Research On Energy-efficient Relay-aided Underwater Acoustic Communications

Oceans are not only crucial for the national construction of people's livelihood,economic development,and sovereignty safeguarding,but also for the entire human's survival and development.As the key technology to connect marine devices and transmit acquired marine data,underwater communication has always been one of the main research directions of marine science and engineering.Among the mainstream underwater communication technologies,underwater acoustic communication(UAC)is the only efficient medium-and long-distance transmission means in complex undersea environments.However,UAC has the disadvantages of low transmission rate and high transmit power on account of several adverse propagation properties of sound and the hostile marine environments.On the other hand,the transmitted volume of marine data grows explosively in recent years,and the energy storage of underwater communication devices are usually limited without effective replenishment means.Therefore,it is urgent to develop high-data-rate and energy-efficient transmitting schemes for UAC.A potential solution to improve the UAC's data rate and energy efficiency is to construct multi-hop transmitting networks through deploying relays between the transmitter and receiver.Existing works have widely studied relay-aided underwater acoustic networks,but the majority of them shield restrictions on the relay number(e.g.,two-hop)or assume that the relay topology obeys some specialized relay distributions(e.g.,equidistant and rectangular-grid).In this thesis,we devote to jointly optimizing the relay distribution and relay number in terms of the energy and delay performance in linear underwater acoustic transmitting networks.In other words,we investigate two fundamental problems that how many relays should be introduced and where should they be deployed.Our main research contents and results are as follows:1.We introduce and analyze the bandwidth-distance and power-distance features of underwater acoustic channels.The absorption and ambient noise models of UAC are both extremely complicated empirical formulae,which impose great challenge on quantitatively evaluating the performance of relay-assisted underwater acoustic transmitting systems.Numerical results indicate that,both the effective bandwidth and the required transmit power can be precisely approximated as functions of the transmission distance by first-order least-squares polynomial approximation on a logarithmic scale.Following this fact,we fist review UAC's path loss and ambient noise,based on which we simulate the effective bandwidth and required transmit power for a given signal-to-noise ratio(SNR).We then precisely approximate to obtain the bandwidth-distance and power-distance models and further evaluate the dependence of involved model parameters on the given SNR.2.We investigate two fundamental problems involved in designing energy-efficiently single-relay linear underwater acoustic transmission networks,namely under which conditions a relay should be introduced and where to deploy it if necessary.Specifically,we first utilize the approximated effective bandwidth and power models to formulate an energy consumption minimization problem.To solve this problem,we leverage the differential analysis to derive a critical transmission distance,defined as open distance.Particularly,a relay should not be deployed if the transmission distance is less than the open distance,and otherwise a relay should be placed at the middle point of the link.Extensive simulation results validate our theoretical analysis and show that properly introducing a relay can dramatically reduce the network energy consumption(up to 71.77%)almost without increasing the end-to-end delay(less than 1.56%).Furthermore,we apply a polynomial fitting method to derive another precise expression for the open distance through least-squares approximation based on the sufficient realistic data for potential applications.3.We investigate two fundamental problems involved in designing energy-efficiently linear underwater acoustic transmitting networks without any restriction on relay distribution or number,namely how many relays should be introduced and where to deploy them.Similarly,we utilize the approximated effective bandwidth and power models to formulate a much more complicated energy consumption minimization problem,and then solve it in two steps.First,based on the open distance,we prove that equidistant distribution is the globally optimal relay distribution.Then,we further derive a series of critical transmission distance,which provide an extremely simple method to decide the optimal relay number and thus the optimal relay positions.Extensive simulation results validate our theoretical analysis and show that equidistantly introducing a proper number of relays can dramatically reduce the network energy consumption(up to 83.80%)almost without increasing the end-to-end delay(less than 4.95%).Through the above research,we provide an extremely convenient and effective method to determine the optimal relay number and positions for the design of energy-efficient underwater acoustic transmitting networks.