Underwater wireless optical communication (UWOC) highly depends on the alignment between the receiver and transmitter in realization. In practice, the facts that laser sources have narrow divergence angles, water body suffers from fluctuation, and precise positioning is very difficult in the underwater environment bring great challenges to the link alignment as well as practical implementation. However, due to the intrinsic optical properties of seawater, photons will be scattered during the transmission process, resulting in the diffusion of light, which relaxes the requirements for strict alignment. Inspired by the uniform spatial distribution of the multi-source arrays with a close spacing, in this paper, we investigate the bit error rate (BER) performance of the multi-input single-output (MISO) laser links in the presence of receiver offset. Based on a system model by considering scattering and absorption effects and noises including background noise and blackbody radiation as well as OOK signaling and an ideal photon counter, we derived a closed-form expression of relationship among BER, transmit power, link range, receiver aperture and offset distance, which is verified by Monte Carlo simulations. Numerical results suggest that, regardless of water types, linear light source arrays parallel to the offset direction can improve the tolerance of receiver offset with specific transmit power and link range for reliable communications. On this basis, we compared the anti-offset performance in the case of different inter-spacings of light source arrays such as dual-source and three-source schemes and also derived the optimal inter-spacing for light source to maximize the acceptable offset distance with reliable communication for dual-source links.
In order to evaluate the performance of underwater wireless optical communication (UWOC) systems, it is of significance to fully understand the impact of spatial diffusion of light beams. Meanwhile, simple and highly adaptable spatial channel modeling is also necessary and essential for performance evaluation and system design. In this paper, we focus on the spatial channel modeling and, in particular, quantify the photon spatial distributions for different water types, link distances, and transmitter/receiver characteristics. Via using the Gaussian distribution to complete the fitting, we have proposed a simple expression to describe the spatial irradiance distribution. The numerical results have shown that the proposed spatial channel model for UWOC systems agrees well with the Monte Carlo simulation results in terms of mean square error (MSE) with or below the order of 10−7 in both turbid coastal and harbor water and demonstrates a high adaptability to the link conditions. Furthermore, on this basis, we extend the study from single source to multi-source scenario and derive the corresponding expression of spatial channel model. Considering the integrity of closely spaced multi-source array, the multi-source model has been further simplified by two-dimensional Gaussian fitting.
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