JP7634462B2 - Underwater acoustic communication device and underwater acoustic communication method - Google Patents

Description

The present invention is a technology related to wireless communication devices that use underwater acoustics.

Acoustic communication is used for wireless communication underwater, as radio waves and light are highly attenuated. It is known that the lower the frequency of sound, the less attenuation there is, and for long-distance communication, frequencies below 100 kHz are used. This makes it difficult to use a wide frequency band, and the propagation time is also longer than with radio waves and light, so improving transmission efficiency, such as throughput, is a major challenge for underwater acoustic communication.

Widely known types of wireless communication with restricted frequency bands are the frequency division method, which divides the uplink and downlink of communication between nodes by frequency, and the time division method, which divides them by time. The time division method has the advantage of being able to easily change the allocation of transmission volume by changing the time allocation of the uplink and downlink, and is advantageous in terms of frequency utilization efficiency. In particular, in use cases where data transmission occurs, such as underwater images and videos, being able to adaptively change the allocation of transmission volume between the uplink and downlink is effective in terms of frequency utilization efficiency.

Known examples of the use of a time division method in underwater acoustic communication include those disclosed in Japanese Patent Application Laid-Open No. 2003-233699 and Japanese Patent Application Laid-Open No. 2003-233699.
In Patent Document 1, transmission timing control is performed so that packet signals transmitted from a plurality of nodes are continuously received in the form of a packet train at the sink node.
In Patent Document 2, for a slot having a first time length, a second time length is determined so that the second time length is longer than the sum of the time length of a propagation delay that occurs during transmission and reception depending on the distance from other communication devices and the time length of the packet contained in the slot, and transmission and reception of previous information based on the slot is instructed according to the second time length, thereby enabling information contained in the packet to be transmitted and received without any loss.

Special table 2019-508000 publication JP 2011-101084 A

In the conventional time division method typified by the above-mentioned patent documents, a guard time equivalent to the propagation time is required to prevent interference between upstream and downstream communications. During the guard time, data communication cannot be performed, which causes a decrease in communication efficiency. Since the propagation speed of underwater acoustic communication is slower than that of radio waves or light, the guard time becomes large, particularly in long-distance communication, and the decrease in communication efficiency becomes significant. For example, in the case of communication over a distance of 3 km, the propagation time of radio waves or light is about 10 microseconds, while the propagation time of underwater acoustic communication is 2 seconds, so that a guard time of 2 seconds or more is required to use the time division method in underwater acoustic communication.
Therefore, an object of the present invention is to improve communication efficiency in underwater acoustic communication.

Representative aspects of the inventions disclosed in this application will be briefly described below.
The disclosed underwater acoustic communication device is characterized in that it comprises a receiving unit that receives an acoustic signal from a counterpart node, a transmitting unit that transmits an acoustic signal to the counterpart node, a timer, a round-trip propagation time estimation unit that uses the received signal received by the receiving unit and the output of the timer to estimate the round-trip propagation time required for the acoustic signal to travel round-trip between the counterpart node, a transmission timing control unit that matches the transmission interval by the transmitting unit with the round-trip propagation time, and a transmission time length control unit that controls the transmission time length by the transmitting unit to be shorter than the round-trip propagation time.
The disclosed underwater acoustic communication method is characterized in that it includes an underwater acoustic communication device including a receiving step in which an acoustic signal is received from a counterpart node, a round-trip propagation time estimation step in which, using the received signal received in the receiving step and the output of a timer, an estimation step of a round-trip propagation time required for the acoustic signal to travel round-trip between the counterpart node and the counterpart node, a transmission timing control step in which a transmission interval of the acoustic signal to the counterpart node is matched with the round-trip propagation time, a transmission time length control step in which a transmission time length to the counterpart node is controlled to be shorter than the round-trip propagation time, and a transmission step in which an acoustic signal is transmitted to the counterpart node.

The present invention can improve communication efficiency in underwater acoustic communication. Problems, configurations, and effects other than those described above will become clear from the description of the embodiment of the invention below.

