JP4333475B2 - Underwater communication system - Google Patents

Description

  The present invention relates to an underwater communication system using ultrasonic waves as a transmission medium, and more particularly to an underwater communication system capable of performing stable communication between a surface station and an underwater station against the influence of multiple reflections in a shallow sea area. About.

  Conventionally, in digital data communication in seawater, a communication system using ultrasonic waves as a transmission medium has been used. In the sea, in addition to the direct wave that arrives directly from the transmission side to the reception side, there is a delayed wave that reflects and arrives at the sea surface or the sea floor. Particularly in the shallow water area, since the distance attenuation of the sound wave is small, a large number of delayed waves having a large power are generated, which causes a data transmission error. Hereinafter, a conventional underwater communication system will be described with reference to FIGS.

FIG. 9 is an explanatory diagram for explaining ultrasonic propagation of an underwater communication system composed of an underwater station and a water station in the prior art.
In FIG. 9, the seafloor topography data and object detection data collected by the underwater station 10 are converted into modulation signals in the ultrasonic frequency band, transmitted from the transmitter 100 provided in the underwater station 10, and 20 is received by the receiver 200 included in the unit 20. The received signal is digitally processed by the surface station 20 and transmitted to another surface station or a land base station via the wireless antenna 3.

  The ultrasonic waves received by the receiver 200 are reflected along the solid line Rd and reflected by the sea surface 1 like the broken line Rs in addition to the direct wave that directly arrives at the receiver 200. Delay waves, delayed waves that arrive after being reflected by the sea surface 1 such as the one-dot chain line path Rb1, delayed waves that arrive after being reflected by the sea floor 2 and the sea surface 1, such as a two-dot chain line path Rb2 There is a delayed wave. Since these delayed waves are superimposed on the direct wave to create a so-called multipath fading environment in which the amplitude and phase of the received signal are changed, they cause data transmission errors.

  FIG. 10 is a pulse response waveform diagram showing the temporal response of the pulse received by receiver 200 in FIG. FIG. 10 (a) is a diagram showing the overall temporal response, and FIGS. 10 (b) and 10 (c) are enlarged views in the vicinity of part A and part B in FIG. 10 (a).

  Here, the depth Dt of the transmitter 100 is 150 m, the distance H from the sea floor 2 to the transmitter 100 is 50 m, the distance Dr from the sea surface 1 to the receiver 200 is 10 cm, and the transmitter 100 and the receiver 200 are. The horizontal distance L is 200 m, the pulse band is 20 kHz to 70 kHz, and the symbol rate is 8000 baud. Of the pulse waveforms shown in FIG. 10B, the pulse A1 is a direct wave transmitted along the solid-line path Rd shown in FIG. 9, and the pulse A2 is along the broken-line path Rs shown in FIG. Is a delayed wave transmitted. Of the pulse waveforms shown in FIG. 10 (c), the pulse B1 is a delayed wave transmitted along the one-dot chain path Rb1 shown in FIG. 9, and the pulse B2 is the two-dot chain line shown in FIG. This is a delayed wave transmitted along the path Rb2. 9 and 10, it can be seen that there is a delayed wave having a large power with a time delay of several hundred symbols, particularly in a shallow sea area.

  A technology that enables stable underwater communication even in such a multipath fading environment is known (for example, Patent Document 1: “Underwater ultrasonic transmission apparatus and ultrasonic transmission method using multiple ultrasonic waves”, Patent Document 2: “Underwater communication system”). The technique described in Patent Document 1 improves the certainty of data reception by interpolating a dead area (null area) generated due to sea surface reflection using carrier waves of different frequencies.

  In the technique disclosed in Patent Document 2, frequency analysis is performed on an FSK (Frequency Shift Keying) modulation wave of a reception signal composed of a start pulse, a data pulse, and an end pulse, and the start pulse is included in the frequency band of the modulation wave. When the presence of the start pulse is confirmed by detecting whether or not the signal exists, the code determination is performed using the frequency analysis result of the detected signal separated in time.

