JP4264832B2 - Passive phase conjugate underwater acoustic communication method and passive phase conjugate underwater acoustic communication system - Google Patents

Description

  The present invention transmits information from a sound source to a receiver array by transmitting a transmission signal composed of a probe signal and a data string continuously from the sound source to a receiver array composed of a plurality of receivers. The present invention relates to a passive phase conjugate underwater acoustic communication method and a system for realizing the method.

  In the operation of autonomous underwater vehicles (AUV), etc., if the AUV status can be monitored or commands sent to the AUV through underwater acoustic communication, the operability will be greatly improved. be able to. In this case, it is necessary to realize long-distance horizontal underwater acoustic communication without being obstructed by an obstacle or the like existing in the middle of propagation of the sound wave.

  As a method for realizing such a long-distance horizontal underwater acoustic communication, a method using a phase conjugate wave has been studied (for example, Non-Patent Documents 1, 2, and 3). A phase conjugate wave is a wave that has the same spatial amplitude distribution as a certain wave and reverses in time, and is sometimes referred to as a time-reversed wave. A sound wave emitted from a sound source is received by a transceiver array composed of a plurality of receivers, and the received sound wave is inverted on the time axis to generate a phase conjugate wave for the received sound wave. When the wave is retransmitted from the transceiver array, the sound wave converges to the original sound source position (focus).

  When a phase conjugate wave is transmitted from the array, the sound wave propagation phenomenon occurs with the time reversed, so that the traveling direction is reversed from the time when the sound wave was transmitted at the position of the original sound source. Is received. In other words, since the phase conjugate wave is a sound wave having the same spatial distribution shape of sound pressure and only the traveling direction is opposite, the position of the original sound source (with the same spatial shape as when transmitted from the sound source ( The sound wave will converge to the focal point.

  Convergence by a phase conjugate wave is a phenomenon realized by temporally and spatially collecting reflected waves and refracted waves generated until the phase conjugate wave emitted from the transceiver array reaches the original sound source. It is known that the convergence by the phase conjugate wave has a very high convergence performance compared to the conventional beam forming technique.

  In order to use a phase conjugate wave for underwater acoustic communication from a transmitter / receiver array (for example, base station side) to an original sound source (for example, AUV side), the following is performed. First, a pulse wave is transmitted from a sound source, a phase conjugate wave of the pulse wave received by each receiver constituting the transceiver array is generated, and prepared as a phase conjugate pulse wave. That is, the phase conjugate pulse wave is stored in a storage device provided in each receiver constituting the transceiver array.

  Next, the phase conjugate pulse wave is multiplied by a data string to be transmitted to obtain a transmission signal from the transceiver array to the sound source. Since this transmission signal contains phase conjugate pulse waves at symbol intervals, if this transmission signal is transmitted from the transceiver array toward the sound source, the sound wave converges to the position (focus) of the original sound source, The original data string (data string transmitted from the transceiver array toward the sound source) is received and reproduced by the sound source.

  If the phase conjugate wave is not used for communication from the transmitter / receiver array to the sound source, the multipath wave composed of the reflected wave and the refracted wave overlaps with the adjacent signal, and the received signal cannot be demodulated. Sometimes. That is, intersymbol interference occurs and the received signal is not accurately demodulated at the sound source. Therefore, in conventional communication that does not use a phase conjugate wave, the above-described multipath wave is removed using an adaptive filter (AF) technique. However, it is known that this method of removing multipath waves by AF has limitations.

  On the other hand, in contrast to the conventional method that does not use the phase conjugate wave, the method that uses the phase conjugate wave converges the signal at the position of the original sound source so that the phase conjugate wave converges to the original sound source position. If received, there is almost no inter-symbol interference and the reception level is high, so the accuracy of communication is high. Further, in communication using this phase conjugate wave, a technique for further adaptively filtering the signal received at the position of the original sound source has been studied. According to this adaptive filter processing method, there is a synergistic effect that adaptive signal processing removes a signal that has not been converged even after using a phase conjugate wave and has become noise-like, and the conventional adaptive filter processing is not performed. It has been clarified that communication can be performed with much higher accuracy than underwater acoustic communication using phase conjugate waves.

  As described above, underwater acoustic communication in which a signal transmitted from the transceiver array is received by the original sound source, that is, communication from the array to a point-like object is actually transmitting a phase conjugate wave from the array. Therefore, it is called active phase conjugate underwater acoustic communication, Active Time Reversal communication, etc. On the other hand, the theory of phase conjugate wave can be applied to underwater acoustic communication in which an array of signals transmitted from a point-like sound source is received (for example, Non-Patent Document 4). And 5).

  In this case, since the information to be sent from the point-like sound source is sent to the array, this array has a function as a receiver. Therefore, an array that receives information sent from a point-like sound source is called a receiver array.

  In order to apply the theory of phase conjugate wave to underwater acoustic communication in which a signal transmitted from a point-like sound source is received by a receiver array, the following is performed. First, a transmission signal composed of a probe signal and a data string reflecting information to be transmitted is continuously transmitted from the sound source. Next, this transmission signal is received by a receiver array composed of a plurality of receivers, and at each receiver constituting the receiver array, a correlation waveform between the received probe signal and the received data string. Ask for. Finally, if the sum of correlation waveforms obtained at each receiver constituting the receiver array is obtained as a received signal, the processing is the same as in communication using a phase conjugate wave, and it is transmitted from the sound source. A data string that is almost identical to the recorded data string is reproduced.