FIG. 1 is a block diagram of an underwater acoustic communication device according to an embodiment of the present invention. FIG. 2 is a diagram showing the timing of communication signals in a conventional time division system. FIG. 3 is a diagram showing the timing of communication signals in the time division system of the present invention. FIG. 4 is an image diagram showing a spectrum of a modulated signal according to the present invention. FIG. 5 is a flow chart of the present invention.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The examples are illustrative for explaining the present invention, and are omitted and simplified as appropriate for clarity of explanation. The present invention can be implemented in various other forms. Unless otherwise specified, each component may be singular or plural.
In order to facilitate understanding of the invention, the position, size, shape, range, etc. of each component shown in the drawings may not represent the actual position, size, shape, range, etc. Therefore, the present invention is not necessarily limited to the position, size, shape, range, etc. disclosed in the drawings.
In the embodiment, a process performed by executing a program may be described. Here, the computer executes the program using a processor (e.g., CPU, GPU), and performs the process defined by the program while using a storage resource (e.g., memory) and an interface device (e.g., a communication port). Therefore, the subject of the process performed by executing the program may be the processor. Similarly, the subject of the process performed by executing the program may be a controller, device, system, computer, or node having a processor. The subject of the process performed by executing the program may be a calculation unit, and may include a dedicated circuit that performs a specific process. Here, the dedicated circuit is, for example, an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), or a CPLD (Complex Programmable Logic Device).
The program may be installed on the computer from a program source. The program source may be, for example, a program distribution server or a computer-readable storage medium. When the program source is a program distribution server, the program distribution server may include a processor and a storage resource for storing the program to be distributed, and the processor of the program distribution server may distribute the program to be distributed to other computers. In addition, in the embodiments, two or more programs may be realized as one program, and one program may be realized as two or more programs.

A representative embodiment of the invention disclosed in this application will be outlined below. As shown in FIG. 1, an underwater acoustic communication device according to a representative embodiment of the invention includes a transmitting unit (101), a receiving unit (102), and a control unit (103).

In the transmitting unit (101), the transmission data (105) is digitally modulated (106) in the transmission signal generating unit (104), then converted into an analog signal of the desired frequency band in the analog processing unit (107), which is then converted into acoustic sound via the acoustic transmitter (108) and output as an acoustic signal (109) directed to another node.

In the receiving unit (102), an acoustic signal (110) sent from another node is converted into an analog signal by an acoustic receiver (111). The converted analog signal is converted into a digital signal after noise suppression of unnecessary frequency bands in an analog processing unit (112). The converted digital signal is subjected to detection of a synchronization signal and Doppler correction in a received signal extraction unit (113), and then digitally demodulated (114) and extracted as received data (115).

When the same frequency band is used for the transmitted acoustic signal (109) and the received acoustic signal (110), it is necessary to prevent interference between transmission and reception, and when it is difficult to avoid spatial interference, it is necessary to divide the time so that the transmission time and the reception time do not overlap. Timing control is essential for time division, and timing control of the transmitted acoustic signal (109) can be easily performed by using a timer (116) in the control unit (103).

Figure 2 shows a typical time division method, and shows the timing of communication signals between node 1 (201) and node 2 (202) and when node 2 is far away (203). The horizontal direction of Figure 2 corresponds to time, and the vertical direction corresponds to distance. In the case of the time division method, during the period (204) when node 1 is transmitting, the transmitted signal from node 2 cannot be received by node 1, so the transmitted signal from node 2 is invalid during the period between the diagonal lines (205) and (206). In addition, during the period between the diagonal lines (207) and (208) when node 2 receives the transmitted signal (204) from node 1, a signal cannot be transmitted from node 2 to node 1. The remaining period (209) is when a signal can be transmitted from node 2 to node 1, and it can be seen that the period (209) is shorter when node 2 is far away (203). In addition, the period when neither node 1 nor node 2 can transmit is called the guard time (210), and corresponds to the propagation delay time between node 1 and node 2. It can be seen that the guard time (210) is longer when node 2 is far away (203). For example, if the distance between node 1 and node 2 is 3 kilometers, a guard time of about 10 microseconds is required for radio waves or light, whereas the speed of sound in water is approximately 1500 meters per second, so a guard time of 2 seconds is required. During this guard time, neither node 1 nor node 2 outputs a signal, so transmission efficiency is significantly degraded, especially in underwater acoustic communication.

Figure 3 shows the timing of communication signals in the present invention. It is possible to eliminate the need for a guard time by matching the period (301) during which the transmission signal (204) of node 1 is received by node 2 with the period of diagonal lines (205) and (206) during which the transmission signal from node 2 is invalid. In this case, the transmission interval (302) matches the round-trip propagation time of diagonal lines (207) and (205). Due to the speed of sound in water, the transmission interval can be set to about 4 seconds for a distance of 3 kilometers, and about 0.4 seconds for a distance of 300 meters. The timing control of this transmission interval is performed using a transmission timing control unit (117) that matches the transmission interval with the round-trip propagation time in Figure 1.

