Underwater acoustic communication system and synchronization method

Abstract

This underwater acoustic communication system comprises: a transmission device that stores, in a preamble of each of a plurality of frames, a known sequence different for each frame, and transmits the plurality of frames in a time series using sound waves; a receiver that acquires, for each reception point, the plurality of frames transmitted in a time series; a plurality of preamble detection units that detect, in a time series for each reception point, the preambles containing the different known sequences; a selection unit that selects, for each reception point, a frame with the preamble containing the same known sequence; a reception time determination unit that determines, for each reception point, the time of reception of the selected frame; a reference time determination unit that determines a reference time; a difference determination unit that determines, for each reception point, the difference between the time of reception of the frame determined for each reception point and the determined reference time; and two or more timing synchronization units that offset, for each reception point, the timing of the plurality of acquired frames on the basis of the difference determined for each reception point.

Description

Underwater acoustic communication system and synchronization method

[0001] The present invention relates to an underwater acoustic communication system and a synchronization method.

[0002] Underwater acoustic communication systems are being considered as systems that perform wireless communication using sound waves underwater (acoustic wireless communication). Systems that perform wireless communication in the atmosphere often use radio waves to transmit frames. However, radio waves are significantly attenuated underwater. In contrast, sound waves are relatively less attenuated underwater. Therefore, underwater acoustic communication systems use sound waves to transmit frames.

[0003] Noise generated by ships and marine life can propagate through water. The received noise intensity varies depending on the location of the noise source relative to the receiver. Noise generated near the receiver significantly reduces the signal-to-noise ratio (SNR) of underwater acoustic communications.

[0004] Therefore, one method of improving noise immunity is to install receivers at two or more points in the water. This is expected to prevent the signal-to-noise ratio of other receivers that are sufficiently far away from the receiver from being significantly reduced, even if the signal-to-noise ratio of one receiver is reduced due to nearby noise.

[0005] The receiving device aggregates each frame received by receivers at multiple receiving points and performs multi-channel reception processing (multipoint reception) on each aggregated frame. Here, if the receiving device combines a combination of synchronized frames among the aggregated frames from the multiple receiving points, a diversity effect can be expected. Furthermore, the receiving device may perform array signal processing by regarding the collection of receivers provided at the multiple receiving points as an array.

[0006] Fig. 7 is a diagram showing an example of the configuration of a communication system. The communication system shown in Fig. 7 is, as an example, a system that performs wireless communication using radio waves in the atmosphere. The communication system includes a transmitting device, K (where "K" is an integer of 2 or greater) receiving points, a backhaul line for each receiving point, and a receiving device. Each receiving point is connected to the receiving device via the backhaul line.

[0007] A transmitting device (transmitting station) transmits a time series of frames using radio waves in the atmosphere. Each frame consists of a preamble and a payload. The preamble is placed at the beginning of the frame. A known sequence of a single pattern is stored in each preamble of the time series frames by the transmitting device.

[0008] The K receiving points are installed, for example, on land. Each receiver at the K receiving points receives a time-series frame from a transmitting device. Here, since the distance from the transmitting device to the receiving point (the distance of the propagation path) differs for each receiving point, the timing at which each receiving point receives the frames simultaneously transmitted from the transmitting device differs for each receiving point.

[0009] 8 is a diagram illustrating an example of the configuration of a receiving device, which includes one preamble detection unit, K frequency synchronization units, K timing synchronization units, and a multi-channel receiving unit (cooperative receiving unit).

[0010] The preamble detection unit acquires received time-series frames from each reception point, and detects the preamble of the time-series frames for each reception point.

[0011] The K frequency synchronization units determine parameters for frequency synchronization processing for the frame based on the results of detecting the frequency of the preamble, and compensate for frequency deviations of oscillators (not shown) operating at each receiving point based on the parameters for frequency synchronization processing.

[0012] The K timing synchronization units determine timing synchronization parameters for the frame based on the detection results of the preamble reception times. The K timing synchronization units compensate for the propagation delay of the detected preamble frame for each reception point based on the timing synchronization parameters. This timing synchronization process is based on the premise that the frame propagation delay (symbol-unit propagation delay) for each reception point is equal to or less than half the length of the frame.