In underwater acoustic communication in which an array receives signals transmitted from the above-described point-like sound source, a phase conjugate wave is not actually transmitted to the outside of the sound source or the array. Therefore, this communication method is referred to as passive phase conjugate underwater acoustic communication, Virtual Time Reversal underwater acoustic communication, or the like.
T. Shimura, Y. Watanabe, H. Ochi, "A Basic Research on the Long Horizontal Active Time Reversal communication", Oceans 2004, pp. 2219-2224 (2004). Takuya Shimura, Yoshitaka Watanabe, Hiroshi Ochi, `` Fundamental study of underwater acoustic communication using Active Time Reversal '' 2004 Ocean Acoustics Society Proceedings, pp.61-64 (2004). Takuya Shimura, Yoshitaka Watanabe, Hiroshi Ochi, “Fundamental study of underwater acoustic communication using Active Time Reversal” IEICE Technical Report, US2004-33, pp. 7-12 (2004). TC Yang, "Differences Between Passive-Phase Conjugation and Decision-Feedback Equalizer for Underwater Acoustic Communications", IEEE J. Oceanic Engineering, Vol 29, No. 2, pp. 472-487. D. Rouseff, et al., "Underwater Acoustic Communication by Passive-Phase Conjugation: Theory and Exprimental Results", IEEE J. Oceanic Engineering, Vol. 26, No. 4, pp.821-831 (2001).

  In the passive phase conjugate underwater acoustic communication, as in the case of the active phase conjugate underwater acoustic communication, even if the phase conjugate wave is used, the signal in the form of noise is still mixed. By increasing the number of receivers constituting the receiver array, this noise-like signal can be reduced. However, increasing the number of receivers constituting the receiver array is technically economical. With some difficulties.

  Accordingly, an object of the present invention is to provide a passive phase conjugate underwater acoustic communication method capable of performing communication with high accuracy without increasing the number of receivers constituting the receiver array, and a system for realizing the method. There is.

  In passive phase conjugate underwater acoustic communication using a receiver array composed of a limited number of receivers, it is necessary to remove a noise-like signal by some method. If this noise-like signal can be removed, it is expected that at least the number of receivers constituting the receiver array can be communicated with high accuracy in passive phase conjugate underwater acoustic communication.

  Up to now, there has been provided means for removing a noise-like signal mixed in the data sequence reproduced from the data sequence reproduced as the sum of correlation waveforms obtained in each receiver constituting the receiver array. No passive phase conjugate underwater acoustic communication method has been reported.

To solve the problems described above, the passive phase conjugate underwater acoustic communication method of the present invention, the steps listed below including.
(A) A first step of transmitting a transmission signal composed of a probe signal and a data string continuously from a sound source (B) A transmission signal is received by a receiver array composed of a plurality of receivers, In each receiver constituting the receiver array, a second step for obtaining a correlation waveform between the received probe signal and the received data string (C) Correlation output from all receivers constituting the receiver array Third step for obtaining the sum of waveforms as a received signal (D) A fourth step for performing noise removal on the received signal using an adaptive filter.
In the method of the present invention, the number of the plurality of receivers constituting the receiver array is the number of remaining noise-like signal components that cannot be converged due to the convergence property of the phase conjugate wave in the received signal. The adaptive filter removes a noise-like signal component remaining in the received signal without being converged.

A system for realizing the above-described passive phase conjugate underwater acoustic communication method is configured as follows. That is, the passive phase conjugate underwater acoustic communication system includes a sound source, a receiver array, an adder, and an adaptive filter. A transmission signal composed of a probe signal and a data string is transmitted from the sound source. The receiver array is composed of a plurality of receivers. The transmission signals are received by the plurality of receivers, respectively, and a correlation waveform between the received probe signal and the received data string is obtained. The adder generates a sum of correlation waveforms output from all receivers constituting the receiver array as a reception signal. The adaptive filter removes noise from this received signal. In the system of the present invention, the number of the plurality of receivers constituting the receiver array is the number of noise-like signal components that cannot be converged due to the convergence property of the phase conjugate wave in the received signal. The adaptive filter removes a noise-like signal component remaining in the received signal without being converged.

  The passive phase conjugate underwater acoustic communication method of the present invention further includes a fourth step of performing noise removal by an adaptive filter in addition to the first to third steps provided by the conventional passive phase conjugate underwater acoustic communication. Yes. The first to fourth steps can be realized in the passive phase conjugate underwater acoustic communication system of the present invention. That is, the first to fourth steps are executed by the above-described sound source, receiver array, adder, and adaptive filter, respectively.

  Therefore, according to the present invention, a noise-like signal that has not been sufficiently converged in the past can be removed by the noise removal process by the adaptive filter executed in the fourth step, which is included in the data string reproduced in the receiver array. The effect is obtained. In other words, the noise removal processing by the adaptive filter makes it possible to reproduce a data string having a small amount of noise-like signal components to such an extent that communication that does not cause noise failure in practice can be performed.

