Analysis Of Delay On Underwater Acoustic Networks And Design Of Media Access Control Protocols

As an important technical support for ocean exploitation,applications based on underwater acoustic networks(UANs)are widely found in military and commercial fields,e.g.disaster prevention,seismic monitoring,undersea exploration,environmental pollution monitoring and ocean data sampling.(UANs)have become a hot research topic and attracted more and more research attention all over the world.Due to the sharing of the underwater channels,media access control(MAC)protocols are one of the key knowledges of underwater acoustic networks design.Underwater channels are one of the most complex wireless channels,so the design of underwater acoustic networks MAC protocols are facing considerable challenge.Underwater channels are subjected to the long and variant propagation delay,high BER,high cost and intermittent communication;underwater nodes are powered by battery and UANs are resources-limited system;underwater nodes are deployed sparsely and the maintenance cost is high;thus,MAC protocols for terrestrial wilress networks can not be used directly based on the described characteristics of underwater channels and nodes.Comparing with terrestrial wireless system,the speed of the acoustic wave in the water is about 1500m/s,combining with the long range of the underwater acoustic systems,causing long and variant propagation delay.With the impact of time-varying characteristic of underwater environment,the impact of the variant delay on MAC protocols should not be neglected.However,the most research works of MAC designs up to now are based on the sumption of the fixed propagation delay between nodes.The thesis is focusing on the long and variant propagation delay of underwater channels and analyzes the impact of delay on handshaking-based MAC protocols via mathematical modulation and simulation analysis.Also we extendedly discuss the impact of the variant delay on overall underwater acoustic networks architecture design.We analyze in detail the impact of delay variance on MAC protocls via Markov chain and derive the closed-form expression for the throughput of MAC based on slotted-FAMA.Simulation results show that variant delay affects the performance of MAC protocols.Our paper proposes an adaptive slot length based on slotted-FAMA.Slot eliminates the asychronism of MAC protocols,and is one of the most important key parameters.Any packet should be sent at the beginning of the slot.Slot is usually defined as the sum of the maximum propagation delay among nodes and the transmission time of CTS packet to avoid the collision of data packets.Both of analytical and simulation results show that our proposed slot length adaptation improves significantly the throughput of MAC in underwater acoustic networks.The sumption of the fixed propagation delay between nodes are not reasonable,we should take the delay variance into the consideration.Long and variant delay prevents the accurate estimation of round trip time(RTT).The expected RTT value can be used to compute the Retransmission Time-Out(RTO)or the waiting time in MAC.The estimation of RTT is also meaningful for Automatic Repeat re-Quest(ARQ)scheme because the system should ensure reliable data transmissions in the presence of high bit error rate in the underwater acoustic channel.By analyzing the impact of RTO on throughput under the effect of delay variance,we conclude that the fixed RTO is inefficient and RTO should be adaptively set to improve the throughput.We present a novel approach of predicting the RTT using a Bayesian dynamic linear model,and then adjust RTO adaptively according to the predicted values.Simulation results show that the predicted values can adapt quickly to the sample RTT values.Under the effect of RTT fluctuations,the Bayesian algorithm offers performance gains in terms of throughput and prediction performance,comparing with Karn's algorithm.Our study highlights the value of predicting the RTT using Bayesian approach in underwater acoustic networks.