# Bit Error Rate of Multi-Gaussian Correlated Asymmetric Bessel Beam Through Turbulent Ocean
###### Abstract  
We investigate the underwater propagation of multi-Gaussian correlated asymmetric Bessel beam with partial coherence in the condition of quadrature amplitude modulation. The oceanic turbulence optical power spectrum is used to characterize turbulence effects under variable temperature and salinity. Based on the derivation of orbital angular momentum mode distribution, the theoretical model of bit error rate (BER) is constructed. Numerical analyses show that the low-temperature oceanic channel is more beneficial to BER reduction than the low-salinity channel. Due to the better resistance to turbulence, low-order modulation is superior in BER performance. As for beam optimization, the increments in wavelength and source coherence width, or the decrements of topological charge and asymmetry factor, help to obtain a lower BER. The research is instructive for the construction of underwater transmission links based on vortex beams. b 2020 2020 25 February 2020 1 March 2020 1 March 2020 2020 2020 2020 2020 2020 2  
bit error rate for underwater communication than the Gaussian beam [10]. [PERSON] et al. reported the generation of caustic structured light based on Bessel superposition, which could be applied in communication because the curved trajectory enhanced the robustness of signal [11]. Since coherence is an important property of light field, its impacts on beam propagation attract much research interest [12; 13]. As for partially coherent beams, source coherence is usually characterized by the correlation function in its cross-spectral density. Compared with the common correlation function of the Gaussian Schell model, the multi-Gaussian Schell model has an additional parameter of beam index in addition to source coherence width, which offers a way to adjust the far-field intensity profile [14]. By introducing the correlation factor of multi-Schell mode, [PERSON] et al. proposed the multi-modified Bessel-Gaussian Schell beam, which had flatter intensity distribution in the far field [15]. [PERSON] et al. investigated the propagation of the partially coherent anomalous hollow beam generated by the multi-Gaussian Schell model source in oceanic turbulence, and elaborated the evolution characteristics of its spectral density distributions [16]. [PERSON] et al. further analyzed the detection probability and channel capacity of the multi-Gaussian correlated Hankel-Bessel beam in oceanic turbulence [17]. In contrast with the direct light field transmission from beam source to receiver, signal modulation regulates the beam characteristics such as amplitude and phase by adopting appropriate modulation strategies, which can enhance the ability of vortex beams to resist interference and improve spectral efficiency [18]. On-off keying (OOK) is a commonly used amplitude modulation technique, which controls the presence or absence of beam carriers to achieve amplitude changes, thereby representing two different signal states. The two different states distinguished by amplitude are used to carry binary information in signal transmission. Although it is easy to implement in terms of hardware, it exhibits limitations on frequency range and transmission distance [19]. Binary phase shift keying is a phase modulation technique with two different phase states. By mapping the phase states, binary information is carried by phase states during signal transmission. The significant difference between the two phase states makes the distinction relatively easy even under a certain degree of noise interference. But it is unsuitable for high-speed communication due to the limited bandwidth utilization [20]. Quadrature amplitude modulation (QAM) is an efficient digital modulation technique, which combines the advantages of amplitude modulation and phase modulation. Through different combinations of amplitude and phase, it can modulate multiple bits simultaneously. This not only enhances the flexibility of signal transmission, but also significantly increases the information capacity available [21]. [PERSON] et al.