JP-A-9-55707

JP 2004-15762 A

  As described above, in particular, in data communication between a water station and an underwater station in a shallow sea area, a delayed wave of large power arrives with a time delay of several hundred symbols. When such a delayed wave arrives, the frequency selectivity of the acoustic propagation path due to fading is very finely divided, and many dead areas are formed. In addition, since the change of the insensitive area largely depends on the positional relationship between the water station and the underwater station, the above-described conventional technology may deteriorate the stability of communication.

An object of the present invention is to provide an underwater communication system that can reduce the influence of intersymbol interference due to multiple underwater reflections and enables stable and efficient communication.
An object of the present invention is to provide an underwater communication system capable of performing stable underwater communication between a surface station and an underwater station against the influence of multiple reflections in a shallow sea area.

The underwater communication system of the present invention is based on means for detecting the temporal response of the acoustic propagation path formed between the surface station and the underwater station, and based on the arrival time interval of the direct wave and the delayed wave from the detected time response. Means for controlling the transmission interval.
In the underwater communication system of the present invention, the underwater station receives a known transmission pulse transmitted from the surface station, converts a received signal into an electric signal, a signal obtained from the receiver, and a known A correlation processing unit for obtaining a correlation value with a transmission pulse, and detecting a delay time between the direct wave and the delayed wave from the correlation value, and a maximum of the delayed wave exceeding a predetermined allowable delay time and a predetermined correlation threshold A delay time detector for obtaining a delay time, a signal generator for generating a data frame sequence of transmission data with the maximum delay time as a maximum frame length, and outputting the data frame sequence at intervals of the maximum delay time, and an output of the signal generator A modulation unit that generates a modulation signal modulated by the D / A converter, a D / A converter that converts the modulation signal into an analog signal, a transmission amplifier that amplifies the analog signal, and an electrical output of the transmission amplifier that is converted into acoustic power. And it includes a wave transmitter for transmitting in water and.

In the underwater communication system of the present invention, the data frame sequence includes a frame synchronization sequence Fs (41 in FIG. 4) representing the beginning of the frame, a data sequence, and a frame synchronization sequence Fe (42 in FIG. 4) representing the end of the frame. The data sequence is located between the frame synchronization sequence Fs (41 in FIG. 4) and the frame synchronization sequence Fe (42 in FIG. 4) in time. A frame synchronization unit for detecting a frame synchronization sequence Fs (41 in FIG. 4) and a frame synchronization sequence Fe (42 in FIG. 4) and extracting a modulation signal of the data sequence; and included in the modulation signal of the data sequence A symbol length detection unit that obtains a symbol length of the data to be transmitted, and a demodulation unit that demodulates a modulation signal of the data sequence and outputs received data.
With such a configuration, since the time interval for transmitting and receiving data is determined in accordance with the temporal response characteristics of the acoustic propagation path that changes sequentially, data transmission and reception in water can be performed stably and efficiently.

In the underwater communication system of the present invention, the frame synchronization sequence Fs (41 in FIG. 4) and the frame synchronization sequence Fe (42 in FIG. 4) use Barker codes or M-sequence codes having a sharp autocorrelation function. . Therefore, reliable frame synchronization and detection of the number of received data symbols can be performed, and transmission / reception of data in water can be performed stably and efficiently.
In the underwater communication system of the present invention, the floating station includes an adaptive equalizer that equalizes received symbol data with a tap interval T / m and a tap number n, where m is a natural number and T is a symbol rate. The allowable delay time in the station delay time detection unit is T · n / m or more.

  In the underwater communication system of the present invention, a decision feedback type adaptive equalizer is used as the adaptive equalizer. In this case, T is the symbol rate, nb is the number of taps of the feedback filter of the decision feedback adaptive equalizer, and the allowable delay time in the delay time detector of the underwater station is T · nb or more. By using such a configuration and method, the influence of intersymbol interference of delayed waves that arrive at relatively short intervals is removed by the adaptive equalizer, and the influence of delayed waves that cannot be equalized by the adaptive equalizer is also included. The transmission and reception of data in water can be performed stably and efficiently.

