JP6707737B2 - Underwater ultrasonic communication device using OFDM modulation with performance deterioration prevention function against position fluctuation - Google Patents

Description

This invention realizes large-capacity underwater digital communication between divers in the ocean or between a diver and a ship, and is applied to safety confirmation, position grasping, etc., and digital information (images acquired by underwater exploration ships, underwater robots, etc.) , Video, etc.) to a mother ship and can be applied to search for marine resources.

The so-called single carrier method is the mainstream of conventional undersea communication methods, in which the amplitude and phase of a sine wave of a certain frequency are changed in units of a certain time, and the change is detected on the receiving side to obtain digital information. Was communicating.
Figure 8 is a newsletter issued by Oxytech Co., Ltd. in September 2013 (No.29) as a paper on a digital image transmission device compatible with the manned submersible research vessel "Shinkai 6500" (owned by Japan Agency for Marine-Earth Science and Technology (JAMSTEC)). ), the processing flow of the same device is shown.

This figure illustrates the mechanism of transmission and reception of a modulated signal at a certain moment. The amplitude characteristic and the phase characteristic (1) of this certain modulated signal are It is shown that the characteristics of the receiver are affected by the characteristics (2), (3), and (4), and that the output of the receiver is significantly different from (1) as in (5). ..
This is because the waveform is distorted by the communication path.
Then, in order to remove the distortion of the waveform, an adaptive equalizer is used to remove the waveform distortion, and as a result, a method of reproducing the transmission signal shown in (6) is shown.

In radio communication, a method has been proposed in which different frequencies are used as carrier waves and a plurality of parallel communications are realized at the same time to increase the transmission capacity.
Among them, a particularly mainstream method is Orthogonal frequency Division Multiplex, so-called OFDM.
In OFDM, it is possible to reduce the difference between adjacent frequencies of different frequencies, and it is possible to transmit data for each maximum number of different frequencies in a certain available frequency band.
In OFDM, a large number of carriers of frequencies for transmitting this data are called subcarriers.

However, in underwater communication, it is difficult to correct the influence of the Doppler effect due to the movement by the adaptive equalizer alone, in so-called mobile communication in which the positions of the transmitter and the receiver change with time. There was a problem that the performance was greatly degraded when moving.
In addition, it is common to use the OFDM method to realize large-capacity transmission, but since the OFDM method performs parallel data transmission using a large number of subcarriers, due to the influence of the Doppler effect, etc. However, there is a problem that the Doppler effect is apt to be affected, such as interference between the two and the performance is significantly deteriorated.

On the other hand, in Patent Document 1, a plurality of frequencies are assigned to a plurality of transmitters and/or receivers, the same or common digital signal is input in parallel to the plurality of transmitters, and the digital signals are output from the plurality of receivers. A digital signal transmission device that enables highly reliable transmission even when interference or interference is received from other sources by selecting and outputting the one normally received from multiple digital signals It is disclosed.

JP 2001-136114 A

However, the invention of Patent Document 1 is not based on the OFDM system, and does not have the viewpoint of performing output combining for each subcarrier forming an OFDM signal.
Therefore, by using multiple OFDM receivers and combining the outputs of the OFDM receivers to reduce noise, etc., it is not possible to improve the accuracy of transmission performance due to position changes of transmitters and receivers. Absent.

Therefore, in view of the above problems, the present invention prevents a decrease in transmission performance due to position fluctuations of a transmitter and a receiver, and provides an underwater ultrasonic communication device using OFDM modulation that is strong against position fluctuations. It is an issue.

The underwater ultrasonic communication device according to the present invention digitizes the received signal, detects the rate of temporal expansion and contraction due to the position change of the transmitter/receiver, and converts the sampling frequency of the received signal by the detection rate, At the same time, it is an underwater ultrasonic communication device using OFDM (Orthogonal Frequency Division Multiplexing) that corrects the frequency shift caused by the sampling frequency conversion. The feature is that the shift is detected and the Doppler correction of the OFDM signal is performed.
Further, the present invention is characterized in that a plurality of OFDM wave receiving devices are used and the outputs of the OFDM wave receiving devices are combined to further reduce noise.

