Designing Multi-User Underwater Acoustic Communication Networks For Enhanced Information Rates:Access,Interference,and Security

With the growing of continuous utilization and development on marine resources,the operational units in the ocean gradually show the trend of “from single point to multi-point network” and “from single network to multi-network”.Each work unit/network in the ocean is connected by sound waves,forming underwater acoustic networks with different functions.Therefore,multiple networks have coexisted in the ocean.The un-derwater acoustic communication networks will not only include the regular users in-side the network,but also encounter the users outside the network who also generate the acoustic waves,the malicious users who eavesdrop or destroy the underwater acoustic communication network,etc.The aforementioned users form a generalized multi-user underwater acoustic communication network,and all these heterogeneous users affect the performance of the underwater acoustic communication network.This thesis aims at improving the information rate of the desired users in the un-derwater acoustic networks with heterogeneous users.Firstly,the models of underwater acoustic communication channels are introduced and summarized.From the physical channel model,the equivalent low-order model of underwater acoustic communication physical channel is analyzed theoretically.Then,user ranging and the grant-free multi-ple access techniques are developed to realize distributed in-network user access and to improve the information rate of network users with low complexity equalizers.External acoustic interference users are considered and a robust interference filter is designed to guarantee the user information rate in the network.Finally,the secrecy information rate is studied when malicious eavesdropping users co-exist in the network.Specific research thrusts are given as follows:(1)Based on the underwater acoustic communication channel models in the litera-ture,low-order equivalent block channel model is proposed for underwater acoustic com-munication physical channels.Starting from underwater acoustic physics,the influence of ocean medium environment on acoustic communication is fully considered in the channel model.The effects of delay spread and Doppler spread on underwater acoustic commu-nications are expressed based on the block transmission model.The low-order equivalent model of underwater acoustic communication physical channel is given,and the equiv-alence between low-order model and underwater acoustic communication model is ver-ified and classified theoretically.This part of research provides the theoretical basis for improving the information rate of underwater acoustic communication network users.(2)To control the network access,a fast inter-user ranging method is proposed for multi-user underwater acoustic communication networks.Considering the requirements of the users of the underwater acoustic communication network for the fast and accurate calculation of the distance between users,a low-complexity precise distance measurement method based on the transmission time,the sound speed,the gradient of the sound speed profile,and the user depth is proposed.The accuracy and robustness of the proposed method are verified.The research of this part provides the basic access to the network for the internal users.(3)Due to the large delay and Doppler spreads of the underwater acoustic channels,a grant free access mechanism suitable for high-speed underwater acoustic communication time-frequency double-selective channel is proposed.Relying on the channel parameter,a user access method is proposed by building a timer to control the transmission.This transmission also facilitates the receiver to collect multipath-Doppler diversity by using low complexity linear equalizers.Our design increases successful access rate of users without any collaborations or communications among users,and thus,the network infor-mation rate is increased.At the same time,the fairness of users and the energy efficiency of the network are studied in the design.(4)There are many existing interferences in the underwater acoustic frequency band.An interference filter is proposed to detect and mitigate the shot interference of external users.Considering the heterogeneity of multi-user underwater acoustic communication network,a block transmission based filter is designed,which is suitable for both multi-carrier and single-carrier communications.In the design,the interference detection is realized based on an adaptive sliding window,and the least trimmed squares equalization is realized based on robust regression theory.The external user interference is estimated and mitigated,which reduces the bit error rate of the desired users.At the same time,dur-ing the whole filtering process,there is no need of any prior knowledge about interference and null subcarrier.Thus,the external interference users do not reduce the information rate of network users dramatically.The adaptive ability and robustness of the filter are proved.(5)To solve the problem of malicious eavesdropping on multi-user underwater acous-tic communication network,a secure transmission strategy suitable for multi-user network cooperative communication is proposed.Considering the uncertainty of channel state in-formation feedback in underwater acoustic communications,this part presents an opti-mal beamforming method based on the second-order statistics of channel state informa-tion.Taking into account the energy efficiency requirements of the network,the optimal beamforming strategies for decoding-and-forwarding(DF)modes and amplifying-and-forwarding(AF)modes are given under the conditions of total power constraints and individual power constraints respectively.The information rate of network users is max-imized in the eavesdropping environment.