Research On Several Key Technologies Of Cooperative Communications For Underwater Wireless Optical Networks

Underwater wireless communications have been commonly employed in numerous fields,such as marine environment monitoring,disaster warning,resource reconnaissance,and underwater target discovery.Conventionally,acoustic waves are adopted to accomplish underwater wireless communications.Although underwater acoustic communication has the advantages of long transmission distance and reliable quality,big-volume and high-speed traffic such as high-resolution image and real-time video acquisitions cannot be effectively supported due to its low bandwidth,slow data rate and long delay.By contrast,underwater wireless optical communication(UWOC)that employs blue-green light,has attracted widespread attention thanks to the characteristics of large bandwidth,high-speed transmission,ultra-low delay,strong anti-interference ability,and high security.Due to the detrimental effects of absorption and scattering,UWOC suffers from large path loss and limited communication range.Cooperative transmissions that adopt relays to assist data transmissions and complete multi-hop communications could expand the transmission distance and improve the system performance.The cooperative transmission has been widely applied to terrestrial radio frequency(RF)communications with mature techniques for practical realization.However,the existing achievements in RF networks may not be applicable for UWOC,since there exist various distinctive factors that could influence the performance of UWOC and great differences in channels and systems between UWOC and RF communications.Therefore,it is necessary to investigate the cooperative transmission technologies for UWOC,which has important scientific significance and practical value.The literature reviews of UWOC have been fully studied and several key issues of the application of cooperative transmission in UWOC have been explored,including the performance analysis in the presence of solar radiation noise and oceanic turbulence,relay assignment and power allocation schemes for single-user,multi-user multi-relay,and multi-hop networks.The detailed contributions of this dissertation are summarized as follows.(1)With the consideration of solar radiation noise and Log-Normal oceanic turbulence,the signal-to-noise ratios(SNRs)of amplify-and-forward(AF)relaying and decode-and-forward(DF)relaying based on line-of-sight(LOS)links and non-line-of-sight(NLOS)links are theoretically analyzed.The exact closed-form expressions for outage probability are derived for one-way and two-way multi-hop networks over Log-Normal oceanic turbulence channels.Numerical results demonstrate the impacts of solar radiation noise and oceanic turbulence on the performance of cooperative UWOC.(2)For single-user cooperative networks based on AF relaying,two algorithms of power allocation that optimize the energy consumption and the overall SNR are proposed,where the effects of absorption,scattering,and solar radiation noise are all considered.Both power allocation problems are proved to be strictly quasi-convex and solved by Levenberg-Marquardt algorithm.The effectiveness of the proposed schemes is evaluated in LOS links and NLOS links,and the effects of solar radiation noise and beam divergence angle on the system performance are analyzed.(3)For multi-user multi-relay networks over Log-Normal oceanic turbulent channels based on DF relaying,a joint optimization scheme of relay assignment and power allocation is proposed.This joint optimization problem is formulated as a mix integer programming problem and solved by an alternating optimization method.Therein,the relay assignment sub-problem is modeled as a weighted matching problem for a bipartite graph and addressed by an improved Kuhn-Munkres algorithm.The power allocation sub-problem is proved to be quasi-convex under certain conditions and solved by an iterative bisection algorithm.Simulation results indicate that the proposed schemes show better performance in the reduction of the average outage probability and hold fast convergences.(4)To integrate UWOC with the existing underwater acoustic networks,an architecture of underwater hybrid optical/acoustic multi-hop sensor network is proposed,and an adaptive surface relay deployment algorithm is designed.More concretely,an affiliated-node discovery algorithm is proposed to calculate the distribution coefficient of the nodes for this multi-hop network.With the fuzzy clustering method,an adaptive surface relay deployment algorithm is presented,where the number of surface relays could be adaptively adjusted.Simulation results demonstrate that the proposed algorithm could significantly decrease the energy consumption and the number of surface relays,which could reduce the cost of construction and operation.