Security enhancement for adaptive optics aided longitudinal orbital angular momentum multiplexed underwater wireless communications
Lei Zhu, Xiangjun Xin, Huan Chang, Xishuo Wang, Qinghua Tian, Qi Zhang, Ran Gao, Bo Liu
Abstract
The frozen-wave-based longitudinal orbital angular momentum multiplexing (LOAMM) system developed in [ IEEE Photonics J. 10 , 7900416 ( 2018 ) 10.1109/JPHOT.2017.2778238 ] has the potential to overcome the crosstalk effects induced by turbulence. In this paper, we propose a defocus measurement aided adaptive optics (DMA-AO) technique for turbulence compensation in a LOAMM underwater wireless optical communication (UWOC) system to investigate the enhancement of physical layer security. Relying on a phase retrieval algorithm and probe beam, three amplitude-only measurements obtained from different back focus planes can realize phase reconstruction of distorted OAM beams. Moreover, the so-called mixture generalized gamma-Johnson S B (GJSB) distribution is proposed to characterize the probability density function (PDF) of reference-channel irradiance of OAM. The GJSB allows for obtaining closed-form and analytically tractable expression for the probability of strictly positive secrecy capacity (SPSC) in a single input single output (SISO) system. Furthermore, the average secrecy capacity (ASC) and probability of SPSC for a multiple input multiple output (MIMO) system are investigated. Compared to the traditional OAM multiplexing system based on Laguerre-Gaussian (LG) beams, the LOAMM system with a probe beam assisted DMA-AO technique has potential advantages for improving the security performance in UWOC.