  In the underwater communication system of the present invention, stable and efficient underwater communication is possible between the surface station and the underwater station with the above-described configuration. In addition, for the influence of multiple reflections in shallow water, more specifically, even for a large power delayed wave that arrives after a relatively long delay time after the direct wave arrives, Stable underwater communication is possible with underwater stations.

  The underwater communication system of the present invention is an underwater communication system that transmits and receives digital data between a surface station and an underwater station using ultrasonic waves as a transmission medium, and the temporal response of an acoustic propagation path between the surface station and the underwater station. An underwater communication system characterized by detecting and controlling a transmission interval based on an arrival time interval of a direct wave and a delayed wave from a detected temporal response. , The receiver receives a known transmission pulse transmitted from the surface station, converts the received signal into an electrical signal, and the correlation processor converts the signal obtained from the receiver and the known transmission pulse. The correlation value is obtained, the delay time between the direct wave and the delay wave is detected from the correlation value, and the maximum delay time of the delay wave exceeding the predetermined allowable delay time and the predetermined correlation threshold is detected by the delay time detection unit. By the signal generator A data frame sequence of transmission data is generated with a maximum delay time as a maximum frame length, a data frame sequence is output at an interval of the maximum delay time, a modulation unit generates a modulation signal modulated by an output of the signal generation unit, The modulated signal is converted into an analog signal by a D / A converter, the analog signal is amplified by a transmission amplifier, and the electrical output of the transmission amplifier is converted into acoustic power by a transmitter and transmitted to the water. This is an underwater communication system.

  In the above-described underwater communication system of the present invention, the data frame sequence is composed of a frame synchronization sequence Fs representing the beginning of the frame, a data sequence, and a frame synchronization sequence Fe representing the end of the frame, and the data sequence is a frame synchronization sequence. Fs and the frame synchronization sequence Fe are sandwiched in time. At the water station, the frame synchronization unit detects the frame synchronization sequence Fs and the frame synchronization sequence Fe, and the modulation signal of the data sequence is detected. The symbol length detection unit obtains the symbol length of the data included in the data sequence modulation signal, the demodulation unit demodulates the data sequence modulation signal, and outputs received data. With this feature, since the time interval for transmitting and receiving data can be determined in accordance with the temporal response characteristics of the acoustic propagation path that changes sequentially, the transmission and reception of data in water can be performed stably and efficiently.

  The underwater communication system of the present invention is characterized in that the frame synchronization sequence Fs and the frame synchronization sequence Fe are Barker codes or M sequence codes. For this reason, reliable frame synchronization and detection of the number of received data symbols can be performed, and transmission / reception of data in water can be performed stably and efficiently.

In the above-described underwater communication system of the present invention, the floating station equalizes received symbol data having a tap interval T / m and a tap number n by an adaptive equalizer, where m is a natural number and T is a symbol rate. The allowable delay time in the delay time detector of the underwater station is set to T · n / m or more.
In the above underwater communication system of the present invention, the adaptive equalizer is a decision feedback type adaptive equalizer, where T is the symbol rate and nb is the number of taps of the feedback filter of the decision feedback type adaptive equalizer. The allowable delay time in the station delay time detection unit is set to T · nb or more.

  By the method described above, the influence of intersymbol interference of delayed waves that arrive at relatively short intervals is removed by the adaptive equalizer, and the influence of delayed waves that cannot be equalized by the adaptive equalizer can also be excluded. Transmission and reception of data in water can be performed stably and efficiently.

  In the underwater communication system of the present invention, it is possible to perform stable and efficient underwater communication between a surface station and an underwater station. In addition, for the influence of multiple reflections in shallow water, more specifically, even for a large power delayed wave that arrives after a relatively long delay time after the direct wave arrives, Stable underwater communication can be performed with the underwater station.

