Development and Application of Underwater Acoustic Transducer Sensor

1　The basic concept and history of underwater acoustic transducer network

The underwater acoustic transducer network is the product of the popularization of global network technology. Now that the land is connected through wired optical or electrical means, and the network is connected through wireless networks or even communication satellites in the air, the underwater network may be the only remaining virgin land that has not been fully cultivated. It is conceivable that one day, when you turn on the computer and connect to the Internet, you can immediately obtain real-time data of ocean currents in the deep atlantic ocean. If an underwater camera is installed, you can even see the colorful fishes of the great upstream on your screen. . This is the task facing the underwater acoustic transducer network: the underwater acoustic network is used as the means of information transmission, the underwater sensor is used as the window for information acquisition, and the underwater acoustic network is finally incorporated into the conventional network in some way to integrate the underwater data sent to the observer. Since sound waves are the only form of energy that can be transmitted over long distances in the water, radio waves have a very short propagation distance in water, and light is also not suitable for underwater environments because of high attenuation and scattering underwater. . The underwater acoustic transducer is a wireless network composed of underwater acoustic waves as the information carrier. It is analogous to a wireless network in the air, except that the information carrier in the air is radio waves, and the information carrier in the water is sound waves. The underwater acoustic network must solve two technical problems, one is underwater acoustic communication transducer, and the other is networking based on acoustic communication. The underwater acoustic communication solves the point-to-point communication between two users (or information sources), and the networking solves the problem of information interaction when multiple users (or information sources) share the water medium channel. As an emerging technology under development, the reason why the development of underwater acoustic network lags far behind the wireless network in the air is largely limited by the development of underwater acoustic communication technology. The earliest underwater acoustic communication can be traced back to amplitude modulation (AM) and single-sideband (SSB) underwater telephones for analog data in the 1950s; there were a few analog systems before the 1970s, due to the amplitude modulation in the underwater acoustic reverberation environment. With the development of VLSI technology, the underwater digital frequency shift keying (FSK) technology was applied in the early 1980s. It is robust to the time and frequency spread of the channel. The underwater acoustic coherent communication appeared in the late 1980s. Compared with non-coherent communication, the coherent underwater acoustic communication technology can improve the bandwidth efficiency of the limited bandwidth underwater acoustic channel. However, due to the harshness and complexity of the underwater acoustic channel, the underwater acoustic coherent communication has not started It was accepted that the product of distance and speed of underwater acoustic communication at that time was about 0.5 km. In the 1990s, due to the development of DSP chip technology and digital communication theory, many complex channel equalization technologies can be realized, which drove the development of underwater acoustic coherent communication technology, and turned to the study of horizontal channel communication, because multipath effect of the channel is much more complicated than that of the vertical channel in the deep sea. In the mid-1990s, the speed and distance product of underwater acoustic communication transducer in the shallow sea environment reached 40 km× kbit, which made the establishment of underwater acoustic transducer. A landmark key component of underwater networks is the emergence of underwater acoustic modems. The earliest concept of underwater acoustic transducer application was the Autonomous Ocean Sampling Network (AOSN) in 1993. The United States has started an annual experiment in 1998 verify the concept of underwater acoustic transducer. Since the mid-1990s, underwater acoustic communication technology and underwater network technology have been developing steadily at the same time. However, due to the particularity and complexity of the water medium (such as high time delay, large attenuation, multipath and frequency shift), it is used on land. Wireless network technology cannot be directly applied to underwater networks, and research on underwater channels, underwater communications, and underwater network protocols is in the ascendant. At the same time, from the 1990s to the present, the development of terrestrial wireless sensor networks based on short-range wireless communication has also been very rapid. It can be said that underwater acoustic sensor network is an extension of the concept of terrestrial sensor network to underwater applications. The underwater acoustic sensor network is composed of multiple sensor nodes. The nodes can be fixed, such as anchored buoys or submersible targets, or mobile, such as underwater robots (UV or AUV). At present, the underwater acoustic sensor network can obtain different information according to the different types of underwater sensors: it can be used for oceanographic data acquisition, marine pollution monitoring, nearshore development, disaster prevention, underwater navigation and positioning assistance, marine resources Survey and scientific research data acquisition, distributed tactical monitoring, mine reconnaissance, and underwater target detection, tracking and positioning. In short, the underwater acoustic sensor network is to obtain underwater information through various sensor nodes in a certain underwater area, and conduct acoustic communication and networking with underwater nodes, and finally pass through specific nodes and re-radio In a wired and wired form, the information obtained in the coverage area is incorporated into the conventional network on the shore and sent to the observer's underwater subnet. You can see several characteristics of the underwater acoustic sensor network: The first is mobility. Because it is movable, it must be an autonomous network that can self-organize and follow a certain network routing method; the second is underwater wireless and underwater acoustic communication, due to the use of underwater acoustic communication, must be adaptive to the characteristics of the marine environment and solve the technical challenges of the physical layer; third, it is energy limited, because it is wireless, so it is battery-powered; fourth, it has data The relay function can transmit the monitoring data to the shore. In order to transmit the data effectively and reliably, a certain network protocol must be followed. The network topology determines the routing method, energy loss, network capacity and reliability of the network, so the network topology must be introduced first.