[0013] When a frame preamble is first detected at any of the K reception points, the K timing synchronization units set the reception point of the receiver that received the preamble as the reference point, and also set the reception time (detection time) of the preamble as the reference time.

[0014] When a frame preamble is detected at a reception point other than the reference point, the K timing synchronization units select, for each reception point, a frame with a reception time closest to the reference time. The K timing synchronization units determine a timing offset amount for each reception point so that the reception time of the selected frame is synchronized (matched) with the reception time of the frame at the reference point. The K timing synchronization units offset the reception time of the acquired frame for each reception point based on the offset amount. The multi-channel reception unit performs multi-channel reception processing on the frames synchronized for the reception points (see Non-Patent Document 1).

[0015] H. Yan, S. Hanna, K. Balke, R. Gupta and D. Cabric, "Software Defined Radio Implementation of Carrier and Timing Synchronization for Distributed Arrays," 2019 IEEE Aerospace Conference, Big Sky, MT, USA, 2019, pp. 1-12, doi: 10.1109 / AERO.2019.8742232.

[0016] There is a demand for improving the effectiveness of multi-channel reception processing in underwater acoustic communication systems. To achieve this, it is conceivable to apply timing synchronization processing in systems that perform wireless communication using radio waves in the atmosphere to timing synchronization processing in underwater acoustic communication systems.

[0017] However, in underwater acoustic wireless systems, due to the slow propagation speed of sound waves in water, the frame propagation delay (symbol-by-symbol propagation delay) varies greatly from one receiving point to another. This means that the assumption that the frame propagation delay at each receiving point is equal to or less than half the frame length does not hold true, making timing synchronization in the pre-processing of multi-channel reception difficult in underwater acoustic wireless systems.

[0018] The speed of sound waves propagating underwater is approximately 1,500 m / s per second. For example, if the difference between the propagation path of a first receiving point and the propagation path of a second receiving point is 100 m, the difference in propagation delay of the sound waves is 66 ms. When frames are communicated in an underwater acoustic wireless system at a symbol rate of 25 kBaud, the difference in propagation delay of 66 ms corresponds to a propagation delay of approximately 1,666 symbols. If the difference in propagation delay corresponds to a propagation delay of several thousand symbols, the amount of propagation delay exceeds the length of the frame (e.g., 1,000 symbols). Note that in a system that performs wireless communication using radio waves in the atmosphere, the difference in propagation delay under the same conditions corresponds to a propagation delay of approximately 0.008 symbols.

[0019] 9 is a diagram showing an example of a combination of frames received at each receiving point installed underwater. The preamble "PA" is a known sequence of a single pattern in an underwater acoustic wireless system.

[0020] A time-series frame arriving at the second reception point is delayed relative to a time-series frame arriving at the first reception point in accordance with the difference between the propagation path at the first reception point and the propagation path at the second reception point, and a time-series frame arriving at the Kth reception point is delayed relative to a time-series frame arriving at the first reception point in accordance with the difference between the propagation path at the first reception point and the propagation path at the Kth reception point.

[0021] In Figure 9, the receiver at the second receiving point receives a frame consisting of a preamble "PA" and a payload "PD2" at a timing close to the timing at which the receiver at the first receiving point receives a frame consisting of a preamble "PA" and a payload "PD3".

[0022] At a timing close to the timing at which the receiver at the first receiving point receives a frame consisting of a preamble "PA" and a payload "PD3", the receiver at the Kth receiving point receives a frame consisting of a preamble "PA" and a payload "PD1".

[0023] In order to improve the effectiveness of multi-channel receiving processing in an underwater acoustic communication system, the receiving device must select each frame that was simultaneously transmitted from the transmitting device from the time series frames acquired at each receiving point, and perform timing synchronization processing on each selected frame.

[0024] However, in an underwater acoustic wireless system, the assumption that the propagation delay of a frame at each receiving point is less than half the length of that frame does not hold true, and therefore, if the timing synchronization process in a system that performs wireless communication using radio waves in the atmosphere is simply applied to the timing synchronization process in an underwater acoustic communication system, the receiving device will perform timing synchronization for the combination of frames with the smallest difference in propagation delay (difference in reception time).