  According to the underwater communication system and the underwater communication system of the present invention, by transmitting and receiving data at the arrival time interval between the direct wave and the delayed wave, the influence of interference due to the delayed wave is reduced, and stable underwater communication with less errors is possible. become.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to FIGS. Note that capital letters 1, 2, I, and F following T in the parameters T1, T2, TI, and TF used in the following description are shown as subscript lowercase letters in the drawing.

  In the following description, considering the underwater communication between the underwater station 10 and the surface station 20 in the positional relationship as shown in FIG. 9, the receiver 110 and the transmitter 100 of the underwater station 10 not shown in FIG. Is assumed to be at the same position, and the transmitter and receiver 200 provided at the water station 20 (not shown in FIG. 9) are assumed to be at the same position. The receiver 110 and the transmitter 100 are located at a depth Dt from the sea surface 1 of 150 m and a distance H from the sea floor 2 of 50 m, and are not shown in FIG. The distance from the sea surface 1 is 10 cm, and the horizontal distance L between the transmitter 100 and the receiver 200 is 200 m. The symbol rate T is 8000 baud, and the frequency band used for data communication is 42 kHz to 58 kHz.

FIG. 1 is a block diagram showing a device configuration of an underwater station 10 used in an underwater communication system according to an embodiment of the present invention.
FIG. 2 is a block diagram showing a device configuration of the floating station 20 used in the underwater communication system according to the embodiment of the present invention.

FIG. 3 is a characteristic diagram illustrating an example of a correlation function obtained by the correlation processing unit of the underwater station used in the underwater communication system according to the embodiment of the present invention. In FIG. 3, the vertical axis represents the correlation value (relative value of the correlation function), and the horizontal axis represents the time normalized by the symbol rate T.
FIG. 4 is a diagram showing a configuration of a data frame sequence used in the underwater communication system according to the embodiment of the present invention.

FIG. 5 is a diagram illustrating the timing of the data frame sequence output from the signal generation unit of the underwater station used in the underwater communication system according to the embodiment of the present invention.
FIG. 6 is a diagram illustrating another timing of the data frame sequence output from the signal generation unit of the underwater station used in the underwater communication system according to the embodiment of the present invention.

  In FIG. 1, a receiver 110 provided in the underwater station 10 receives a known pulse signal transmitted from the surface station 20. As this pulse signal, for example, a chirp signal is used. The pulse signal received by the receiver 110 is amplified by the reception amplifier 111, converted from an analog signal to a digital signal by the A / D converter 112, and output to the correlation processing unit 113. The correlation processing unit 113 performs a correlation operation using the known pulse signal transmitted from the surface station 20 as a replica signal, and calculates a correlation function.

  FIG. 3 is a characteristic diagram illustrating an example of a correlation function obtained by the correlation processing unit 113 when the underwater station 10 and the surface station 20 are located in the above-described positional relationship. Here, the pulse signal is a chirp signal having a pulse width of 1 ms having the above frequency band, and the time axis on the horizontal axis indicates the time normalized by the symbol rate T. Based on the correlation function obtained as shown in FIG. 3, the delay time detecting unit 114 shown in FIG. 1 has a maximum delay time ΔT of a delayed wave exceeding a predetermined allowable delay time and a predetermined correlation threshold. Ask for. For example, assume that a direct wave is detected at T1 with an allowable delay time of 30 symbols and a correlation threshold of 1/10 of the correlation value at T1. The delay time detection unit 114 further detects a delay wave at T2, and outputs ΔT = T1-T2 as the maximum delay time.