2　 Topological structure of underwater acoustic sensor network

Like the wireless sensor network structure on land, the topological structure of the underwater hydroacoustic sensor network can be divided into two categories: centralized network (centralized network), and distributed peer-to-peer network (distributed peer-to-peer network). In a centralized network, the communication between nodes is realized through a central node, and the network is connected to the backbone network through this central node. The main disadvantage of this configuration is that there is a single point of failure, that is, the failure of this node will lead to the failure of the entire network. And because the range of a single modem is limited, the coverage of the centralized network is limited. Figure 1 is a schematic diagram of the topology of a centralized network. Peer-to-peer network means that there is no central node to "administer" them, and each node has a relatively equal authority. According to the different routing methods, there are some differences in the peer-to-peer network. A fully connected peer-to-peer network provides direct "point-to-point" connections to two arbitrary nodes in the network. This topology reduces the need for routing. However, when the nodes are scattered in a large area, there is a need for communication. The power has greatly increased. And there will also be a "near and far" problem, that is, when a node A is sending a data packet to a remote node, it will block the neighboring nodes of node A from receiving other signals.

The multi-hop peer-to-peer network only communicates between adjacent nodes, and a message is completed by multiple hops between nodes from the source to the destination. The multi-hop system can cover a larger area, because the range of the network depends on the number of nodes, and is no longer limited by the range of a single modem. Figure 2 is a schematic diagram of the multi-hop peer-to-peer network topology. The  network is a network for wireless mobile applications, which belongs to a multi-hop peer-to-peer network. It does not need to build infrastructure in advance, also known as infrastructureless network (infrastructural network). Its characteristics are: autonomous network, dynamic topology, bandwidth limitation and variable link capacity, multi-hop communication, distributed control, nodes with limited energy, and limited security. Because it does not rely on infrastructure, it can be deployed quickly and cover a larger area. Because the infrastructure that can be relied on in the water is limited, and the movable AUV will be an important part of the underwater acoustic sensor network (AUV can enhance the performance of the underwater sensor network), its self-organization ability and dynamic topology, make The AdHoc network is very suitable to be used in underwater acoustic sensor networks. Although the AdHoc network is suitable for the application of hydroacoustic network,Its safety issue has always been a research topic. In fact, the underwater hydrophone sensor network should be a hybrid of a centralized network and a peer-to-peer network. In literature [16], a two-dimensional and three-dimensional hydroacoustic sensor network is introduced. Two-dimensional refers to the dimension of information obtained. In the two-dimensional underwater acoustic sensor network, sensor nodes and data transponders (Sink) are placed on the seabed, in a small area with Sink as the center, and the data of each sensor can be in the horizontal link To reach the Sink in a direct or multi-hop way (Multi-hop peer-to-peer network), and the sensor data can only reach the surface station if it is forwarded on the vertical link through Sink. Because only the information of a certain area of the seabed can be obtained, it is called a two-dimensional sensor network. In the three-dimensional underwater acoustic sensor network, the depth of the submersible target can be controlled, so that the multi-sensor nodes in a certain area are located at different depths, so the ocean information of a certain area and different depths can be obtained, so it is called a three-dimensional underwater acoustic sensor network. In the network topology, it is also a multi-hop peer-to-peer network. AUV can reach different depths in the ocean, combined with a fixed bottom sensor network, can also form a three-dimensional underwater acoustic sensor network. It is worth pointing out that because of underwater acoustic sensor networks, there is always a problem of accessing other conventional networks on the water. There is a special node called surface station, gateway or master node to complete this work. It must not only have an acoustic modem for communication with underwater networks, but also a radio or cable modem for communication with satellite or shore-based networks. The surface station can use the buoy as the carrier, or the surface ship as the carrier. The network topology determines the routing method, energy loss, network capacity and reliability of the network. Studies have shown that a network composed of multiple sensor nodes distributed at equal intervals along a straight line consumes more power than a multi-hop peer-to-peer network according to the routing method of a fully connected peer-to-peer network; and the network capacity is also affected by the network topology.