[0025] 9, the combination of frames with the smallest difference in propagation delay is the combination of a frame consisting of a preamble "PA" and a payload "PD3" at the first reception point, a frame consisting of a preamble "PA" and a payload "PD2" at the second reception point, and a frame consisting of a preamble "PA" and a payload "PD1" at the Kth reception point. The receiving device performs timing synchronization processing for these combinations of frames transmitted at different timings.

[0026] However, in Fig. 9, for example, a combination of frames each consisting of a preamble "PA" and a payload "PD3" is a combination of frames simultaneously transmitted from a transmitting device, and therefore, a receiving device should perform timing synchronization processing for this combination of simultaneously transmitted frames.

[0027] In this way, when time series frames transmitted from a transmitting device using acoustic waves underwater are acquired at two or more receiving points, there is a problem that the time series frames acquired at each receiving point cannot be correctly synchronized.

[0028] In view of the above circumstances, the present invention aims to provide an underwater acoustic communication system and synchronization method that, when time series frames transmitted from a transmitting device using acoustic waves underwater are acquired at two or more receiving points, can correctly synchronize the time series frames acquired at each receiving point.

[0029] One aspect of the present invention includes a transmitting device that stores a different known sequence for each of a plurality of predetermined frames in the preamble of each frame and transmits the plurality of frames in time series using sound waves; a receiver that acquires the plurality of frames transmitted in time series for each of two or more receiving points; a plurality of preamble detectors that detect the preambles in time series for each of the receiving points, each of which stores a different known sequence for each of the frames; a selector that selects, for each of the receiving points, frames of the preambles in which the same known sequence is stored; and a receiver that selects, for each of the receiving points, frames of the preambles in which the same known sequence is stored. a reference time determination unit that determines a reference time common to the two or more reception points based on the reception times of the frames determined for each reception point; a difference determination unit that determines, for each reception point, a difference between the reception times of the frames determined for each reception point and the determined reference time; and two or more timing synchronization units that offset the timing of the acquired frames for each reception point based on the difference determined for each reception point.

[0030] One aspect of the present invention is a synchronization method executed by an underwater acoustic communication system, the synchronization method including a transmitting step of storing a different known sequence for each of a plurality of predetermined frames in the preamble of each frame and transmitting the plurality of frames in time series using sound waves; a receiving step of acquiring the plurality of frames transmitted in time series for each of two or more receiving points; a plurality of preamble detection step of detecting the preambles in which the different known sequence for each of the frames is stored in time series for each of the receiving points; and a synchronization method of detecting the preambles in which the same known sequence is stored for the two or more receiving points for each of the receiving points. a reception time determination step of determining the reception time of the selected frame for each of the reception points; a reference time determination step of determining a reference time common to the two or more reception points based on the reception time of the frame determined for each of the reception points; a difference determination step of determining, for each of the reception points, a difference between the reception time of the frame determined for each of the reception points and the determined reference time; and two or more timing synchronization steps of offsetting the timing of the acquired multiple frames for each of the reception points based on the difference determined for each of the reception points.

[0031] According to the present invention, when a time series of frames transmitted from a transmitting device using acoustic waves underwater are acquired at two or more receiving points, it is possible to correctly synchronize the time series of frames acquired at each receiving point.

[0032] FIG. 1 is a diagram showing an example of the configuration of an underwater acoustic communication system in a first embodiment. FIG. 2 is a diagram showing an example of the configuration of a receiving device in the first embodiment. FIG. 3 is a diagram showing an example of reception timing for each reception point in the first embodiment. FIG. 4 is a flowchart showing an example of the operation of the receiving device in the first embodiment. FIG. 5 is a diagram showing an example of the configuration of a receiving device in a second embodiment. FIG. 6 is a diagram showing an example of the hardware configuration of a communication device in each embodiment. FIG. 7 is a diagram showing an example of the configuration of a communication system. FIG. 8 is a diagram showing an example of the configuration of a receiving device. FIG. 9 is a diagram showing an example of a combination of frames received at each reception point installed underwater.

[0033]

[0023] An embodiment of the present invention will be described in detail with reference to the drawings. (First Embodiment) Fig. 1 is a diagram showing an example of the configuration of an underwater acoustic communication system 1 (acoustic wireless communication system) according to an embodiment. The underwater acoustic communication system 1 is a system that performs wireless communication using sound waves underwater.