  In other words, even in a situation where it is difficult to provide a sufficient number of receivers constituting the receiver array, a noise-like signal that still cannot be converged using a method using a phase conjugate wave, It can be removed sufficiently in the fourth step. As a result, even when it is difficult to provide a sufficient number of receivers constituting the receiver array, it is as high as when using a receiver array including a sufficient number of receivers. Communication can be performed with accuracy.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure shows one configuration example according to the present invention, and only schematically shows the arrangement relationship of each component to the extent that the present invention can be understood. It is not limited to. In addition, overlapping description of the same components in each drawing may be omitted.

  First, the principle of active phase conjugate underwater acoustic communication will be specifically described with reference to FIGS. 1 (A) to (D). FIG. 1 (A) shows a transmitter / receiver array 20 installed in a position far away in the horizontal direction from the AUV 10 by transmitting a probe wave S from the AUV 10 equipped with the sound source into the seawater 12 on the seabed 14. It shows how it is received. The transceiver array 20 includes a total of five receivers 20-1 to 20-5 that are linearly connected in the vertical direction.

  In FIG. 1A, a transceiver array 20 including a total of five receivers is shown, but the number of receivers is not necessarily limited to five. Further, although the transceiver array 20 is arranged in a line in the vertical direction in FIG. 1A, it is not necessary to arrange in a line in this way, and the arrangement shape may be arbitrary. As the number of receivers included in the transceiver array increases, the convergence performance of the phase conjugate wave, which will be described later, increases, and the quality of underwater acoustic communication increases. Installation work becomes difficult. On the other hand, if the number of receivers included in the transmitter / receiver array is small, the manufacture and installation work of the transmitter / receiver array is reduced, but the convergence property of the phase conjugate wave is deteriorated and the quality of the underwater acoustic communication is deteriorated.

  The probe wave S transmitted from the AUV 10 equipped with the sound source is reflected (reflected wave) by the sea floor 18 or sea water surface 16 or is refracted or diffracted due to non-uniform sound velocity distribution of the sea water 12. (Diffracted wave) and scattered by an obstacle present in the seawater 12 (scattered wave) to reach each receiver (receiver 20-1 to receiver 20-5) constituting the transceiver array 20 . In the following description, the reflected wave, diffracted wave, and scattered wave described above may be referred to as a multipath wave.

  When the probe wave S is transmitted from the sound source installed in the AUV 10, a multipath wave is generated while propagating through the seawater 12, while the transmitter array 20 is transmitted from the sound source installed in the AUV 10. The probe wave S immediately after that is received by the transceiver array 20 as a probe wave R which is a received wave due to deformation of the wavefront.

  FIG. 1 (C) is a diagram conceptually showing an example of a time waveform of the probe wave R received by each of the receivers 20-1 to 20-5 constituting the transceiver array 20. is there. The received waves received by the receivers 20-1 to 20-5 in order from the top are shown. For example, in the receiver 20-1, as shown at the top of FIG. 1 (C), the wave portion indicated by P with an arrow first arrives, and finally the wave indicated by Q with an arrow is reached. It shows how the part arrives. Similarly, each of the receivers 20-2 to 20-5 receives a wave having a time waveform as shown in the fifth column from the second column from the top in FIG. 1C.

  For example, a sonic pulse wave (or burst wave) that is symmetrical with respect to the time axis is used as the probe wave S transmitted from the sound source mounted on the AUV 10, but it is multipath while propagating in the seawater 12. When a wave is generated and received by the transceiver array 20, the multipath wave is mixed, so that the received probe wave R is complicated as shown in FIG. It becomes a time waveform. In addition, since the probe wave received by each receiver (receiver 20-1 to receiver 20-5) constituting the transceiver array 20 is different from each other in the spatial position where the receiver is set, Each time waveform has a different shape.

  FIG. 1B shows a state where a phase conjugate wave S ′ with respect to the probe wave R is generated and transmitted at each receiver constituting the transceiver array 20 and the phase conjugate wave R ′ is received by the AUV 10. Yes.

  Each receiver constituting the transceiver array 20 receives the probe wave R and generates a signal wave (phase conjugate wave) S ′ in which the time axis of the received signal wave is inverted. The phase conjugate wave S ′ immediately after being transmitted from the transceiver array 20 generates a multipath wave while propagating through the seawater 12, and in the vicinity of the AUV 10, the phase conjugate wave S immediately after being transmitted from the transceiver array 20 is generated. The wavefront is deformed to become a phase conjugate wave R and is received by AUV 10.

  FIG. 1D is a diagram conceptually illustrating an example of a time waveform of a phase conjugate wave generated in the receiver 20-5 from the receiver 20-1 constituting the transceiver array 20. For example, the phase conjugate wave generated by the receiver 20-1 has a time waveform shown at the top of FIG. This time waveform is represented as a wave with the time axis reversed, as can be seen from the fact that the time waveform shown at the top of FIG. That is, contrary to when the receiver 20-1 receives the probe wave, the wave portion indicated by Q with an arrow first propagates first, and the wave portion indicated by P with an arrow finally propagates. The state of doing is drawn. A phase conjugate wave is generated by inverting the sign of the parameter (time t) indicating the time waveform of the wave.