The round-trip propagation time can be estimated by using a time stamp method in which timer (116) information of the control unit (103) in FIG. 1 is included in the transmission data (105) and transmitted. At this time, the timers (116) of node 1 and node 2 must be synchronized. For example, the nodes can be synchronized with GPS before being submerged in water, and a high-precision clock such as a quartz oscillator or atomic clock can be used as the timer (116) to maintain accurate synchronization. The round-trip propagation time estimation (118) using the time stamp method may be a simple estimation in which the one-way propagation delay time is doubled to obtain the round-trip propagation time, or the propagation round-trip propagation time may be estimated by including both the measurement result of the one-way propagation delay time and the timer information in the transmission data (105). The former estimation method of doubling assumes that the propagation delay times on the outbound and return routes are the same, so a stable propagation environment is desirable. The latter requires the signal to be sent back and forth, so there is a concern that the estimated information on the round-trip propagation time may become out of date.

When there is a relative speed between node 1 and node 2, the round trip propagation time changes, so it is necessary to estimate (118) the round trip propagation time for the next period while controlling (117) the transmission timing. The relative speed between node 1 and node 2 can be known by calculating the relative speed from the estimated Doppler amount, since the Doppler amount can be estimated when synchronizing in the received signal extraction unit (113). If the relative speed between node 1 and node 2 is known, the round trip propagation time for the next period can be estimated, so it is advisable to use this estimated round trip propagation time.

To realize the timing of the communication signal shown in FIG. 3, it is necessary to form a transmission packet short with respect to the round-trip propagation time. Here, the number of bits contained in the transmission packet needs to be formed with a somewhat larger number of bits, taking into account the code length of error correction. In addition, forming an extremely short packet will lead to a deterioration in transmission efficiency because it contains a synchronization signal and a header signal. Taking these factors into consideration, it is assumed that the transmission packet is formed with, for example, 5000 bits or more. If the modulated signal is assumed to be BPSK, one bit is one symbol, so one packet is assumed to be 5000 symbols or more. FIG. 4 is a diagram illustrating the frequency spectrum (401) of the modulated signal in long-distance communication in the present invention and the frequency spectrum (402) of the modulated signal in short-distance communication. In underwater acoustic communication, the lower the frequency, the lower the attenuation, so it is advantageous to use a low frequency in the case of long-distance communication. In addition, if the output of the acoustic transmitter (108) is too strong, a phenomenon called cavitation occurs in which bubbles are generated in water, so there is a limit to the sound pressure level of the transmitted acoustic signal. When performing long-distance communication with a limited sound pressure level, it is advantageous to narrow the modulation band because the sound pressure level per frequency increases. On the other hand, in short-distance communication, since signal attenuation and sound pressure level limits are not a concern, it is possible to use a wideband modulation signal that extends to high frequencies. Here, as an example, assume that a modulation band of 5 kHz to 10 kHz is used for long-distance communication with a communication distance of 3000 meters, and a wideband modulation band of 5 kHz to 55 kHz is used for short-distance communication with a communication distance of 300 meters. Since the modulation band is 5 kHz in long-distance communication, assuming that one packet is 5000 symbols or more, one packet will be generated in more than one second. Since this one second is shorter than the round-trip propagation time of 3000 meters, which is 4 seconds, it can be said that the timing of the communication signal shown in FIG. 3 can be realized. In addition, the allocation of the transmission time (204) of node 1 (201) and the transmission time (209) of node 2 (203) in FIG. 3 can be set in a trade-off relationship between 1 second and 3 seconds, so that the time allocation is made according to the amount of data to be transmitted, which makes communication efficient. This time allocation is made by the transmission time length control unit (119) in FIG. 1. The transmission time length control unit (119) controls the transmission time length so that the transmission time length of the own node is shorter than the round-trip propagation time, and further so that the sum of the transmission time length of the own node and the transmission time length of the other node is within the round-trip propagation time. The transmission time length control unit (119) makes the time allocation according to the amount of data to be transmitted by the own node and the other node within the range of this condition. Since 50 kHz can be used as the modulation band in short-distance communication, if it is assumed that one packet is 5000 symbols or more, one packet will be generated in 0.1 seconds or more. Since 0.1 seconds is shorter than the 300 meter round trip propagation time of 0.4 seconds, the communication signal timing shown in FIG. 3 can be realized. The switching of the modulation band according to the distance is performed by the communication modulation band control unit (120) in FIG. 1. On the other hand, if the same modulation band of 5 kHz as for long distance communication is used for short distance communication, the packet time of 1 second will be longer than the round trip propagation time of 0.4 seconds, so the communication signal timing shown in FIG. 3 cannot be realized. When the timing in FIG. 3 cannot be realized in this way, it is necessary to perform transmission timing control by bypassing the transmission timing control unit (117) in FIG. 1, which matches the transmission interval with the round trip propagation time.