  Based on the maximum delay time ΔT obtained from the delay time detection unit 114 shown in FIG. 1, in the signal generation unit 120 shown in FIG. 1, the length TF of the data frame transmitted at a time is less than the maximum delay time ΔT. Thus, data is extracted from the transmission data 30, and a data frame sequence 40 as shown in FIG. 4 is generated. The data frame sequence 40 includes a frame synchronization sequence 41 located at the head of the frame, a training sequence 45 used for initializing tap coefficients of the adaptive equalizer 220 described later, a data sequence 43 extracted from the transmission data 30, a frame It consists of a frame synchronization sequence 42 located at the end. In order to perform reliable synchronization, a pause 44 may be inserted between the frame synchronization sequence 41 and the training sequence 45 or between the data sequence 43 and the frame synchronization sequence 42.

  The signal generation unit 120 outputs the data frame sequence generated in this way to the modulation unit 121 one after another at an interval TI equal to or greater than the maximum delay time ΔT. That is, the data frame sequence 40 is output from the signal generation unit 120 at the timing shown in FIG.

  When the maximum delay time ΔT is very long, data is extracted from the transmission data 30 so that the data frame length TF is ½ or less of the maximum delay time ΔT as shown in FIG. One data frame sequence 40 may be output at an interval TI equal to or greater than the maximum delay time ΔT. Similarly, three or more data frame sequences 40 may be output at an interval TI equal to or greater than the maximum delay time ΔT. This improves the accuracy of frame synchronization and equalization, and can reduce transmission errors.

  The modulation signal modulated by the modulation unit 121 is converted from a digital signal to an analog signal by the D / A converter 122, amplified by the transmission amplifier 123, converted to acoustic power by the transmitter 100, and transmitted to the water. Is done. The signal transmitted to the water is received by the receiver 200 provided in the surface station 20, amplified by the reception amplifier 211, and then passed through the low-pass filter (LPF) 212 to the A / D converter 213. In this way, an analog signal is converted into a digital signal. Here, the cutoff frequency of the low-pass filter (LPF) 212 may be a Nyquist frequency determined from the sampling frequency of the A / D converter 213.

  The converted digital signal is scaled by a digital automatic gain control circuit (AGC) 214 and input to a band pass filter (BPF) 215 to remove out-of-band noise. The received signal that has passed through the band pass filter (BPF) 215 is then input to the frame synchronization unit 216.

  In the frame synchronization unit 216, the frame synchronization sequence 41 and the frame synchronization sequence of the data frame sequence 40 are detected by correlation processing, and the training sequence 45 and the data sequence 43 sandwiched between the frame synchronization sequence 41 of the data frame sequence 40 are detected. Extracted and output to the demodulator 218. The symbol length detection unit 217 calculates the number of symbols included in the extracted training sequence 45 and data sequence 43 with reference to the correlation processing result in the frame synchronization unit 216. Whether or not the data sequence 43 is correctly extracted depends on the accuracy of the frame synchronization. Therefore, the frame synchronization sequence 41 and the frame synchronization sequence 42 have a sharp correlation function like a Barker code or an M sequence code. It is desirable to use something.

  The signal demodulated by the demodulator 218 has a tap interval T / m (where m is a natural number) of the adaptive equalizer 220 by the decimator 219 by the length corresponding to the number of symbols obtained from the symbol length detector 217. Downsampled and input to the adaptive equalizer 220. The adaptive equalizer 220 initializes the tap coefficients using the training sequence 45, equalizes the data sequence 43, and outputs the reception data 31. The adaptive equalizer 220 may be a linear adaptive equalizer. Preferably, the decision feedback type adaptive equalizer has better estimation accuracy and the number of trainings by the training sequence 45 may be reduced. it can.

  Here, the operation of the adaptive equalizer 220 will be described. When the tap interval of adaptive equalizer 220 is T / m (where m is a natural number) and the number of taps is n, adaptive equalizer 220 arrives within T · n / m after the direct wave arrives. Intersymbol interference due to a delayed wave is an object of equalization, but a delayed wave that arrives after T · n / m or more cannot be equalized.

