UWOC系统的误码率、信道容量和中断概率的计算误差,在以盐度波动为主的情况下系统有更好的性能。此外,瞄准误差的存在降低了UWOC系统性能。本研究的结论对水下光通信的研究有一定的意义和价值。
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Article Outline
丁桂璇, 杜星, 杜浩, 王生, 敖磊, 李崔春. 不稳定分层海洋湍流对厄米-高斯光通信系统的影响[J]. 光学学报, 2024, 44(6): 0601008. Guixuan Ding, Xing Du, Hao Du, Sheng Wang, Lei Ao, Cuichun Li. Effects of Unstable Stratified Ocean Turbulence on Hermite-Gaussian Optical Communication System[J]. Acta Optica Sinica, 2024, 44(6): 0601008.