Energy Efficient Schemes For Underwater Acoustic Sensor Networks

Underwater acoustic sensor (UAS) networks are comprehensive networked systems, composed of UAS nodes with a short communication distance, the sea buoy nodes, and the two-way acoustic link between them, with the ability of the infor-mation acquisition, processing, classification, and compression, as well as delivering data in a multi-hop manner from the undetwater node back to the land or ship base station. UAS networks play an important role in ocean information acquisition, environment monitoring, resource exploration, disaster warning, military recon-naissance, and national security. China has vast coastlines, rivers and lakes, and the position of the marine economy in national economic construction is increasing year by year, which provides a huge space for the development and application of UAS networks in China.A typical feature of UAS networks is their limited energy. After the initial setup of UAS nodes, it needs too much cost due to the immature underwater recharging technology, and the difficulty of battery replacement and the salvage at sea, so it is often hoped to reduce network energy consumption as much as possible in order to prolong network lifetime under the condition that the network throughput and reliability are guaranteed at the same time.This paper focuses on the topic of energy optimization of the UAS networks, and designs from different angles energy optimization shemes that are suitable for UAS networks. The main academic contributions this paper are as follows:(1) The basic characteristics of underwater acoustic communication channel especially those feathers that are different from the terrestrial wireless communication channel such as the big delay, low bandwidth, frequency dependent transmission loss and environmental noise, etc, are introduced. Current commonly used channel models, especially the information theory based relationship between received signal-to-noise ratio and the available bandwidth, are studied to illustrate the important differences between UAS networks and traditional wireless networks in the system design. In a word, underwater acoustic channel has extension on the three dimensions of time, space and frequency, and is much more complicated than terrestrial radio channel. The needed transmission energy consumption is also different from wireless commu-nication, and therefore it provides the necessary foundation and prerequisite for designing energy efficient schemes suitable for UAS networks. (2) The optimal routing paths that minimize the total energy consumption for a given target signal-to-noise ratio (SNR) for multi-hop underwater acoustic networks were considered. A2-dimensional multi-hop network energy model was established, and an empirical closed-form expression of the optimal frequency-distance relation-ship was derived through the method of curve fitting to simplify the energy model. Then the energy efficient routing paths for both variable transmission power (VTP) and fixed transmission power (FTP) schemes were investigated. It is shown that the total energy consumption of the network is minimized only if all the hop distances are equal for a linear multi-hop network. In the case of equispaced relays, the analytical results on the optimal number of hops and the optimal distance that minimize the total energy consumption were also provided. Simulation results verified the correctness of this theory. The results obtained has certain guiding significance to the design and implementation of the nearshore environmental monitoring networks.(3) The UAS network of uniform distribution is considered, where the source node transmits data to the destination node using some routing algorithm in the form of multi-hop. To solve the problems of "when, who and how to cooperate and its effect", so as to maximize reduce total network energy consumption, the concept of cooperation factor is introduced, optimal allocation problems of the energy con-sumption of radio and cooperative transmission phase is taken into account, and an energy efficent cooperative communication scheme is designed. The effect of the energy allocation, the number of cooperative nodes, the average bit error rate and communication distance on the total energy consumption of the network is discussed through numerical simulations. Results show that there exists a critical distance; when the communication distance is greater than the critical distance, cooperative com-munication is more energy efficient than direct transmission; it is also found that the relay node can control the number of cooperative nodes via adjusting its transmission energy, so as to minimize the total network energy consumption. Results obtained may play some inspiration role in the design of energy effcient cooperative UAS networks.(4) An energy efficent clustering two-layer data gathering framework for UAS networks is put forward. The lower layer is compressed sensing based compressed sampling layer, where the cluster head using random sampling to the member nodes, who sent the data through the random access channel technology to the cluster head; The upper layer is traditional technique based data convergence layer, where data gathering center adopts full sampling to all cluster heads, who sent the data through determining channel access technology to the gathering center. The energy effcient data compression sampling scheme is designed, the methods of determining the number of clusters and probability of how many node will participate in data sampling are given, illustrated with concrete examples. Simulation tests show that the mecha-nism can effectively reduce the amount of sampling node, so as to reduce the network energy consumption. This data compression framework may provide reference to the design of long-term deep water environment monitoring networks.