  In the vicinity of the AUV 10, the phase conjugate wave S ′ transmitted from the transceiver array 20 is deformed from the time waveform immediately after being transmitted from the transceiver array 20 to become a phase conjugate wave R ′. As a property inherent to the phase conjugate wave, the phase conjugate wave R ′ converges to the AUV 10. However, in order to achieve complete convergence, the probe wave S is transmitted from the AUV 10 in the middle of the AUV 10 and the transmitter / receiver array 20 and has an effect such as reflection, diffraction, and scattering on the propagation of the sound wave. Thus, the process received by the transceiver array 20 and the process where the phase conjugate wave S ′ is transmitted from the transceiver array 20 and received by the AUV 10 need to be in the same state.

  For example, due to ocean currents, the refractive index distribution of seawater 12 is generated when the probe wave S is transmitted from the AUV 10 and received by the transceiver array 20, and the phase conjugate wave S ′ is transmitted from the transceiver array 20. The phase conjugate wave R ′ does not converge completely if it is different from when it is received by the AUV 10. However, the time from when the probe wave R is received by the transceiver array 20 until the phase conjugate wave S ′ is transmitted is not so long. In actual underwater acoustic communication, the refractive index distribution of the seawater 12, etc. The factors that give effects such as reflection, diffraction, and scattering to the propagation of sound waves are not greatly changed. Therefore, the phase conjugate wave R ′ converges to the AUV 10 with sufficiently high accuracy.

  An AUV 10 comprising the original sound source from the transceiver array 20 (for example, a base station) by utilizing the property that the phase conjugate wave inherently converges at the position of the sound source that transmitted the probe wave. To perform communication to send information to

  First, a probe wave (sound pulse) S is transmitted from the AUV 10 and received by the receivers 20-1 to 20-5 constituting the transceiver array 20. Each of the receivers 20-1 to 20-5 performs time reversal processing on the received probe wave R. That is, a phase conjugate wave with respect to the received probe wave R is generated. Then, a data string reflecting information to be transmitted from the transceiver array 20 serving as a base station toward the AUV 10 is multiplied by the phase conjugate wave to generate a transmission signal. Since this transmission signal includes pulsed phase conjugate waves at symbol intervals, if this transmission signal S ′ is transmitted from the transceiver array 20, this transmission signal R ′ converges at the position of AUV 10. . Therefore, the transmission signal R ′ is efficiently received by the AUV 10, and as a result, the data string transmitted from the transceiver array 20 is reproduced with high communication quality.

  In this way, if a phase conjugate wave is used to send information from the transceiver array 20 to the AUV 10, the data string is reproduced with high fidelity in the AUV 10, so that there is almost no intersymbol interference and high. A reception level is realized. Therefore, highly accurate communication can be realized.

  As described above, in underwater acoustic communication for transmitting information from the transceiver array 20 to the AUV 10, from a plurality of receivers (receiver 20-1 to receiver 20-5) constituting the transceiver array 20 The phase conjugate wave S ′ is transmitted as an actual sound wave. Underwater acoustic communication that sends information from the transceiver array 20 to the AUV 10 carries information transmitted from a plurality of receivers (receiver 20-1 to receiver 20-5) constituting the transceiver array 20. This is a transmission system in which sound waves are transmitted and received by the AUV 10 located at a point in the seawater. Therefore, such underwater acoustic communication is referred to as communication from the transceiver array to one point.

  On the other hand, underwater acoustic communication for sending information from an AUV present at one point in seawater toward the receiver array is called communication from one point toward the receiver array. As described above, passive phase conjugate underwater acoustic communication is used for communication from one point to the receiver array. Thus, passive phase conjugate underwater acoustic communication will be described with reference to FIGS. 2 (A) to 2 (E). In passive phase conjugate underwater acoustic communication, the phase conjugate wave itself is not transmitted toward seawater.

  Fig. 1 (A) is a drawing referenced to explain the principle of active phase conjugate underwater acoustic communication, but in terms of transmitting sound waves from the AUV toward the array, the passive phase conjugate described below. Since it is common in underwater acoustic communication, for convenience, passive phase conjugate underwater acoustic communication will be described with reference to FIG. 1 (A) again. However, in passive phase conjugate underwater acoustic communication, the phase conjugate wave is not transmitted into the seawater from the array side, so see Figure 1 (A) for an explanation of passive phase conjugate underwater acoustic communication. In this case, the transceiver array 20 is replaced with the receiver array 20. In addition, in FIG. 1A, the receiver array 20 is arranged in a line in the vertical direction, but as in the case of the active phase conjugate underwater acoustic communication described above, it is necessary to arrange the receiver array 20 in a line in this way. However, the shape of the arrangement may be arbitrary.

  The sound wave transmitted from the AUV 10 was a probe wave in the active phase conjugate underwater acoustic communication described above, whereas in the passive phase conjugate underwater acoustic communication, the probe signal and the data sequence are continuous. The difference is that the transmission signal is configured as follows. However, in the following description of the passive phase conjugate underwater acoustic communication, the transmission signal transmitted from the AUV 10 is represented as a transmission signal S within a range where no confusion occurs, and the transmission signal immediately before the receiver array 20 Is expressed as a transmission signal R.

