| With the increasing demand for ocean exploration,Underwater Acoustic Networks(UANs)have become one of the research hotspots in recent years.UANs provide essential technical support for developing marine resources and promoting marine research with broad application prospects.However,the marine environment is extremely complex,and the underwater acoustic channel has unique characteristics,such as narrow bandwidth,long propagation delay,and high attenuation.The protocol design of each layer suffers from these characteristics in UANs.Among them,Media Access Control(MAC)is one of the core contents of UAN,designed to solve the access problem of nodes sharing communication resources.Performing stable and efficient MAC protocols can achieve low-cost and conflict-free communications,which is vital for improving UANs’ performance.However,existing underwater acoustic MAC protocols lack a general performance evaluation model,which greatly challenges researchers to select and modify an appropriate basic scheme from multiple candidates.This results in high trial-and-error costs and design difficulty.In addition,during the optimization design of the basic protocol,most existing protocols merely focus on data collisions caused by underwater acoustic channel characteristics,ignoring particular problems caused by scenario characteristics,resulting in poor practicability of MAC protocols in real UANs.To solve the above problems,this thesis first models and evaluates three basic underwater MAC protocols,including a non-competitive protocol,a random-access protocol,and a handshake protocol.Based on the model’s guidance,appropriate basic protocols are selected and optimized for three specific application scenarios: a fixed network assisted by a single mobile node,a network with multiple mobile and fixed nodes,and an entire mobile network to improve UAN performance in different scenarios.The main research contents of this thesis are as follows:1)A basic underwater acoustic MAC protocol performance evaluation model.Evaluating the performance of multiple basic MAC protocols provides practical guidance for designing new MAC protocols and reduces trial-and-error costs.However,existing models provide inaccurate performance results since they ignore the effects of unique UANs’ characteristics and MAC protocols’ diversity on data loss and service time.In this thesis,we propose a basic underwater MAC Performance Evaluation Model(MPEM)for UANs,which considers both unique UANs’ characteristics and MAC protocols’ diversity.We design the Successful Transmission Probability(STP)and Packet Service Time(PST)models in MPEM to estimate nodal throughput,delay,and energy consumption.STP model analyzes data loss types of different MAC protocols by considering long propagation delay,half-duplex communication,and random backoff to achieve a superior STP result.Based on the STP model,we employ the Markov chain to deduce the retransmission number in the PST model.In this way,MPEM ensures effectiveness and applicability in real-ocean environments.Simulation results show that based on the given network scenario parameters,MPEM can accurately evaluate different MAC protocols’ performance boundaries,select an appropriate basic protocol for the current scenario,and provide optimization suggestions for the selected protocol.2)A Time Division Multiple Access(TDMA)-based low-delay adaptive scheduling underwater acoustic MAC protocol.Based on the analysis results of MPEM,TDMA-based MAC protocols have significant robustness and are easy to implement in static UANs.As the demand for ocean exploration increases,more and more application scenarios use a single mobile node to assist a static UAN to perform complex tasks.Compared with a fully static UAN,such a UAN generally faces dynamic bidirectional data streams.However,existing TDMA-based MAC protocols assume that UANs only have a unidirectional data stream,which fails to adapt to the change of data streams’ direction and forwarding path,resulting in the long end-to-end delay in a static UAN assisted by a mobile node.To solve the above issue,this thesis optimizes the non-competitive TDMA-based MAC protocol.The proposed protocol analyzes the relationship between data stream direction and scheduling sequence.Then,slot length model and packet delay model are proposed to deduce the end-to-end delay of different data streams.Based on these two models,we present a scheduling sequence and slot length allocation algorithm to adaptively provide the minimum end-to-end delay for current bidirectional data streams.Extensive simulation results show that the proposed protocol efficiently addresses severe queue congestion of the state-of-the-art protocols.Compared with other traditional protocols,the proposed protocol can reduce the delay of dynamic bidirectional data streams by 53.6% at least.3)An Orthogonal Frequency Division Multiplexing(OFDM)-based next-hop sensing underwater acoustic MAC protocol.In static-mobile convergence UANs,the MAC protocol’s design suffers from both the network and physical layer protocols,facing unknown next hop and dynamic communication environments.Based on the above scenario characteristics and analysis results of MPEM,we optimize non-competitive and handshake protocols based on geo-routing protocol and OFDM technology.The proposed protocol formulates the OFDM resource allocation as a joint optimization problem based on transmission mode,subcarrier spacing,guard interval,and transmission power to decrease transmission delay and energy consumption.Then,a Karush-Kuhn-Tucker conditions-based heuristic algorithm is proposed to solve this problem.When assistant geo-routing protocols with the next-hop selection,the proposed protocol considers queue congestion and channel quality to modify the routing protocol’s backoff scheme,reducing packet collisions and improving communication efficiency.Extensive simulation results show that the proposed protocol is more suitable for geo-routing protocols and OFDM technology than state-of-the-art protocols.It can reduce energy consumption by around 30% while improving the packet reception rate by about 20%.4)A Multi-Carrier Code-Division Multiple Access(MC-CDMA)-based mobile underwater acoustic MAC protocol.In a mobile swarm UAN,the underwater acoustic communication faces large navigation noise and is sensitive to communication delay due to the high mobility of Autonomous Underwater Vehicles(AUVs).MC-CDMA has the advantages of strong anti-interference capacity and high spectrum efficiency,which is expected to solve the above problems.Therefore,we combine the analysis results of MPEM and optimize a handshake-based underwater acoustic MAC protocol based on MC-CDMA,to achieve efficient concurrent communication in mobile UANs.In MC-MAC,we design an adaptive clustering algorithm based on mobile velocity,propagation distance,packet size,and packet level,which dynamically differentiates the importance of nodes and determines the optimal number of clusters,reducing the computational complexity and enhancing the flexibility of resource allocation.On this basis,a super multi-objective communication resource allocation algorithm is proposed,which allocates the spreading codes’ length,subcarriers’ range,codes’ number,and transmission power for each node to optimize the throughput,delay,and energy consumption in mobile UANs.Extensive simulation results show that the proposed protocol fully uses the advantages of MC-CDMA technology,providing efficient concurrent communication for mobile UANs.It can increase throughput by about 60% while reducing energy consumption by around 70%. |
