Research On Connectivity Control And Access Technologies For Directional Underwater Acoustic Sensor Networks

With the rapid development of sensor technology,underwater acoustic sensor networks(UASNs)have attracted more and more attention in recent years for their huge advantages in military and civil fields such as sea area warning and resource detection,which has become an important metric to measure a country’s sea power.However,they face many challenges,using acoustic waves to communicate underwater,such as minimal communication bandwidth,low signal propagation speed,multi-path effect,etc.These limitations mainly result from the poor underwater acoustic channel.As the most complicated environment,the underwater acoustic channel leads to a high bit error rate and high delay for underwater acoustic communication,bringing in difficulties for reliable transmission of the physical layer and the design of upper-layer protocols.These challenges could be effectively addressed through directional communication tech-nology,which principle is to restrict the energy of acoustic signal to a specific angle range.Therefore,a signal can spread to a more extended range and obtain a higher signal-to-interference-plus-noise-ratio(SINR)when the transmission power is identical.In other words,when the transmission distance is the same,the signal can bring a lower transmission energy consump-tion.In addition,directional communication technology also overcomes the broadcast charac-teristics of the underwater acoustic channel,reducing the whole network’s conflict domain.In this way,it reduces the mutual interference between nodes.Furthermore,it improves the SINR at the receiver,thus improving the communication reliability of the physical layer.A directional underwater acoustic sensor network(DUASN)is a type of UASNs that con-sists of underwater acoustic sensor nodes with directional communication capability.According to the generation mode of a directional beam,the orientation system can be divided into fixed type and steerable type.The beam direction of the former will stay fixed after the network is set up? however,the latter can tone the beam direction of the transducer(array)through a mechani-cal structure.Therefore,topology control and MAC protocol are indispensable for UADSNs to work well.Based on the background mentioned above,this dissertation mainly does the following works:Firstly,aiming at the topology control technology for DUASNs with fixed beam orien-tation,the problem of assigning transducer orientation to fully connect the whole network on the condition that the beamwidth of π/3 is studied as well as the problem of connecting a DU-ASN under consumption constraint.In this research,the issues are reduced from the problem of Hamiltonian Path in Hexagonal Grid Graph(HPHGG)? thus,the non-deterministic polyno-mial(NP)completeness is proved.Based on the above analyses,a greedy algorithm is proposed by constructing a node-set with unique properties,where the symmetric connectivity between nodes in the set is guaranteed.There is at least one link between any two groups to connect them.Finally,the whole network is symmetrically connected with minimum energy consumption.Secondly,a full-duplex directional collision avoidance MAC(FDDCA-MAC)protocol is proposed for DUASNs with steerable beam orientation.With FDDCA-MAC,the channel fre-quency division multiplexing is realized by equipping the sensor node with an omnidirectional underwater acoustic transducer and a multi-modal underwater acoustic transducer with direc-tional communication ability.The two transducers work in different frequency bands and can transmit and receive independently.Therefore,the deafness problem brought by directional communication technology is resolved by notifying an access node with the idle transducer.In this way,the network performances under different topologies are significantly improved in terms of throughput,end-to-end delay,and energy consumption,benefiting from interference suppression brought by the directional communication technology to focus the beam.Finally,for directional sensor networks,a pair of nodes must beam-form towards each other to communicate successfully,i.e.,a node needs to know the direction of its intended communi-cation peer,so neighbor discovery is a crucial issue.Given this problem,a traffic-fair directional MAC(TFD-MAC)protocol is proposed based on the cooperation of a vector hydrophone and a multi-modal transducer.The protocol discovers and senses the events around the node through the vector hydrophone and dynamically updates the neighbor table.In addition,TFD-MAC adopts the data train technology to achieve higher network performance,where a successful reservation indicates multiple data packet transmissions at a time.However,this mechanism leads to an unfair probability for nodes to access the channel to a certain extent.TFD-MAC proposed several mechanisms,e.g.,postponed CTS,to address this problem.A sink will wait for a contention window to collect RTS after it replies to the ACK? it selects a transmitter from previously collected RTS packets according to the fairness algorithm and replies a CTS to the transmitter? thus,traffic fairness of the node and higher channel utilization is obtained.