  FIG. 2A shows a time waveform of the transmission signal S transmitted from the AUV 10. The probe signal P and the data string D are configured continuously. 2 (B-1) and 2 (B-5) show time waveforms of reception signals received by the receiver 20-1 and the receiver 20-5 constituting the receiver array 20, respectively. That is, the time waveform of the received signal received by each of the receivers 20-2 to 20-4 constituting the receiver array 20 is omitted. Here, as in the case of the description of the active phase conjugate underwater acoustic communication described above, the description will be made assuming that there are five receivers constituting the receiver array 20, but it is not necessary to limit the number to five. .

  In FIG. 2 (B-1), the time waveform P ′ indicates the probe signal received by the receiver 20-1, and the time waveform D ′ indicates the data string received by the receiver 20-1. Similarly, in FIG. 2 (B-5), the time waveform P ′ indicates the probe signal received by the receiver 20-5, and the time waveform D ′ indicates the data string received by the receiver 20-5. .

  The time waveform shown in FIG. 2 (C-1) is the time waveform of the probe signal P ′ received by the receiver 20-1 in FIG. 2 (B-1) and the receiver in FIG. This is a correlation waveform with the time waveform of the data string D ′ received at 20-1. Also, the time waveform shown in FIG. 2 (C-5) is the time waveform of the probe signal P ′ received by the receiver 20-5 in FIG. 2 (B-5), and in FIG. This is a correlation waveform with the time waveform of the data string D ′ received by the receiver 20-5. These correlation waveforms are generated by a digital signal processor provided in the receiver 20-1 and the receiver 20-5.

  When the sum of correlation waveforms output from all the receivers (20-1 to 20-5) constituting the receiver array 20 is obtained by an adder, the data string given by the time waveform shown in FIG. available. This data string has the same time waveform as the data string constituting the transmission signal transmitted from the AUV 10. That is, it is known that the data signal transmitted from the AUV 10 is reproduced by obtaining the sum of correlation waveforms by an adder (for example, Non-Patent Document 5).

  The process of obtaining the correlation waveform from the received signals received by each receiver constituting the receiver array and obtaining the sum of these correlation waveforms by the adder is the same as the phase conjugate wave generation in the active phase conjugate underwater acoustic communication described above. Corresponds to the mathematical processing of For this purpose, the data string transmitted from the AUV 10 is generated in the receiver array 20 as a correctly reproduced reception signal. Thus, in passive phase conjugate underwater acoustic communication, the phase conjugate wave is not actually transmitted from the receiver array 20 into the seawater.

  The present invention is characterized in that adaptive filter processing is further applied to the data string reproduced in the receiver array 20. As shown in FIG. 2 (E), a data string having a time waveform from which a noise-like signal is removed by adaptive filter processing is reproduced from the reproduced data string shown in FIG. 2 (D). As described above, the present inventors have confirmed through simulation that the effects described below can be obtained by further performing adaptive filter processing on the data string reproduced in the receiver array 20.

<Simulation results in shallow water>
With reference to FIG. 3, the result of simulating the effect of adaptive filter processing in passive phase conjugate underwater acoustic communication in shallow water will be described. FIG. 3 is a diagram for explaining condition setting in this simulation. In this simulation, the effect of adaptive filter processing was verified for passive phase conjugate underwater acoustic communication in shallow water where the speed of sound in the sea can be regarded as constant regardless of water depth.

  The depth H, which is the distance from the bottom 38 of the seabed 34 to the seawater 36 of the seawater 32, is 100 m, and the distance between the AUV 30 and the receiver array 40 is 25 km. In addition, the total number of receivers constituting the receiver array 40 is seven, and a total of seven receivers from the receiver 40-1 to the receiver 40-7 are arranged in order from the side close to the sea surface 36. The interval between each receiver was 15 m.

In the calculation of the propagation of sound waves in the seawater 32, the Pekeris solution of the normal mode method was used assuming that the speed of sound in the seawater 32 is 1500 m / s (the value of c ₁ described later) regardless of the depth ( (See, for example, CB Officer, Introduction to the Theory of SOUND TRANSMISSION with Application to the Ocean, New York, McGraw-HILL Book Company. Pp. 117-145).

That is, the velocity potential given by the following equation (1) is used, where r is the position vector at the position of any receiving point and r _s is the position vector indicating the position of the AUV 30. Here, A (k _n ) is an amplitude coefficient and is given by the following equation (2). H is the water depth (distance from the sea surface 36 to the sea bottom 38), k _n is the horizontal wave number of the n-th mode, β ₁ and β ₂ are the wave numbers in the vertical direction in the sea water 32 and the sea floor 34, respectively. is there. β ₁ and β ₂ are given by the following equations (3) and (4), respectively. j is imaginary unit, c ₁ and c ₂ are the speed of sound of each in sea water 32 and in the seabed 34, [rho ₁ and [rho ₂ are the density of each sea 32 and the seabed 34. And b is given by the following formula (5).

Here, the simulation was performed with c ₁ = 1500 m / s, c ₂ = 1600 m / s, ρ ₁ = 1000 kg / m ³ , and ρ ₂ = 1500 kg / m ³ .