3　 Related concepts of underwater acoustic sensor network layer

The underwater acoustic sensor network is indeed a brand-new field, but the concept it follows is the same as that of the commonly used network protocol stack. Table 1 is the commonly used network layer concepts. For the sake of simplicity, this article only discusses the basic three layers: physical layer, data link layer and network layer. The problem to be solved by the physical layer is how to use the transmission medium

The characteristics (ie, channel characteristics) and the corresponding modulation methods enable effective data transmission. Acoustic communication based on water medium is a typical physical layer problem in the network protocol layer. At the transmitting end, the information bits must be turned into signals (acoustic signals) that can be transmitted by the channel, and at the receiving end, the signals in the medium must be changed back to information bits. This is the task of the underwater acoustic modem, which mainly involves three aspects: Media conversion (such as: electro-acoustic signal conversion), frequency band utilization efficiency, channel adaptability. The modulation methods commonly used in underwater acoustic communication are divided into two categories, one is non-coherent modulation, such as frequency shift keying (FSK), and the other is coherent modulation method, such as phase shift keying (PSK) and quadrature amplitude modulation. (QAM). Non-coherent modulation has good robustness to harsh underwater acoustic environment, but the rate is low; coherent modulation method has high coding efficiency and high frequency band utilization, but the transmission distance is limited. Some technologies are both the physical layer.

The propagation medium of the underwater acoustic sensor network is water, which is very different from the medium air of the terrestrial sensor network. Therefore, the network protocol that can be used effectively on land cannot be applied to the underwater acoustic network. We will start with the acoustic propagation characteristics of water and discuss the effects of sound.Learning the factors of communication and analyze the difficulties it causes to the various layers of the network protocol stack.

4.1 "Physical factors affecting underwater acoustic communication

4.1.1 "Long propagation delay and large delay variance The propagation speed of electromagnetic waves in the air is 200,000 times the propagation speed of sound waves in water. The slow sound speed makes the propagation delay very large, with a delay of about 0.67 s per kilometer, and at the same time The time-varying characteristics of the underwater acoustic channel make the delay variance very large. The former affects the throughput of the network, and the latter makes some time-based protocols inoperable.

4.1.2 "Large propagation loss (also called path loss)

According to Urick’s propagation model, propagation loss is the sum of losses caused by expansion and attenuation. Attenuation loss includes the effects of absorption, scattering and sound energy leaking out of the sound channel. Absorption is caused by the conversion of sound energy into thermal energy, which increases with frequency and distance. Expansion loss refers to the expansion of acoustic energy caused by wavefront expansion. It mainly includes spherical expansion (omnidirectional expansion) of point sources in deep sea environments. The propagation loss increases with the square of the distance; and cylindrical expansion in shallow water environments. Expanding on the horizontal plane, the propagation loss increases with distance. Since the propagation loss of acoustic signals increases with the increase of frequency and distance, the available frequency band of the underwater acoustic channel is very limited, and the propagation distance is also limited. Therefore, in the underwater communication network, if you want to carry out long-distance communication, you can only choose a low code rate; if you want to choose a high code rate, you can only carry out short-distance communication. Generally speaking, to make the propagation distance reach 10-100km, the available bandwidth is in the range of 2-5kHz; the medium-distance transmission is 1-10km, and the bandwidth is on the order of 10kHz; if the used frequency band is greater than 100kHz, the propagation distance must be less than 100m.

4.1.3 "Severe multiple routes

The multipath phenomenon is caused by the existence of more than one propagation path between the sound source and the receiver, and it often occurs in shallow seas and long-distance propagation. Simply put, a signal from a single sound source can receive multiple signals arriving at different times at the receiving end due to the existence of multiple paths. Multi-path will cause fluctuations in signal amplitude and phase. Due to the different propagation time of different paths, it will cause serious signal distortion, will lead to the decorrelation of received signals between different receivers, and multi-path will also cause bandwidth broadening. These will severely degrade the communication signal and cause inter-symbol interference. Multipath is also related to the position and distance between the sound source and the receiver. Taking the seabed plane as a reference, the multi-path influence of the vertical channel is small, and the multi-path influence of the horizontal channel is large.

Environmental noise is a collection of many factors, which are related to tides, turbulence, sea winds and waves, and thunderstorms. Ship noise is also an important noise source. Unlike the situation where the noise of the deep sea is relatively certain, the environmental noise of the shallow sea, especially the coastal waters, bays and ports, will change significantly with time and place. The noise is mainly composed of ship and industrial noise, aeolian noise and biological noise. Environmental noise will reduce the signal-to-noise ratio of the signal and affect the performance of underwater acoustic communication. 4.1.5” Doppler Dispersion Severe Doppler shift is caused by the relative movement of the sound source and receiver. Since the speed of sound is 200,000 times slower than the speed of electromagnetic waves, a very small speed can cause doppler frequency shift, and because of the channel, the underwater acoustic carrier frequency is lower. These two factors add up to make the influence of Doppler in water than The wireless communication in the air is much larger. If Doppler only produces a simple frequency transformation, the compensation of the receiver is relatively easy. However, due to the existence of multiple paths, when the acoustic signal hits the sea surface one or more times, different Doppler shifts will occur between each path, which is difficult to compensate. When high-speed data communication, it will generate inter-symbol interference and reduce the frequency band efficiency.