Research Of Efficient Transmission Protocols For Underwater Sensor Networks

Underwater sensor networks (UWSN) have great applications on civil and military fields such as marine observation, resource exploration, disaster prediction, submarine detection and so on. As the rise of the marine economy and the growing importance of marine rights, USWN have become a new hot research topic in recent years.The major characteristic of UWSN is using acoustic communication. Compared with traditional terrestrial sensor networks (TSN) that use radio communication, underwater acoustic communication has low bandwidth, high bit error rate (BER), and long signal propagation delay. The data transfer rate of UWSN is no more than 10k/bps. Moreover, the speed of underwater acoustic signal is 1500m/s, which is five orders of magnitude lower than radio signal, while the link distance of UWSN could be more than one kilometers, so signal propagation delay of links could be second grade. As a conclusion, data transmission efficiency of current UWSN is far lower than TSN. Finally, energy restriction is another important issue for UWSN.The unique features of UWSN bring challenges for transfer protocol design. Existing schemes, including medium access control (MAC) protocols, routing and cross-layer algorithms, could not completely overcome the difficulties of underwater acoustic communication. MAC protocols can be divided into two categories: competition-based schemes and scheduling schemes. Competition-based protocols focus on decreasing conflict probability and improving network throughput. Long propagation delay will reduce the efficiency of CSMA and MACA; moreover, collisions will waste precious energy. The key issue of scheduling protocols is to design efficient and collision-free channel assignment schemes, CDMA and TDMA is widely applied in this field. Distribute algorithm is a better solution for this kind of protocols, the reason is that, although centralized algorithm is easy to design, it relies on the center node excessively, which is not flexible and will bring hidden troubles of network security. Aimed at different application requirements of UWSN, cross layer data transfer protocols are proposed gradually. Cross-layer schemes should not only focus on channel assignment and routing design, load balancing, congestion control and energy saving are also important issues in protocol design. There are two meanings of energy saving in UWSN: first, energy waste or absolute energy consumption should be reduced; second, nodes'energy consumption should be balanced, if imbalance, large amount of nodes will die in advance and make the network paralysis, so energy consumption should be balanced to prolong network lifetime as far as possible.Aimed at the unique characteristics of UWSN and underwater acoustic channel, a series of efficient data transfer protocols are designed and analyzed in this paper, including MAC protocols and cross-layer transfer protocols. The specific research contents are:(1) Aimed at small-scaled UWSN that all nodes share a common channel, we propose a hybrid MAC protocol HCR, which combines a competition-based protocol and a TDMA protocol. HCR utilizes the advantages of different protocols: when traffic load is low, competition-based protocol is used and end-to-end latency is small; when traffic load is high, TDMA is used to keep high network throughput.(2) Aimed at linear topology UWSN such as Temperature salinity depth sensor chain (CTD) in marine observation, we propose two energy-balanced data transfer algorithm: optimum gradation number (OGN) algorithm and probability based transmission (PBT) algorithm. The basic idea is that node could adjust sending distance by control transmission power, and then each node chooses optimal next hop node to keep balanced energy assumption and prolong network lifetime.(3) Aimed at large-scaled and cluster-based UWSN, we propose an improved TDMA protocol CB-MAC. The goal of CB-MAC is to improve in-cluster throughput. The characteristic of long propagation delay in UWSN is utilized and channel utility of cluster head node is increased. A deffer time is used to allocate transmission time for each node; moreover, a guard time is added for each node to avoid collisions.(4) Aimed at large-scaled and complex topology UWSN, we propose a continuous time allocation based TDMA protocol CT-TDMA. CT-TDMA abandons traditional slotted scheme and uses a local conflict graph to describe node's partial topology; then heuristic rules are used to allocate transmission moments for all nodes, and achieve efficient, collision-free data transmission.(5) All protocols and algorithms are simulated in detail and compared with existing approaches. Effectiveness, reliability and performance of our protocols are validated by simulation results.The goal of this paper is to design and validate efficient, reliable and easily deployed data transfer protocols for UWSN. These protocols could overcome the defects of existing approaches, and have significant meaning for the practical application and promotion of UWSN.