  The frequency of the carrier wave used for passive phase conjugate underwater acoustic communication was 500 Hz, and the bandwidth used for this communication was 100 Hz. As the probe signal, a pulse wave that has passed through a roll-off filter having the characteristics shown in FIGS. 4A and 4B is used. FIG. 4 (A) shows the time response characteristics of the roll-off filter. The horizontal axis shows time in units of seconds, and the vertical axis shows amplitude in units of arbitrary memory. FIG. 4B shows the frequency response characteristic of the roll-off filter, where the horizontal axis indicates the frequency, and the vertical axis indicates the amplitude scaled by an arbitrary memory.

  As shown in FIG. 4 (A), the roll-off filter used in this simulation has a form of 0 every 0.01 seconds in order to transmit data at a symbol interval of 0.01 seconds. As shown in FIG. 4B, the characteristic curve indicating the frequency response characteristic is a filter having a frequency characteristic corresponding to a bandwidth of 100 Hz.

  As the modulation method used for passive phase conjugate underwater acoustic communication, the simulation used the quadrature amplitude modulation (QAM) method, which is a 16 QAM method that assigns 4 bits to each of amplitude and phase.

  The simulation results will be described with reference to FIGS. 5 (A) and 5 (B) and FIGS. 6 (A) and 6 (B). FIGS. 5A and 5B are correlation diagrams showing demodulation results of the 16 QAM system. The horizontal axis indicates the in-phase component, the vertical axis indicates the quadrature component, and the positions indicated by black dots indicate the 16 QAM signal point arrangement. 6 (A) and 6 (B) are diagrams showing demodulation results in comparison with correct values in time series, the horizontal axis indicates the positions where symbols arranged on the time axis, and the vertical axis indicates the signal The amplitude is shown.

  In the simulation, a 1500-symbol data string was transmitted and compared with and without an adaptive filter process with 85 taps. The correlation diagram shown in FIG. 5 (A) shows the result when the adaptive filter processing is not performed, and the correlation diagram shown in FIG. 5 (B) shows the result when the adaptive filter processing is performed. In the correlation diagrams shown in FIGS. 5 (A) and 5 (B), demodulation results from the 1000th symbol to the 1500th symbol of the data sequence are shown.

  The bit error generated between the 1000th symbol and the 1500th symbol of the data string was compared with the S / N ratio of the output signal as follows. When the adaptive filter processing shown in FIG. 5 (A) is not performed, the number of bits in which an error has occurred is 355 bits, whereas the adaptive filter processing shown in FIG. 5 (B) is performed It was 2 bits. Further, the S / N ratio of the output signal was 6.4 when the adaptive filter processing was not performed, whereas it was 17.7 when the adaptive filter processing was performed.

  From this result, it was confirmed that the number of errored bits was reduced from 355 bits to 2 bits by adaptive filtering, and the S / N ratio of the output signal was improved from 6.4 to 17.7. Comparing Fig. 5 (A) and Fig. 5 (B), the signal point arrangement indicated by black dots is uniformly scattered in the former with respect to the 16 QAM signal point arrangement, whereas in the latter, the signal point arrangement is It can be seen that 16 are gathered together in 16 places.

  Further, when the demodulation results from the 1400th symbol to the 1450th symbol of the data string are compared with the correct values in time series, FIG. 6A and FIG. FIG. 6 (A) shows the result when the adaptive filter processing is not performed, and FIG. 6 (B) shows the result when the adaptive filter processing is performed. In any of the figures, correct values are indicated by crosses and connected by broken lines. The demodulation results are indicated by ● and connected by a solid line.

  Comparing the demodulation result when the adaptive filter processing shown in FIG. 6 (A) is not performed with the demodulation result when the adaptive filter processing shown in FIG. It can be seen that the degree is higher when the adaptive filter processing shown in FIG. 6B is performed. That is, the magnitude of the positional deviation in the vertical axis direction between the X mark and the ● mark shown in FIG. 6 (B) is the size of the positional deviation in the vertical axis direction between the X mark and the It can be seen that a smaller value closer to the correct value is demodulated.

  From the above results, it is shown that demodulation is not performed well without adaptive filter processing. In other words, if the receiver array is composed of as few as seven receivers, as assumed in this simulation, a communication path is secured between the AUV 30 and the receiver array 40 without adaptive filtering. It means you can't.

  Even in passive phase conjugate underwater acoustic communication, the effect of intersymbol interference can be reduced by the convergence property of the phase conjugate wave, but if a sufficient number of receivers are included in the receiver array, The magnitude of the noise-like signal component that remains without being converged as the received signal cannot be ignored. Since the noise-like signal component remaining without being converged can be removed by the adaptive filter, it is interpreted that good passive phase conjugate underwater acoustic communication has been realized by performing the adaptive filter processing as described above. Is done.

<Deep sea simulation results>
Next, with reference to FIGS. 7 (A), (B), and (C), the results of simulation of the effect of adaptive filter processing in passive phase conjugate underwater acoustic communication in the deep sea will be described. FIGS. 7A, 7B, and 7C are diagrams for explaining the condition setting in this simulation. In this simulation, unlike the simulation in the shallow sea area described above, the adaptive filter processing is used for passive phase conjugate underwater acoustic communication in the deep sea area where the speed of sound in the sea cannot be ignored. The effect was verified.

