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On BER performance for misaligned underwater wireless MISO laser links

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.