According to the present invention, it is possible to correct temporal expansion and contraction of a signal caused by a position change of a transducer when a sound wave propagating in water is received.
Furthermore, by detecting and correcting the Doppler shift amount, residual distortion can be corrected and transmission performance can be improved.
Further, by using a plurality of OFDM wave receiving devices and combining the outputs of the OFDM wave receiving devices, it is possible to suppress the noise component, and it is possible to realize an underwater ultrasonic communication device that is resistant to noise and distortion. ..

The schematic diagram which shows the structural example of the communication apparatus of this invention. Schematic diagram showing an embodiment of a communication device of the present invention Schematic diagram showing an embodiment of a communication device of the present invention The system block diagram regarding the wave receiving device of the communication apparatus of this invention. Explanatory drawing about the signal expansion/contraction detection part of the wave receiving device of the communication apparatus of this invention. Explanatory drawing about the signal expansion/contraction detection part of the wave receiving device of the communication apparatus of this invention. Explanatory drawing of one embodiment of the structure of the OFDM signal of the present invention Explanatory drawing of one embodiment of the structure of the OFDM signal of the present invention The figure which shows the flow of the whole OFDM receiving process of this invention Explanatory drawing of the synthesizing part which synthesize|combines the output value by the some demodulation process of this invention. Underwater acoustic communication system using conventional single carrier Illustration of resampling process

FIG. 1 is a diagram showing a configuration example of an underwater ultrasonic communication device.
(11a)(11b)(12) are devices called transducers that receive or transmit sound waves or ultrasonic waves in water.
The underwater ultrasonic communication device includes two wave receiving transducers (11a) and (11b), a wave transmitting transducer (12), and a main body (13).
The two wave-receiving transducers (11a) and (11b) simultaneously receive waves, and the signals are demodulated in the main body (13) of the underwater ultrasonic communication device and combined with the two received signals.
By connecting the main body (13) to a telecommunication terminal (14) such as a waterproof smartphone, it is possible to transmit or receive information such as voices, images and moving images via a transducer.

FIG. 2A shows an example of using the underwater ultrasonic communication device, which is a mother ship on the sea (20), an underwater ultrasonic communication device on the master side (10) submerged in the sea, and a diver (21a) (21b) on the user side. FIG. 3 is a diagram showing a relationship between the ultrasonic communication device (10).
Digital transmission is realized between the master side and the user side by ultrasonic wave transmission in the water, the situation of the divers (21a) and (21b) can be grasped by the mother ship (20) and instructions from the mother ship (20) can be received. It is transmitted to (21a) and (21b).

FIG. 2B shows an example of use of the underwater ultrasonic communication device, from the mother ship on the sea (20) to the underwater ultrasonic communication device (10) on the master side submerged in the sea, and the unmanned exploration ship and the underwater robot (22) in the sea. 3] A diagram showing the relationship of the underwater ultrasonic communication device (10).
By ultrasonic transmission in the water, digital communication is realized between the mother ship (20) side and the unmanned exploration ship or underwater robot (22) side, and the unmanned exploration ship or underwater robot (22) can be controlled from the mother ship or Information such as images can be transmitted to the mother ship (20).

FIG. 3 is a block diagram showing the configuration of the underwater ultrasonic communication device (10) according to the present invention, particularly the wave receiving device (30).
A transmission wave signal is generated from the wave transmission device (36) and propagated to the transmission channel (37). The signal passing through the transmission channel (37) is demodulated by the wave receiving device (30).
In the embodiment of this figure, the center frequency of ultrasonic waves is assumed to be 20 KHz.
A signal having a center frequency of 20 KHz is generally called a passband signal.
The wave receiving device (30) first performs a down conversion process (31) for converting the center frequency from 20 KHz to 0 Hz.
Actually, an AD converter for converting the received analog signal into a digital signal is required before or immediately after the down conversion, but it is omitted here.
The output signal of the down conversion processing (31) is called a baseband signal and is generally expressed as a complex number.
After that, the expansion/contraction ratio detecting unit (32) for detecting the expansion/contraction of the down-converted signal with time detects the expansion/contraction ratio.
This description will be given later in FIG.
The received signal is corrected using the expansion/contraction ratio in the resample and derotation unit (33).
The correction consists of a re-sampling process that corrects the sampling frequency of the signal sampled at a fixed sampling frequency that is the AD converter in the previous stage, and a derotation process that corrects the frequency shift in the down conversion (31) in the previous stage. .
The re-sampling process is a process of multiplying the original sampling period Ts of the system by β and converting it to Ts′.