Now, we will explain bypassing the transmission timing control unit (117). As an example, the control unit 103 may compare the round trip propagation time with a predetermined threshold, and bypass the transmission timing control unit (117) when the round trip propagation time falls below the predetermined threshold. Bypassing the transmission timing control unit (117) does not perform control to match the round trip propagation time with the transmission interval, and acoustic communication is performed with a guard time as shown in FIG. 2. However, at the same time, there is no need to restrict the transmission time length to within the round trip propagation time, and since the guard time itself is small at short distances where the round trip propagation time is less than the threshold, the decrease in communication efficiency is only slight even if a guard time is provided.

Figure 5 shows an example of a flowchart in the present invention. A node that starts communication (hereinafter referred to as a communication start node) first determines a carrier frequency (501). When determining the carrier frequency, the noise conditions of each frequency are checked and a frequency with a good noise condition is selected. After determining the carrier frequency, the communication start node transmits a response request to the destination node (502). If there is no response from the destination node to the response request (503), the communication start node reviews the carrier frequency because the noise conditions of the determined carrier frequency may be poor around the destination node. If there is a response (504), the communication start node estimates the round-trip propagation time based on the time from sending the response request to receiving the response (505). Here, when responding, it is efficient to include information such as the noise conditions and reception level at the destination node and the amount of data to be transmitted. The distance between the nodes can be known by estimating the round trip propagation time (505), and the communication environment can be known from the information contained in the response, so the communication initiation node determines communication parameters such as the transmission interval, communication modulation band, and transmission time length based on this information (506). The communication initiation node transmits the determined communication parameter information to the destination node (507). After receiving the communication parameter information, the destination node sends a response (508), and then transitions to communication parameters (509) according to the received information. After receiving the response (508) from the destination node, the communication initiation node transitions to a predetermined communication parameter (509). If there is no response (510), the communication initiation node resends the communication parameter information to the destination node. The flow up to this point uses a predetermined modulation band, and it is desirable for the modulation band to be narrow in order to suppress the effects of noise. Also, if the noise environment at each frequency is similar, it is advisable to set the carrier frequency as low as possible, taking into account signal attenuation.

After the communication initiation node and destination node transition to predetermined communication parameters, communication is performed with the timing shown in Figure 3 as a representative example. At this time, if the round trip propagation time changes, the received signal will deviate from the desired timing, so it is possible to know whether the round trip propagation time has changed, and by using a timestamp method, it is possible to know the absolute value of the round trip propagation time. If the desired reception timing is met (511), communication continues, but if the desired reception timing is not met (512), the round trip propagation time is re-estimated (505) using the timestamp method or the like, and the communication parameters are redetermined (506).

The above explanation has been given with an example of one-to-one time division communication. For example, if a parent node communicates with multiple child nodes, the parent node can assign different frequency bands to the multiple child nodes and communicate with each child node using time division.

As described above, the disclosed underwater acoustic communication device comprises a receiving unit (102) that receives an acoustic signal (110) from a counterpart node, a transmitting unit (101) that transmits an acoustic signal (109) to the counterpart node, a timer (116), a round-trip propagation time estimation unit (118) that estimates the round-trip propagation time required for the acoustic signal to travel round-trip between the counterpart node and the counterpart node using the received signal received by the receiving unit (102) and the output of the timer (116), a transmission timing control unit (117) that matches the transmission interval by the transmitting unit (101) with the round-trip propagation time, and a transmission time length control unit (119) that controls the transmission time length by the transmitting unit (101) to be shorter than the round-trip propagation time.
By thus matching the transmission interval with the round-trip propagation time and controlling the transmission time length to fall within the round-trip propagation time, guard time becomes unnecessary, and communication efficiency in underwater acoustic communication can be improved.
In particular, in long-distance communications, the longer round-trip propagation time can be turned into an advantage in that the transmission interval and transmission time length can be made longer.

The disclosed underwater acoustic communication device further comprises a communication modulation band control unit (120) that controls the band used for communication based on the round-trip propagation time.
For this reason, efficient communication can be selected according to distance; for example, for long-distance communication where the round-trip propagation time is long, the modulation band is narrowed to increase the sound pressure level per frequency, and for short-distance communication where the round-trip propagation time is short, a wideband modulation signal is used.