[0034] The underwater acoustic communication system 1 includes a transmitting device 2, K receiving points 3, a backhaul line 4 for each receiving point 3, and a receiving device 5a. Each receiving point 3 includes N ("N" is an integer equal to or greater than 1) receivers 31. The number "N" of receivers 31 may be different for each receiving point 3. A cooperative receiving area 6 (multi-channel receiving area) is predetermined underwater.

[0035] The transmitting device 2 is mounted on a moving body (not shown) and moves within a cooperative reception area 6 defined underwater. K receiving points 3 and K backhaul lines 4 are installed in the cooperative reception area 6. The K backhaul lines 4 are connected to the receiving device 5a. Some or all of the backhaul lines 4 may be wired communication lines. Some of the backhaul lines 4 may be wireless communication lines that use radio waves in the atmosphere. The receiving device 5a may be installed within the cooperative reception area 6 or outside the cooperative reception area 6.

[0036] A transmitter 2 (transmitting station) transmits time-series frames using acoustic waves underwater. Each frame is composed of a preamble and a payload. The preamble is placed at the beginning of the frame.

[0037] The transmitting device 2 stores a known sequence with a different pattern for each frame in each of the preambles of a predetermined number "L" ("L" is an integer equal to or greater than 2) of frames in the first embodiment. Therefore, the predetermined number "L" corresponds to the number of patterns of the known sequence stored in the preamble. The known sequence is, for example, a pseudo-noise code. The pseudo-noise code may be a sequence resulting from dividing a long M sequence, or may be a predetermined sequence with high orthogonality. The predetermined sequence with high orthogonality is, for example, a Gold sequence (Gold code). The transmitting device 2 stores predetermined data in the payload. The transmitting device 2 transmits a time series of frames using sound waves underwater.

[0038] Each receiver 31 at the K receiving points 3 receives time-series frames from the transmitting device 2. Here, since the distance from the transmitting device 2 to the receiving point 3 (the distance of the propagation path) differs for each receiving point 3, the timing at which the frame transmitted from the transmitting device 2 is received by the receiver 31 at each receiving point 3 differs for each receiving point 3.

[0039] The receiving device 5a (aggregate station) acquires time-series frames from each receiving point 3. The receiving device 5a detects the preamble of the frame for each receiving point 3. The receiving device 5a compensates for the frequency deviation of an oscillator (not shown) operating at each receiving point 3 based on the preamble detection result. The receiving device 5a compensates for the difference in propagation delay of the frame of the detected preamble for each receiving point 3 based on the preamble detection result. In other words, the receiving device 5a offsets the reception time of the acquired frame for each receiving point 3. The receiving device 5a performs multi-channel reception processing on the frames synchronized for the reception points.

[0040] 2 is a diagram showing an example of the configuration of a receiving device 5a in the first embodiment. The receiving device 5a includes L preamble detection units 51, a selection unit 52, a reception time determination unit 53, a reference time determination unit 54, a difference determination unit 55, K frequency synchronization units 56, K timing synchronization units 57, and a multi-channel receiving unit 58 (cooperative receiving unit) in the first embodiment.

[0041] The number "L" of known series patterns stored in the preamble (the number "L" of a predetermined number of frames), i.e., the number "L" of preamble detectors 51, is predetermined according to the propagation delay for each reception point 3. Here, the number "L" is predetermined so that the apparent reception interval of the frames is longer than the maximum propagation delay of the frames. The number "L" is expressed as in equation (1).

[0042]

[0043] Here, the "maximum propagation delay amount" is the propagation delay amount in symbol units, and is the largest propagation delay amount among the propagation delay amounts of frames at the K receiving points 3. In the first embodiment, the "maximum propagation delay amount" is expressed as in equation (2).

[0044]

[0045] Here, "f ｓｙｍ " represents the symbol rate used in frame communication. "d" represents the distance between the two receiving points 3 located at the furthest positions from each other in the cooperative reception area 6. "v" represents the speed of sound waves in water (in meters per second).

[0046] As an example, if the reception points 3 are installed at 100 m intervals in the cooperative reception area 6, the symbol rate is 25 kBaud, and the frame length (symbol length) is 1000 symbols, the "maximum propagation delay amount" is determined to be approximately 1667 symbols using equation (2). The number "L" is determined to be 4, which is the smallest integer greater than approximately 3.33, using equation (1).