As shown in FIG. 9, the sample points of ● are converted into ◯.
As can be seen from the figure, the complementary processing using the original sampling points of ● is required, and it is usually implemented by FIR filter with variable coefficient.

"Derotation processing" means the correction of frequency, but here the signal is expressed by a complex number, and since vibration is mathematically expressed as rotation, the meaning of reverse rotation is meant for rotation that indicates rotation. Is represented by the name derotation.

The description of the derotation process is as follows.
The complex exponential function for down conversion is shown by the following equation.
When this is sampled at t=n·Ts (Ts is a sampling period), the following equation is obtained.
Here, there is an error in this sampling period Ts, and when the sampling period is converted using Ts′=β·Ts,
Becomes That is, by conversion by β,
This means that the frequency has been shifted by the amount of.
Therefore, when the sampling period is converted by the resampler using Ts′=β·Ts, the frequency shift of the formula 5 is generated. Therefore, the frequency shift of the formula 5 is corrected by the following formula. can do.
This correction is called derotation processing.

Through the above processing, the expansion and contraction of the received signal in the time direction including the Doppler effect due to the movement of the transmitter and the receiver are corrected.
After that, the Doppler shift is corrected in the block process of (34), and the OFDM demodulation is performed in the block process of (35).

5A and 5B are diagrams for explaining an embodiment of an OFDM signal.
The horizontal axis corresponds to the time direction, and the vertical axis corresponds to the frequency direction.
A plurality of vertically plotted marks at a certain time are called subcarriers and correspond to one complex number.
Information is transmitted using the complex number.
A plurality of vertically plotted marks, that is, a plurality of subcarriers, are at the same time and are a set of simultaneously transmitted complex numbers.
In this embodiment, 2N+1 subcarriers are simultaneously transmitted in parallel.
The subcarriers 51 of ▲ are called SCATTERED PILOT (SP) in this embodiment.
SPs are inserted in the frequency direction at intervals of two indexes 1, 5,... In the time direction.
Since SP is assigned a predetermined complex number, it is possible to detect how the complex value of SP was converted during transmission from the transmitting side to the receiving side by using the SP part in the receiving device. it can.
Therefore, it is possible to show the influence of the transmission channel on each frequency in the frequency domain.
This is generally called the channel transfer function CHANNEL TRANSFER FUNCTION (CTF).
Further, the subcarrier 52 of □ is a pilot signal continuously placed at a position of a certain specific frequency in the time direction, and is called a CONTINUOUS PILOT (CP) in this embodiment.
Similar to SP, CP is assigned a predetermined complex number, so it is possible to detect the CTF at that location using the CP part in the receiving device.
The subcarrier 53 marked with a circle, which is indicated by other subcarriers, is a complex value determined according to the data to be transmitted. Generally, a complex value is obtained by digital modulation such as BPSK, QPSK, 16QAM, 64QAM. You can decide.
Since the CPs are continuously arranged in the time direction, it is possible to always detect the change in the CTF in the time direction.
In the block (34) of FIG. 3, by extracting this CP after the FFT and detecting the time change of the CTF, the change in the time direction, that is, the amount of frequency shift due to the Doppler effect can be detected, and the phase to be corrected is determined. Can be calculated.
By feeding back this correction phase amount to the time signal, Doppler correction can be realized.

FIG. 5B is an example of the OFDM signal of FIG. 5A in which the CP portion is surrounded by a square and emphasized.
Since the same complex value is transmitted in the CP part, it has the same value at the same subcarrier position without Doppler frequency shift.
A phase shift between OFDM symbols can be detected by measuring a change in two CP values continuous in the time direction on a subcarrier having this CP.
For example, if the CP values that are continuous in the time direction are X0 and X1,
Becomes
This is a complex number, so
Then, by obtaining φ, the phase rotation amount of the adjacent subcarriers can be obtained.
Assuming that the GI length of the OFDM signal is Tg and the OFDM effective symbol length is Tu, it means that the phase of φ changes in time (Tg+Tu).
Therefore, the Doppler shift amount is
It can be estimated that
Since the instantaneous Doppler shift estimated value is obtained for each CP subcarrier, the noise component included in the estimated value can be reduced by performing averaging processing on all CP subcarriers.