  That is, as shown in FIG. 3, in order to equalize the delayed wave at time T2 that arrives at a delay of about 380 symbols from the time T1 when the direct wave arrives, the adaptive equalizer 220 has at least 380 taps or more. There must be a number of taps. If m is increased in order to improve equalization accuracy or the symbol rate T is increased in order to increase the transmission speed, the number of taps of several thousand taps or more must be required, which makes real-time processing difficult. become.

  Therefore, in the underwater communication as shown in FIG. 9, the delayed wave arriving along the broken-line path Rs can be equalized by the adaptive equalizer 220, but arrives along the one-dot chain line Rb1 and the two-dot chain line Rb2. The delayed wave is difficult to equalize and causes deterioration in communication stability. Hereinafter, the length of the data frame sequence and the stability of communication will be described with reference to FIGS.

  FIG. 7 is a characteristic diagram showing an example of an equalization error function of the decision feedback type adaptive equalizer of the surface station used in the underwater communication system of the embodiment of the present invention. In FIG. 7, the vertical axis represents the mean square error, and the horizontal axis represents the time normalized by the symbol rate T.

  FIG. 8 is a reception point arrangement diagram of four-phase shift key modulation reception data output from the decision feedback adaptive equalizer in the system of the embodiment of the present invention and the conventional system. FIG. 8A shows the result obtained by the conventional method, and FIG. 8B shows the result obtained by the method of the embodiment of the present invention. 8A and 8B show the arrangement of the reception points on the complex plane, the real part Re shows the in-phase component having the same phase, and the imaginary part Im shows the quadrature component.

FIG. 7 is a characteristic diagram showing an example of the equalization error function of the decision feedback adaptive equalizer when a decision feedback adaptive equalizer is used as the adaptive equalizer 220. Here, the characteristics of the transmission path without noise are the same as those in FIG. 10, and the number of taps in the feedforward filter of the decision feedback adaptive equalizer is 8, the tap interval is T / 2, and the number of taps in the feedback filter. 3 and the tap interval T. The modulation method was a four-phase shift keying method, and the signal energy to noise power ratio per bit was 12 dB. As is apparent from FIG. 7, the influence of the delayed wave A2 (see FIG. 10B) is removed from the data frame sequence for 1500 symbols, but the delayed waves B1 and B2 (see FIG. 10C) After the time of interference, the equalization error becomes very large.
As is apparent from FIG. 8, it is understood that the underwater communication system according to the embodiment of the present invention enables stable underwater communication with few errors.

  In the decision feedback type adaptive equalizer of the above example, since the delay wave from the arrival time of the direct wave to the delay of 3 symbols can be equalized, the intersymbol interference of the delay wave A2 in FIG. However, the delayed wave in FIG. 10C cannot be equalized. Therefore, the maximum delay time ΔT may be detected for a delayed wave that arrives with a delay of 3 symbols or more after the direct wave arrives. That is, when the floating station 20 includes an adaptive equalizer 220 that equalizes received symbol data with a tap interval T / m and a tap number n, where m is a natural number and T is a symbol rate, The allowable delay time in the delay time detector 114 of 10 may be T · n / m or more, and when the adaptive equalizer 220 is a decision feedback type adaptive equalizer having the number of feedback filter taps nb, The allowable delay time in the delay time detection unit 114 of the underwater station 10 may be T · nb or more.

  As described above, according to the present invention, the maximum delay time ΔT of the delayed wave that cannot be equalized by the adaptive equalizer 220, for example, arrives along the alternate long and short dashed path Rb1 and the alternate long and two short dashed line Rb2 shown in FIG. Therefore, the length of the data frame sequence 40 to be transmitted / received is set to ΔT or less, and transmission / reception is performed at intervals of ΔT or more, so that the influence of intersymbol interference of delayed waves that arrive relatively directly from the direct wave is avoided. And stable underwater communication becomes possible.