[0047] The L preamble detection units 51 detect preambles in which a different known sequence is stored for each frame in time series for each reception point 3. Here, the preamble detection unit 51-1 detects a preamble in which the known sequence "PA1" is stored for each reception point 3. The preamble detection unit 51-L detects a preamble in which the known sequence "PAL" (for example, "PA4" if L=4) is stored for each reception point 3.

[0048] The selection unit 52 selects, for each of the K reception points 3, a time-series preamble frame in which the same known sequence (e.g., "PA4") is stored for each of the K reception points 3. The reception time determination unit 53 determines the reception time of the selected frame for each of the reception points 3. The reference time determination unit 54 determines a reference time common to the K reception points 3 based on the reception times of the frames determined for each of the reception points 3. For example, the reference time determination unit 54 determines the first or last reception time of the reception times of the frames determined for each of the reception points 3 as the reference time common to the K reception points 3. For example, the reference time determination unit 54 may determine the median of the reception times of the frames determined for each of the reception points 3 as the reference time common to the K reception points 3. The difference determination unit 55 determines, for each of the reception points 3, the difference between the reception time of the frame determined for each of the reception points 3 and the determined reference time, as a parameter for timing synchronization processing.

[0049] The K frequency synchronization units 56 determine parameters for frequency synchronization processing for that frame based on the results of detecting the frequency of the preamble, and compensate for frequency deviations of oscillators (not shown) operating at each receiving point based on the parameters for frequency synchronization processing.

[0050] The K timing synchronizers 57 compensate for differences in propagation delay of the detected preamble frames for each reception point 3 based on parameters of the timing synchronization process. That is, the K timing synchronizers 57 offset the timing of the acquired L frames for each reception point 3 based on the differences determined for each reception point 3. For example, the K timing synchronizers 57 pad the beginning of the preamble in which the known sequence (e.g., "PA4") is stored with zero values, the number of samples corresponding to the difference, for each reception point 3. For example, the K timing synchronizers 57 may remove, for each reception point 3, the number of sample values ​​corresponding to the difference from among the sample values ​​in the received signal before the beginning of the preamble in which the known sequence (e.g., "PA4") is stored.

[0051] 3 is a diagram showing an example of reception timing for each reception point 3 in the first embodiment. Preamble 101 “PA1”, preamble 101 “PA2”, preamble 101 “PA3”, and preamble 101 “PA4” are known sequences that differ for each frame.

[0052] The time series frames arriving at reception point 3-2 are delayed relative to the time series frames arriving at reception point 3-1 according to the difference between the propagation path at reception point 3-1 and the propagation path at reception point 3-2. Also, the time series frames arriving at reception point 3-K are delayed relative to the time series frames arriving at reception point 3-1 according to the difference between the propagation path at reception point 3-1 and the propagation path at reception point 3-K.

[0053] In Figure 3, at a timing close to the timing at which each receiver 31-1 at receiving point 3-1 receives a frame consisting of preamble 101-3 "PA3" and payload 102-3 "PD3", each receiver 31-2 at receiving point 3-2 receives a frame consisting of preamble 101-2 "PA2" and payload 102-2 "PD2".

[0054] At a timing close to the timing at which receiver 31-1 at receiving point 3-1 receives a frame consisting of preamble 101-3 "PA3" and payload 102-3 "PD3", receiver 31-K at receiving point 3-K receives a frame consisting of preamble 101-1 "PA1" and payload 102-1 "PD1".

[0055] 3, for example, a combination of frames made up of a preamble 101-3 "PA3" and a payload 102-3 "PD3" is a combination of frames simultaneously transmitted from the transmitting device 2. Therefore, the timing synchronization unit 57 performs timing synchronization processing on this combination of simultaneously transmitted frames.

[0056] Next, an example of the operation of the receiving device 5a will be described. Fig. 4 is a flowchart showing an example of the operation of the receiving device 5a in the first embodiment. The transmitting device 2 stores a different known sequence for each of L frames in the preamble of each of the frames (step S101). The transmitting device 2 transmits the L frames in chronological order (step S102).

[0057] Each receiver 31-1 at the receiving point 3-1 acquires the L frames transmitted in time series (step S103-1), and each receiver 31-K at the receiving point 3-K acquires the L frames transmitted in time series (step S103-K).