Next, details of the expansion/contraction ratio detection unit (32) will be described with reference to FIGS. 4A and 4B.
The expansion/contraction ratio detection unit (32) uses an OFDM signal in which SPs are arranged every four, such as 1, 5, 9... In the time direction indexes shown in FIG.
First, the OFDM signal including the SP is transformed from the time domain to the frequency domain by the fast Fourier transformer (40).
As a result, a plurality of subcarrier signals indicated by 1, 5, 9,... In the time direction index of FIG. 5 are obtained.
Of these, SP(51) can be used to obtain the channel transfer function (CTF).
The delay profile can be obtained by inverse fast Fourier transforming this series of CTF values.

FIG. 4B is a diagram showing this delay profile.
When the OFDM signal propagates through the propagation channel without reflection or the like, a large peak waveform is obtained as shown by the solid line at the bottom of FIG. 4B.
The position of this peak corresponds to the delay time of the FFT signal in the FFT time domain.
The position (PP) of the delay time at which the peak occurs in (43) can be detected.
In the present embodiment, it is possible to perform the same processing with every fourth index in the time direction, and the difference between the position PP(n-1) of the previously detected delay time and the current PP(n) ( By taking 44), the fluctuation of the delay time can be detected, and the expansion/contraction of the received signal can be detected by this value.

FIG. 6 is a diagram illustrating the overall flow based on FIGS. 3 to 5.
(61) shows an OFDM signal in the time axis direction when the transceiver does not move, and the white part is the part called the guard interval GI.
(62) shows the delay profile, and since the transducer is not moved, it does not change with respect to the dotted line indicating the FFT window position.
(63) corresponds to the case where the position of the transducer is changed, and corresponds to the case where the distance between the transducers increases with time.
Therefore, as shown in (64), the position of the peak of the delay profile indicating the leading position of the OFDM signal gradually moves to the right from the dotted line indicating the FFT window position (the shift width increases).
That is, it is possible to detect the position change of the wave transmitter/receiver by monitoring the change in the deviation width.

Assuming that SPs are arranged every 4 OFDM symbols in the time direction as shown in FIG. 5, 5 OFDM symbols are used as a processing unit in FIG. 6, and the processing window is shifted for every 4 OFDM symbols. It is carried out.
(65) is the extraction of 5 OFDM symbols which is one processing unit.
(66) shows the delay profile corresponding to the 5 OFDM symbols.
Here, the deviation of the first delay profile of the 5 OFDM symbols in (65) is shown as ds and the deviation of the fifth delay profile as de.
As described above, by taking the difference between de and ds, the expansion/contraction of the OFDM signal due to the movement of the transceiver can be detected.
This expansion and contraction can be corrected by the resample and derotation unit (33) as described above.
(67) and (68) show the corrected OFDM signal and its delay profile.
It is possible to correct a large deviation with this correction, but after that, the Doppler correction described above is performed in (34), the OFDM symbol in (69) is generated, and normal OFDM demodulation processing is performed in (35). To do.
The OFDM demodulation processing unit (35) shown in FIG. 3 outputs the distortion-corrected output signal EQ, the estimated channel transfer function CTF, and the noise amplitude estimated value NOISE.

With the above configuration, it is possible to realize an underwater ultrasonic communication device using OFDM modulation having a performance deterioration prevention function against position fluctuations.

Further, by causing the wave receiving device (30) to perform a plurality of demodulation processes and synthesizing the output values thereof, the SN ratio of the distortion-corrected output signal EQ can be improved.

FIG. 7 is a diagram showing an embodiment in which output values are combined by a plurality of demodulation processes (30a, 30b, 30c, 30d) performed in the wave receiving device (30).
The signal power can be estimated by squaring the absolute value of the channel transfer function CTF of the output by the plurality of demodulation processes (30a, 30b, 30c, 30d) in the wave receiving device (30).
Further, the estimated value of the noise power can be obtained by calculating the square of the absolute value of the estimated value NOISE of the noise amplitude.
In FIG. 7, in order to obtain an average estimated value of noise power, averaging is performed for each frequency of each subcarrier to calculate an average value of noise power of each subcarrier. By taking the ratio of the noise power average and the signal power, the so-called signal-to-noise power ratio CN can be calculated.
In the synthesis circuit of FIG. 7, the output signals EQ1, EQ2, EQ3, EQ4 of the plurality of demodulation processes (30a, 30b, 30c, 30d) by the wave receiving device (30) are synthesized according to the following equations, and the synthesized signal I am calculating the MRC.