  PE (Parabolic Equation) method (for example, see Ding Lee and Martin H. Schultz, Numerical Ocean Acoustic Propagation in Three Dimensions, Singapore, World Scientific, Chapters 3 and 4) for calculation of sound wave propagation in seawater 52 Was used.

Here, the simulation was performed by dividing the sound speed dependence in the sea into two cases. First, the simulation was performed assuming the sound speed dependence shown in FIG. 7 (A). This case is referred to as case (A) for convenience of explanation. In addition, the simulation was performed assuming the sound speed dependency shown in FIG. 7 (B). This case is referred to as case (B) for convenience of explanation. That is, the sound velocity c ₁ in the seawater 52 is given as a function regarding the depth shown in FIGS. 7A and 7B in the cases (A) and (B), respectively.

  7A and 7B, the horizontal axis indicates the sound velocity in units of m / s, and the vertical axis indicates the depth from the sea surface 56 in units of m. Case (A) is a case in which the speed of sound decreases with depth from sea level 56 to 1400 m, and again increases with depth at depths higher than that. On the other hand, the case (B) is a case in which the sound speed increases as the depth from the sea surface 56 increases.

  Hereinafter, the simulation results in case (A) and case (B) will be described.

[Case (A)]
With reference to FIG. 7 (C), the simulation conditions of case (A) will be described. The water depth H, which is the distance from the sea floor 58 of the sea floor 54 to the sea water surface 56 of the sea water 52, was 5000 m, and the distance between the AUV 50 and the receiver array 60 was 500 km. Further, the number of receivers constituting the receiver array 60 is 10 m, and the total number of receivers is 19. In FIG. 7 (C), nine receivers are drawn for the sake of convenience, but in actuality, 19 receivers are provided. The depth h of the receiver array 60 is 4000 m. The depth h of the receiver array 60 refers to the distance from the sea level 56 to the center of the receiver array 60. In this case, the center of the receiver array 60 is from the top constituting the receiver array 60. Say the distance to the 10th receiver position. Also, the sound velocity c ₂ in the seabed 54, the respective density [rho ₁ and [rho ₂ seawater 52 and the seabed _{54, c 2 = 1600 m / s} , ρ 1 = 1000 kg / m 3, ρ 2 = 1500 kg / m ^The simulation was performed as ³ .

  The characteristics of the roll-off filter are the same as those given in FIGS. 4 (A) and 4 (B) assumed in the simulation of the shallow sea region described above. The PE method was used.

  In addition, the 16 QAM method was adopted as a modulation method used for passive phase conjugate underwater acoustic communication in simulations in the deep sea. Also, in the deep sea simulation, a 1500-symbol data string is transmitted and the adaptive filter processing is performed with or without the same adaptive filter with 85 taps used in the shallow sea simulation. Compared.

  With reference to FIGS. 8 (A) and (B), simulation results for case (A) in the deep sea area will be described. FIGS. 8A and 8B are correlation diagrams showing demodulation results of the 16 QAM scheme similar to FIGS. 5A and 5B.

  The correlation diagram shown in FIG. 8 (A) shows the result when the adaptive filter processing is not performed, and the correlation diagram shown in FIG. 8 (B) shows the result when the adaptive filter processing is performed. In the correlation diagrams shown in FIGS. 8 (A) and (B), as in FIGS. 5 (A) and (B), the demodulation results from the 1000th symbol to the 1500th symbol of the data sequence are shown. Yes.

  The bit error generated between the 1000th symbol and the 1500th symbol of the data string was compared with the S / N ratio of the output signal as follows. When the adaptive filter processing shown in FIG. 8 (A) is not performed, the number of bits in which an error has occurred is 296 bits, whereas the adaptive filter processing shown in FIG. 8 (B) is performed It was 6 bits. Further, the S / N ratio of the output signal was 2.1 when the adaptive filter processing was not performed, whereas it was 15.1 when the adaptive filter processing was performed.

  From this result, it was confirmed that the number of bits in which errors occurred was reduced from 296 bits to 6 bits by adaptive filter processing, and the S / N ratio of the output signal was improved from 2.1 to 15.1. Comparing Fig. 8 (A) and Fig. 8 (B), with respect to the signal point arrangement of 16 QAM system, the signal point arrangement indicated by black dots is uniformly scattered in the former, whereas the signal point arrangement in the latter is It can be seen that 16 are gathered together in 16 places.

[Case (B)]
In the simulation of case (B), the depth H, which is the distance from the bottom 58 of the seabed 54 to the sea level 56 of the seawater 52, is 3500 m, and the distance between the AUV 50 and the receiver array 60 is 1000 km. Further, the number of receivers constituting the receiver array 60 is 10 m, and the total number of receivers is 19. The depth h of the receiver array 60 is 2000 m. Also, the sound velocity c ₂ in the seabed 54, the respective density [rho ₁ and [rho ₂ seawater 52 and the seabed _{54, c 2 = 1600 m / s} , ρ 1 = 1000 kg / m 3, ρ 2 = 1500 kg / m ^The simulation was performed as ³ .

