Monte-Carlo-based optical wireless underwater channel modeling with oceanic turbulence

Highlights

• Turbulence, absorption and scattering effects are unified.

• Light intensity distribution at receiving end is analyzed.

• Accurate power loss value is provided for underwater optical link.

Abstract

Underwater optical wireless communication has attracted widespread attention due to its high bandwidth. The complex compositions of seawater affect the transmission of underwater optical signals. However, for underwater optical wireless channel modeling, the turbulence effect cannot be integrated into the Monte Carlo simulation framework for absorption and scattering. Based on the generalized Snell’s Law, we extend the multiple phase screen model to unify turbulence effects, absorption and scattering effects into the Monte Carlo simulation framework, which allows us to study the integration of absorption, scattering and turbulence effects on underwater channels simultaneously. The received intensity probability density distribution, spatial light intensity distribution and power loss are investigated. To the best of our knowledge, this is the first time to explore the integration of absorption, scattering and turbulence effects on underwater optical wireless channel.

Introduction

Underwater communication is one of the key technologies employed in the ocean [1], [2]. Since electromagnetic waves are severely attenuated in underwater, acoustic communication is currently the most widely used technology for underwater applications. Although underwater acoustic communication provides long coverage distance, the communication rate is limited to the order of Kbps. Visible light in the blue–green band can be used as a visible light communication window owing to its small attenuation. In recent years, underwater optical wireless communication has attracted wide attention due to its high speed and good directivity [3], [4], [5], [6], [7].

The composition of seawater medium is complex, containing chlorophyll, yellow matter and various minerals. Photon interactions with impurities in the water lead to scattering effect on the transmitted beam, which is the main reason for underwater light pulse broadening and inter-symbol crosstalk [8], [9], [10]. The Beer–Lambert law is often employed to analyze the absorption effect due to its simplicity. But the assumption that all scattered photons are lost leads to the underestimation of the received optical power. Petzold measured the scattering of light by seawater [11]. The scattering phase functions such as Henyey–Greenstein (HG), two-term Henyey–Greenstein (TTHG), Fournier–Forand (FF) can simulate scattering characteristics of particles in water [12], [13], [14]. By using Monte Carlo simulation for underwater optical wireless channel modeling, we can obtain the time broadening effect and the phenomenon of light beam spreading caused by absorption and scattering during underwater transmission. It is noted that the time broadening effect and the beam spreading phenomenon caused only by the particle scattering effect are time-invariant [8], [9], [15].

In addition to absorption and scattering, there is also turbulence effect in underwater environment, which is caused by the temperature, salinity and density fluctuations [16], [17]. The inhomogeneity of the refractive index due to the turbulence effect leads to the variation of light propagation direction and scintillation of the light intensity at the receiving end, which is one of the main factors of the receiver’s bit error rate characteristics [18], [19], [20], [21]. The turbulence effect becomes more severe as the transmission distance increases. Hence, for long-distance underwater optical wireless communication, the turbulence effect cannot be ignored. The ocean power spectrum is usually used to characterize the ocean turbulence [22], [23], [24]. The oceanic turbulence between transmitter and receiver can be replaced by multiple phase screen model based on the power spectrum inversion method [25], [26]. The irradiance fluctuations of received light beam can be obtained by using scalar diffraction theory.

The influence of the real seawater environment on the transmitted light beam is a combination of absorption, scattering and turbulence effects. However, for underwater optical wireless channel modeling, there is always a gap between the simulation of underwater absorption, scattering and turbulence effects. The calculation of photon’s trajectories in Monte Carlo simulation for absorption and scattering is ray theoretic. Previous studies on absorption and scattering ignore the effect of turbulence. On the contrary, multiple phase screen model used in simulating the oceanic turbulence is wave theoretic. For this reason, the effect of underwater turbulence cannot be added into the Monte Carlo simulation for absorption and scattering effects. There is a relative paucity of studies investigating the combination of the absorption, scattering and turbulence. Based on the generalized Snell’s Law [27], we extend the multiple phase screen model to unify turbulence effect into the Monte Carlo simulation framework, which enables us to simultaneously study the integration of absorption, scattering and turbulence effects on underwater optical wireless channels.