The receiving section detects a Doppler amount of the received signal, and the round trip propagation time estimating section estimates the round trip propagation time based on the Doppler amount.
Therefore, efficient communication can be achieved even when the distance varies due to movement of the partner node.

The communication modulation band control unit also determines the band to be used for communication by using the noise conditions for each frequency.
Therefore, before transmitting and receiving data with a partner node, a frequency band with good conditions can be selected, and a decrease in efficiency due to noise can be avoided.

The transmission time length control unit also notifies each other of the transmission data capacity with the other node, and determines the transmission time length from the mutual transmission data capacity.
Therefore, the transmission time length can be allocated according to the amount of data each party transmits.

The disclosed underwater acoustic communication device switches between matching the transmission interval to the round trip propagation time and not matching the transmission interval to the round trip propagation time, depending on the round trip propagation time.
Therefore, it is possible to easily select an advantageous communication method depending on the distance to the partner node.

Furthermore, when communicating with a plurality of remote nodes, the communication modulation band control unit assigns different frequency bands to each of the plurality of remote nodes.
Therefore, even when communicating with a plurality of partner nodes, time division communication without guard time can be performed with each node individually.

The present invention is not limited to the above-mentioned embodiment, and various modifications are included. For example, the above-mentioned embodiment is described in detail to easily explain the present invention, and is not necessarily limited to the embodiment having all the described configurations. Moreover, the present invention is not limited to the deletion of the configurations, and it is also possible to replace or add the configurations.
For example, some or all of the functions of the transmitting unit 101, the receiving unit 102, and the control unit 103 may be realized by executing a program.

101: transmitting unit, 102: receiving unit, 103: control unit, 104: transmitting signal generating unit, 105: transmitting data, 106: transmitting digital modulation unit, 107: transmitting analog processing unit, 108: acoustic transmitter, 109: transmitting acoustic signal, 110: receiving acoustic signal, 111: acoustic receiver, 112: receiving analog processing unit, 113: receiving signal extracting unit, 114: digital demodulating unit, 115: receiving data, 116: timer, 117: transmitting timing control unit, 118: propagation round trip time estimating unit, 119: transmitting time length control unit, 120: communication modulation band control unit

Claims (8)

A receiving unit for receiving an acoustic signal from a corresponding node;
a transmitter for transmitting an acoustic signal to the other node;
A timer and
a round trip propagation time estimation unit that estimates a round trip propagation time required for an acoustic signal to travel round trip between the corresponding node and the corresponding node, using a reception signal received by the receiving unit and an output of the timer;
a transmission timing control unit that causes a transmission interval by the transmitting unit to coincide with the round-trip propagation time;
a transmission time length control unit that controls a transmission time length by the transmitting unit to be shorter than the round-trip propagation time. 2. The underwater acoustic communication device according to claim 1,
The underwater acoustic communication device further comprises a communication modulation band control unit that controls a band used for communication based on the round trip propagation time. 2. The underwater acoustic communication device according to claim 1,
The receiving unit detects a Doppler amount of the received signal,
The underwater acoustic communication device, wherein the round trip propagation time estimating unit estimates the round trip propagation time based on the Doppler amount. 3. The underwater acoustic communication device according to claim 2,
The underwater acoustic communication device according to the present invention, wherein the communication modulation band control unit determines a band to be used for communication by further using a noise condition for each frequency. 2. The underwater acoustic communication device according to claim 1,
The underwater acoustic communication device, characterized in that the transmission time length control unit notifies each other of the transmission data capacity with the corresponding node and determines the transmission time length from the mutual transmission data capacity. 2. The underwater acoustic communication device according to claim 1,
An underwater acoustic communication device, characterized in that it switches whether or not the transmission interval is made to coincide with the round-trip propagation time depending on the round-trip propagation time. 3. The underwater acoustic communication device according to claim 2,
The underwater acoustic communication device according to claim 1, wherein the communication modulation band control unit, when communicating with a plurality of partner nodes, assigns a different frequency band to each of the plurality of partner nodes. Underwater acoustic communication equipment
a receiving step of receiving an acoustic signal from a remote node;
a round trip propagation time estimation step of estimating a round trip propagation time required for an acoustic signal to travel round trip between the corresponding node and the corresponding node, using the reception signal received in the reception step and an output of a timer;
a transmission timing control step of matching an interval of transmitting an acoustic signal to the corresponding node with the round-trip propagation time;
a transmission time length control step of controlling a transmission time length to the counterpart node to be shorter than the round trip propagation time;
a transmitting step of transmitting an acoustic signal to the corresponding node;
13. An underwater acoustic communication method comprising:

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