  Here again, the roll-off filter has the same characteristics as those given in FIGS. 4 (A) and 4 (B) assumed in the simulation of the shallow sea region described above. The PE method described above was used.

  Also, the 16 QAM system was adopted as the modulation system. In addition, a 1500-symbol data string was transmitted, and the case where adaptive filter processing was performed using the same adaptive filter with 85 taps used in the simulation in the shallow sea area was compared with the case where it was not.

  With reference to FIGS. 9 (A) and (B), simulation results for case (B) in the deep sea area will be described. FIGS. 9 (A) and (B) are correlation diagrams showing demodulation results of the 16 QAM system similar to FIGS. 5 (A) and 5 (B).

  The correlation diagram shown in FIG. 9 (A) shows the result when the adaptive filter processing is not performed, and the correlation diagram shown in FIG. 9 (B) shows the result when the adaptive filter processing is performed. In the correlation diagrams shown in FIGS. 9 (A) and (B), the demodulation results from the 1000th symbol to the 1500th symbol of the data string are shown, as in FIGS. 5 (A) and (B). Yes.

  The bit error generated between the 1000th symbol and the 1500th symbol of the data string was compared with the S / N ratio of the output signal as follows. When adaptive filter processing shown in Fig. 9 (A) is not performed, the number of bits in which errors occurred was 339 bits, whereas when adaptive filter processing shown in Fig. 9 (B) was performed It was 0 bit. Further, the S / N ratio of the output signal was 0.8 when the adaptive filter processing was not performed, but was 18.1 when the adaptive filter processing was performed.

  From this result, it was confirmed that the number of errored bits was reduced from 339 bits to 0 bits by adaptive filter processing, and the S / N ratio of the output signal was improved from 0.8 to 18.1. Comparing Fig. 9 (A) and Fig. 9 (B), the signal point arrangement indicated by the black dots in the former is uniformly scattered in the 16 QAM system. It can be seen that 16 are gathered together in 16 places.

  From the above results, it is shown that demodulation is not performed well without adaptive filter processing even in passive phase conjugate underwater acoustic communication in the deep sea. In other words, assuming that the receiver array is composed of a small number of 19 receivers assumed in this simulation, a communication path is secured between the AUV 50 and the receiver array 60 unless adaptive filter processing is performed. It means you can't.

  In passive phase conjugate underwater acoustic communication in the deep sea area, the effect of intersymbol interference can be reduced by the convergence property of the phase conjugate wave as in the case of passive phase conjugate underwater acoustic communication in the shallow sea area. If a sufficient number of receivers are not provided, the magnitude of the noise-like signal component that remains without being converged as a received signal cannot be ignored. Since the noise-like signal component that has not been fully converged can be removed by the adaptive filter, as described above, by performing the adaptive filter processing, excellent passive phase conjugate underwater acoustic communication is realized even in the deep sea area. It is interpreted.

It is a figure where it uses for description of the principle of underwater acoustic communication by a phase conjugate wave. It is a figure where it uses for description of the principle of passive phase conjugate underwater acoustic communication. It is a figure where it uses for description about the condition setting in the simulation in a shallow sea area. It is a figure which shows the characteristic of a roll-off filter. It is a correlation diagram which shows the demodulation result in a shallow sea area. It is a figure which compares a demodulation result with a correct value in time series. It is a figure where it uses for description about the condition setting in the simulation in a deep sea area. FIG. 10 is a correlation diagram showing a demodulation result in case (A). FIG. 10 is a correlation diagram showing a demodulation result in case (B).

Explanation of symbols

10, 30, 50: Autonomous Underwater Vehicle (AUV)
12, 32, 52: Seawater
14, 34, 54: Undersea
16, 36, 56: Sea level
18, 38, 58: Bottom of the sea
20: (Sending) receiver array
40, 60: Receiver array

Claims (2)

A first step of transmitting a transmission signal composed of a probe signal and a data string continuously from a sound source;
The transmission signal is received by a receiver array composed of a plurality of receivers, and a correlation waveform between the received probe signal and the received data string is obtained at each receiver constituting the receiver array. The second step,
A third step for obtaining as a received signal the sum of correlation waveforms output from all receivers constituting the receiver array;
Look including a fourth step of performing noise cancellation by the adaptive filter to the received signal,
The number of the plurality of receivers constituting the receiver array is the number of noise-like signal components that cannot be converged due to the convergence characteristics of the phase conjugate wave in the received signal,
The passive phase conjugate underwater acoustic communication method according to claim 1, wherein the adaptive filter removes the noise-like signal component remaining in the received signal without being converged . A sound source that transmits a transmission signal composed of a probe signal and a data string continuously;
A receiver array comprising a plurality of receivers for receiving the transmission signal and obtaining a correlation waveform between the received probe signal and the received data sequence;
An adder for generating a sum of correlation waveforms output from all receivers constituting the receiver array as a received signal;
An adaptive filter for removing noise from the received signal ,
The number of the plurality of receivers constituting the receiver array is the number of noise-like signal components that cannot be converged due to the convergence characteristics of the phase conjugate wave in the received signal,
The passive phase conjugate underwater acoustic communication system characterized in that the adaptive filter removes the noise-like signal component remaining in the received signal without being completely converged .

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