Abstract:

To investigate the oceans for scientific and financial reasons, the idea of using free space optical communications underwater has developed fast. Underwater wireless optical communications is a newly emerged technology which uses the optical carriers to transmit the information in underwater mediums. In comparison to acoustic waves that were used previously in underwater wireless communications, employing optical waves increases the speed and bandwidth of data transmission significantly. However, using optical waves is limited to short distances due to high water absorption in optical frequencies. Fortunately, the lower absorption of water in green-blue wavelengths in comparison to other wavelengths makes the visible light as a good option to create high bandwidth communications in tens of meter. This short distance is used for applications such as maintaining oil equipments, monitoring ports and connetions of submarines to lands in wireless sensor networks. The unique features of UWOC channels causes the channel models of free space optical communications not to be suitable for UWOC. To have acceptable data rate and bit error rate for underwater optical communications, it is necessary to investigate the effective parameters on light propagation in the water.

In this research, first the physical model of UWOC channel based on the Monte Carlo method including absorption and scattering phenomena and their effects on different aspects of communications system performance such as power loss, time dispersion (channel bandwidth), transmitter and receiver misalignment and angle of arrival distribution in clear and harbor water are investigated. In addition to underwater channel characteristics, the parameters of optical communications system affect the system performance. Among different system parameters, the effect of divergence angle of Gaussian transmitter and its limitation cases including plane and spherical beams, size of aperture, field of view and sensitivity of detector on the power loss, channel bandwidth and receiver offset are investigated.

In addition to absorption and scattering, variation of water refractive index causes turbulence underwater. Despite the existing physical models including absorption and scattering in previous researches, a physical turbulence model for the applications of UWOC is not presented. In this research, a turbulence model based on the refractive index variations in horizontal UWOC link with Monte Carlo method is presented which is far less computationally intensive than approaches based on computational fluid dynamics and it is more simple and flexible for UWOC applications. Results show that the proposed model predicts correctly the lognormal distribution for the probability density function of received light intensity fluctuations in weak, moderate and strong turbulence regimes while in the saturation regime it follows the negative exponential distribution. Variations of scintillation index with channel parameters including link span and refractive index variation and with system parameters including divergence angle of Gaussian transmitter beam, aperture size and field of view of detector are investigated. Moreover, the proposed model predicts the turbulence induced power loss especially in longer link span and higher refractive index variations.

To verify the proposed turbulence model, experimental tests are achieved. A new practical method to produce turbulence is introduced and weak turbulence up to 12 meter link span is investigated. Two main effective factors on turbulence strength including refractive index variations and link span are considered in the experiments. The results of experiments and the proposed model are in good agreement