Research On Online Fountain Code And Reliable Transmission Mechanism For Underwater Acoustic Networks

Underwater Acoustic Networks(UANs)are an important part of the Smart Ocean,and a key technology to realize the digitalization,ecology,safety and harmony of the ocean.UANs have a wide range of applications in underwater environmental monitoring,resource exploration,and marine military,thus,UANs have become a research hotspot in recent years.UANs adopt acoustic communication,the propagation speed of acoustic wave is slow and varies due to the influence of the marine environment.Therefore,the propagation delay of acoustic signals is long and highly time-varying.The attenuation of acoustic signals in water increases exponentially with frequency due to the absorption of water,long-range underwater communications typically use low-frequency signals,resulting in narrow bandwidth and low data rate.Acoustic signal attenuation and severe noise interference bring about a high bit error for hydroacoustic channels.With the use of mobile nodes such as underwater gliders and autonomous underwater vehicles(AUVs),the Doppler effect of UANs is aggravated.The transmission of acoustic signal in water is interfered severely by the multipath effect due to the refraction of seawater and reflection from the seabed.Sensor nodes are deployed in complex and dynamic underwater environments,and the nodes are generally powered by batteries.Batteries are difficult to replace and recharge,resulting in limited energy for underwater nodes.Although some scholars have begun to study full-duplex communication for UANs in recent years,there are few engineering prototypes of acoustic Modem with full-duplex communication,so most research organizations are still using acoustic Modem with half-duplex communication.Half-duplex communication results in long communication delay and low channel utilization,and the proven reliable transmission mechanisms on land cannot be directly applied to UANs.The movement of underwater nodes with water currents as well as the use of various types of mobile nodes cause dynamic changes in the topology of UANs.As a result,these characteristics of UANs lead to higher transmission errors,loss and incomplete transmission of data collected by sensor nodes.The reliable transmission of data has become an urgent problem.To solve this problem,this dissertation introduces the Online Fountain Code(OFC)in UANs to enhance the reliability of data transmission.OFC is a new rateless code with on-line control and lower overhead than conventional fountain codes.Although OFC has advantages over fountain codes,OFC still transmits a lot of useless coded packets and a large number of feedback packets.Therefore,OFC cannot be directly applied to UANs with constrained resource,mobile nodes,low data rate,long propagation delay and half-duplex communication.In this dissertation,based on the coding and decoding characteristics of OFC,we analyze the reasons for generating useless coded packets and excessive feedback packets,and improve OFC.To recover important data as soon as possible,OFC with unequal error protection is studied considering the dynamical network topology.Based on the improved OFC,a reliable transmission mechanism is proposed for UANs.The specific research and innovations are as follows.1.Focusing on the characteristics of UANs,such as limited energy,narrow available bandwidth and long delay,the application of OFC in UANs is systematically analyzed based on random graph theory,definite integrals and limits,and two optimization objectives of OFC are proposed.According to the optimization objectives,we set the decoding state and feedback strategy,and propose a Recursive Online Fountain Code With Limited Feedback(ROFC-LF)for UANs.In the dissertation theoretical analysis on the number of coded packets needed for ROFC-LF using random graph theory and simulation experiments are conducted.The results show that,compared to OFC,the ROFC-LF improves the encoding efficiency by 10%,while the number of required encoded packets,the number of feedback packets,the overhead,and the computational complexity are reduced by 12%,44%,62%,and53%,respectively.Compared to IOFC,the encoding efficiency of ROFC-LF is improved by 4%,while the number of required encoded packets,the number of feedback packets,the overhead and the computational complexity are reduced by 3%,22%,31% and 42%,respectively.Therefore,the ROFC-LF has better performance in terms of overhead,feedback,computational complexity and coding efficiency.2.Considering the impact of the number of connected components in the build-up phase of the ROFC-LF coding mechanism on the numbers of encoded packets and feedback packets in the completion phase,this dissertation provides a systematic analysis on the build-up phase of the ROFC-LF coding mechanism.Based on the analytical results,ROFC-LF is improved by using the sequential coding strategy in the build-up phase,and the Sequential Recursive Online Fountain Code with Limited Feedback(SROFC-LF)is presented.The number of encoded packets of the SROFC-LF coding mechanism is also analyzed based on the random graph theory and simulated.The theoretical analysis and simulation results show that SROFC-LF has better performance in terms of both overhead and feedback compared to conventional OFCs such as OFC,IOFC,ZDOF-BDM,IOFC-SLDD,HTLO-GDS and OFCNB-EBDS.Compared with ROFC-LF,the SROFC-LF is better in terms of overhead,feedback and coding efficiency.Nevertheless,the ROFC-LF is better in terms of computational complexity and intermediate recovery rates of original packets.3.Considering that the different portions of compressed data such as underwater images or videos have different effects on reconstruction quality,to recover important data as soon as possible,based on the two proposed coding mechanisms of ROFC-LF and SROFC-LF,the problem of cycles in the build-up phase is analyzed based on the random graph theory.Then both the coding and decoding algorithms are improved by combining the unequal error protection.A weighting strategy is introduced in the build-up phase,and a prioritization strategy is introduced in the completion phase,and a circle-avoidance ROFC-LF with unequal error protection as well as a SROFC-LF with unequal error protection are proposed.Further,two coding mechanisms with unequal error protection are analyzed based on random graph theory and simulation experiments are conducted.The results show that the circle-avoidance ROFC-LF with unequal error protection has better performance in recovering both important data and all data compared to the three unequal error protection mechanisms,OFC-UEP,UEPOFC-SWS and OFC-URT.Compared with OFC-UEP,the SROFC-LF with unequal error protection has better performance in recovering both important data and all data.Compared to the UEPOFC-SWS and OFC-URT,although the SROFC-LF with unequal error protection is slower in recovering important data,for recovering all data,this coding mechanism requires fewer coded packets,reduces by 7.2% and 16%respectively.In conclusion,SROFC-LF is suitable for UANs to transmit data of equal importance while the circle-avoidance ROFC-LF with unequal error protection is more suitable for UANs to transmit multimedia data of unequal importance.4.Aiming at the high BER of UANs,the mathematical models of acoustic channels and packet error rates are constructed,and the number of encoded packets needed to decode all original packets in UANs with SROFC-LF is analyzed using random graph theory.For the characteristic of acoustic Modem with half-duplex communication,a transmission control policy is set at the sending node to control the number and type of encoded packets to be sent.Based on the transmission control strategy,the number of encoded packets required for SROFC-LF to decode all original packets is optimized by simulation,and a reliable transmission mechanism for SROFC-LF is proposed.The energy consumption and delay is further analyzed through mathematical modeling and an optimization algorithm is used to find the optimal solutions for the parameters such as the number of encoded packets,the packet length and feedback times,thus this dissertation constructs a reliable transmission mechanism with low energy consumption.