  The underwater communication system and the underwater communication system of the present invention are used for transmitting and receiving digital data between two distant points in the water, and are mainly used for underwater vehicles and the like for undersea exploration and investigation of the underwater environment in shallow water. It is used to send and receive digital data between the underwater station and a surface station such as a communication buoy floating near the sea surface.

The block diagram which shows the apparatus structure of the underwater station used for the underwater communication system of the Example of this invention. The block diagram which shows the apparatus structure of the water station used for the underwater communication system of the Example of this invention. The characteristic view which showed an example of the correlation function obtained in the correlation processing part of the underwater station used for the underwater communication system of the Example of this invention. The block diagram of the data frame series used for the underwater communication system of the Example of this invention. The timing diagram of the data frame series output from the signal generation part of the underwater station used for the underwater communication system of the Example of this invention. The another timing diagram of the data frame series output from the signal generation part of the underwater station used for the underwater communication system of the Example of this invention. The characteristic view which showed an example of the equalization error function of the decision feedback type | mold adaptive equalizer of the surface station used for the underwater communication system of the Example of this invention. The receiving point arrangement | positioning figure of the 4-phase shift keying modulation | alteration data output from the decision feedback type | mold adaptive equalizer in the system of the Example of this invention, and a conventional system. Explanatory drawing for demonstrating the ultrasonic propagation of the underwater communication system which consists of an underwater station and a surface station in a prior art. In FIG. 9, a pulse response waveform diagram showing a temporal response of a pulse received by the receiver 200.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Sea surface, 2 ... Submarine, 3 ... Radio antenna, 10 ... Underwater station, 20 ... Water station, 30 ... Transmission data, 31 ... Reception data, 40 ... Data frame series, 41, 42 ... Frame synchronization series, 43 ... Data Sequence, 44 ... pause, 45 ... training sequence, 100 ... transmitter, 110 ... receiver, 111, 211 ... reception amplifier, 112, 213 ... A / D converter, 113 ... correlation processing unit, 114 ... delay time Detection unit, 120 ... Signal generation unit, 121 ... Modulation unit, 122 ... D / A converter, 123 ... Transmission amplifier, 200 ... Receiver, 212 ... Low-pass filter (LPF), 214 ... Digital automatic gain control circuit (AGC), 215... Band pass filter (BPF), 216... Frame synchronization unit, 217... Symbol length detection unit, 218.

Claims (2)

In an underwater communication system that transmits and receives digital data between a water station and an underwater station using ultrasonic waves as a transmission medium,
The underwater station receives a known transmission pulse transmitted from the surface station, converts the received signal into an electrical signal, a signal obtained from the receiver, and the known transmission pulse. A correlation processing unit for obtaining a correlation value of the delay wave, detecting a delay time between the direct wave and the delay wave from the correlation value, and a maximum delay time of the delay wave exceeding a predetermined allowable delay time and a predetermined correlation threshold A delay time detection unit for obtaining a data frame sequence of transmission data with the maximum delay time as a maximum frame length, and outputting the data frame sequence at an interval of the maximum delay time; and the signal generation unit A modulation unit that generates a modulation signal modulated by the output of the output, a D / A converter that converts the modulation signal into an analog signal, a transmission amplifier that amplifies the analog signal, and an electric power of the transmission amplifier Underwater communication system characterized by comprising a wave transmitter for transmitting into the water to convert the output into acoustic power. The underwater communication system according to claim 1, wherein the data frame sequence includes a frame synchronization sequence Fs representing a frame head, a data sequence, and a frame synchronization sequence Fe representing a frame end, and the data sequence is The data station is located between the frame synchronization sequence Fs and the frame synchronization sequence Fe in time, and the water station detects the frame synchronization sequence Fs and the frame synchronization sequence Fe to detect the data sequence. A frame synchronization unit that extracts a modulation signal of the data sequence; a symbol length detection unit that calculates a symbol length of data included in the modulation signal of the data sequence; a demodulation unit that demodulates the modulation signal of the data sequence and outputs received data; An underwater communication system comprising:

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