| Underwater acoustic communication networks(UACN)are envisioned as the most efficient way to enable applications for oceanographic data collection and transmit the obtained data to processing center timely.The underwater acoustic channel is characterized with narrow available frequency band and low carrier propagation speed.Additionally,the underwater equipment is energy limited and hard to rechargeable.Thus,efficient MAC protocols are critical needed to arrange the limited channel resources properly to improve the network performances such as the throughput and lifetime.For the specialties of underwater acoustic channel,many reliable protocols designed for the terrestrial radio network can not be used in UACN directly.Further,most of the researches are in theoretical and simulations,rarely can be applied in UACN experiments,except some basic ALOHA and CSMA protocols.This dissertation carried out some researches on UACN's data link layer both in theoretical and experiment,and they were concentrated on the practicability and application aspect.Some MAC protocols were designed,with higher performances than that of ALOHA and CSMA protocols.Lake trail and sea trail were conducted to validated the practicability of these proposed protocols and investigate their performances.To address the hidden terminal and exposed terminal problems,which can hardly be solved in traditional single channel rules,a full-duplex and dual channel based MAC protocol(FD-MAC)was designed for UACN.In this protocol,underwater channel was divided into two portions virtually,in which,control packets and data packets were transmitted respectively using different frequency band and different modulate/demodulate approaches.Under this scheme,nodes can handle the control packet from the other nodes simultaneously when the data packet transmission is ongoing,thus,the hidden terminal and exposed terminal problems can be solved efficiently.Full-duplex modem is the key component to implement the FD-MAC protocol,and such modem was designed and implemented in this dissertation.Dual mode communication and sound baffle techniques were invited to restrain local emission interferences,and vector hydrophone was used to improve the receive gain.Experiments were conducted in the Yellow Sea,and the ability of full-duplex communication in condition of point-to-point and 3 users were tested in this trail,as well as the ability of solving the hidden terminal and exposed terminal problems.The results shows that the modem has a good performance in full-duplex communication and the FD-MAC protocol is practicable,the FD-MAC protocol can solve the hidden terminal and exposed terminal problems efficiently.Global clock synchronization is hard to be implemented in underwater environment and periodically broadcasting the synchronize signal is in low efficiency.To address this problem,a relative clock based TDMA(RC-TDMA)protocol was designed for the centralized networks.In this protocol,global clock synchronization was not needed.Working schedule was arranged by the main node according to the propagation delays of the different sub nodes to the main node,which were acquired during the initial stage.This schedule was mapped to every node's local time line respectively,and was modified during the data transmission process.Problems such as the asymmetrical energy consumption of main node and sub-nodes,new node added and old node lost,and different work slot needs for the sub-nodes were well solved in RC-TDMA.Theoretical analysis and simulations show that performances of the RC-TDMA will be affected by network cycle and guard interval,and their value needs to be chosen according to the net load.Results of the pool experiment and lake trail show that the RC-TDMA simplifies the implementation of TDMA protocol,and owns a better performance than that of ALOHA and CSMA protocol which will become much more obvious in heavy net load.A conflict-free MAC(CF-MAC)protocol was designed for the regional underwater observation networks.In this protocol,temporary channel access is assigned by the gateway or AUVs using the top-down channel assignment,which can avoid the channel contention collisions and hand-shaking delays.Advantages of this scheme include the stable throughput in heavy net load and the end-to-end delay is not affected by the load changes.Design of the two ACK types can reduce one propagation round if the data received correctly.This protocol is especially suitable for the network of using an AUV to collect the sensor data.A small test network was established in Qiandao Lake,4 nodes were employed to work as an observer in different region and depths,equipped with CTD to record the water temperature,and this data was collected by the gateway.The test was lasted 4 days,the results show that the CF-MAC worked well and the collected data can be transmitted to the offshore center reliablely.A distance aware CSMA(DA-DSMA)protocol was designed by taking advantage of the long propagation delay and near-far effects in UACN.The basic idea of DA-CSMA is allow the new transmitting course to jump the queue.That means the receiver can response to the new sender when it waits for the coming data,if possible,the new data transmission can be taken out immediately.DA-CSMA protocol has two advantages than the CSMA protocol.Firstly,data collision of repeatedly RTS sending,which induced by the sender's deaf-hearing of CTS,can be avoided.Secondly,throughput can be improved by reusing the waiting time.DA-CSMA protocol can gain a similar performance as that of FD-MAC protocol while working in half-duplex mode.This simplifies the implementation of protocol in hardware.Semi-physical simulations for the DA-CSMA protocol were carried out on network simulator and the protocol working procedure was investigated.During the simulation,distances between the nodes were controlled manually.Simulation results show that DA-CSMA protocol can obtain a better performance in networks with large distance differences,since the probability of jumping the queue is much higher in these networks. |
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