This synthesis formula is a method of synthesizing outputs in consideration of the CN ratio of each of the above demodulators, and “Digital Front-End in Wireless Communication and Broadcasting.” Chapter 18. Diversity and error compensation in Cambridge University Press. The equation (18.35) described in OFDM transceivers: principles and implementation by Tomohisa Wada is referred to.

As described above, according to the underwater ultrasonic communication device using the OFDM modulation of the present invention, the expansion/contraction ratio, which is the ratio at which the received signal expands/contracts with time, is detected due to the position change of the transmitter/receiver. Expansion/contraction ratio detection unit, resampling unit that converts the sampling frequency of the digitized received signal according to the output value of the expansion/contraction ratio detection unit, and corrects the frequency shift caused by the resampling processing of the resampling unit. The derotation part that compensates for the effect of the position shift of the transmitter/receiver, and includes a Doppler shift detection unit that detects the Doppler shift due to the temporal change of a part of the subcarrier signal that constitutes the OFDM signal. By performing Doppler correction of the OFDM signal by the output of the detection unit, there is an effect of realizing stable reception performance even if the position of the transmitter/receiver changes.
Further, by using a plurality of the underwater ultrasonic communication devices and synthesizing their outputs, there is an effect of further realizing stable wave receiving performance.

10 Underwater Ultrasonic Communication Device 11a Wave Receiving Transducer 11b Wave Receiving Transducer 12 Wave Sending Transducer
13 Main Body of Underwater Ultrasonic Communication Device 14 Telecommunications Terminal 20 Mother Ship 21a Diver 21b Diver 30 Wave Receiving Device 31 Down Conversion 32 Expansion/contraction Ratio Detection Section 33 Resample and Derotation Section 34 Doppler Shift Correction 35 OFDM Demodulation 40 Fast Fourier Transform 51 SCATTERED PILOT (SP)
52 CONTINUOUS PILOT (CP)
53 complex values

Claims (5)

An analog/digital converter that converts the received signal to a digital value at the sampling frequency,
An expansion/contraction ratio detection unit that detects a ratio (expansion/contraction ratio) in which the received signal expands/contracts with time due to a position change between the transmitter/receiver,
A resampling unit for resampling and converting the received signal converted into a digital value according to the expansion/contraction ratio,
A derotation unit that corrects the frequency shift caused by resampling conversion,
A Doppler shift detection unit that detects the Doppler shift amount based on a temporal change of a part of subcarriers that constitute an OFDM signal (orthogonal frequency division multiplexing);
Equipped with
By performing Doppler correction of the OFDM signal based on the Doppler shift amount,
In the underwater ultrasonic communication device for improving the accuracy of the received signal with respect to the position variation between the transmitter and the receiver,
The derotation department
The amount of frequency shift generated by resampling conversion (for example,
)
An underwater ultrasonic communication device, which is realized as multiplication of complex conjugate values of. The Doppler shift detection unit,
The underwater ultrasonic communication device according to claim 1, wherein the Doppler shift amount is detected by a temporal phase change of a continuous-use pilot continuously arranged in the time direction included in the OFDM signal. For each subcarrier that makes up the OFDM signal from the underwater ultrasonic communication device,
The underwater ultrasonic communication device according to claim 1, further comprising: a noise power estimation unit that estimates the power power of a noise component included in the received signal. Performs demodulation processing for each of a plurality of subcarriers that make up the OFDM signal from the underwater ultrasonic communication device,
The underwater ultrasonic communication device according to any one of claims 1 to 3, further comprising: a diversity combining unit that performs output combining based on a plurality of output values. The diversity combining unit,
For each subcarrier that constitutes the OFDM signal from multiple underwater ultrasonic communication devices,
The underwater ultrasonic communication device according to claim 4, wherein output synthesis is performed according to an output value of the noise power estimation unit.