[0058] The preamble detection unit 51-1 detects a preamble containing the known sequence "PA1" for the reception point 3-1. The preamble detection unit 51-L detects a preamble containing the known sequence "PAL" (for example, "PA4" if L=4) for the reception point 3-1 (step S104-1).

[0059] The preamble detection unit 51-1 detects a preamble containing the known sequence "PA1" for the reception point 3-K. The preamble detection unit 51-L detects a preamble containing the known sequence "PAL" (for example, "PA4" if L=4) for the reception point 3-K (step S104-K).

[0060] The selection unit 52 selects, for each of the K reception points 3, a preamble frame in which the same known sequence is stored (step S105). The reception time determination unit 53 determines the reception time of the selected frame for each of the reception points 3 (step S106). The reference time determination unit 54 determines a reference time common to the K reception points 3 based on the reception time of the frame determined for each of the reception points 3 (step S107). The difference determination unit 55 determines, for each of the reception points 3, the difference between the reception time of the frame determined for each of the reception points 3 and the determined reference time (step S108).

[0061] The timing synchronization unit 57-1 offsets the timing of the L frames acquired at the reception point 3-1 based on the difference determined for the reception point 3-1. The timing synchronization unit 57-K also offsets the timing of the L frames acquired at the reception point 3-K based on the difference determined for the reception point 3-K (step S109).

[0062] As described above, the predetermined number "L" is predetermined to be an integer greater than the result of dividing a propagation delay amount equivalent to twice the maximum propagation delay amount of a frame (maximum propagation delay amount in symbol units) at two or more (K) receiving points 3 by the symbol length of the frame. If the result of this division is, for example, 3.3, the predetermined number "L" is 4. The maximum propagation delay amount is also determined by the longest distance "d" between the receiving points 3 and the symbol rate "f" of the frame. ｓｙｍ " is multiplied by the velocity "v" of sound waves in water, which is the propagation delay amount.

[0063] The transmitting device 2 stores a different known sequence (e.g., "PA1", "PA2", ...) in the preamble of each of a predetermined number (L) of frames. The transmitting device 2 transmits the L frames in time series using sound waves underwater. The transmitting device 2 similarly transmits the following L frames in time series. In this manner, the transmitting device 2 repeats the process of transmitting the L frames.

[0064] One or more receivers 31 acquire L frames transmitted in time series at each of the K receiving points 3. One or more receivers 31 at each receiving point 3 acquire the following L frames transmitted in time series. In this manner, the one or more receivers 31 repeat the process of acquiring L frames.

[0065] L preamble detection units 51 detect preambles in time series, each containing a different known sequence for each frame, for each reception point 3. A selection unit 52 selects, for each reception point 3, time-series preamble frames in which the same known sequence is stored for each of the K reception points 3. A reception time determination unit 53 determines the reception time of the selected frame for each reception point 3. A reference time determination unit 54 determines a reference time common to the K reception points 3 based on the reception time of the frame determined for each reception point 3.

[0066] The difference determination unit 55 determines, for each reception point 3, the difference between the reception time of the frame determined for each reception point 3 and the determined reference time. The K timing synchronization units 57 offset the timing of the acquired L frames for each reception point 3 based on the difference determined for each reception point 3.

[0067] In this way, the propagation delay may exceed the frame length. Since the number of known sequence patterns stored in the preamble is large, the apparent interval between frame receptions becomes long.

[0068] As a result, when time-series frames transmitted from the transmitting device 2 using sound waves underwater are received at two or more receiving points 3 (multiple points), the receiving device 5a can correctly synchronize the time-series frames received at each receiving point 3. This also makes it possible to effectively perform multipoint reception. For example, it is possible to improve the performance of MIMO (Multi-Input Multi-Output) and beamforming.

[0069] Second Embodiment The second embodiment differs from the first embodiment mainly in that the position of the transmitter 2 is estimated. The second embodiment will be described focusing on the differences from the first embodiment.

[0070] 5 is a diagram showing an example of the configuration of a receiving device 5b in the second embodiment. The receiving device 5b includes L preamble detection units 51, a selection unit 52, a reception time determination unit 53, a reference time determination unit 54, a difference determination unit 55, K frequency synchronization units 56, K timing synchronization units 57, a multi-channel receiving unit 58 (cooperative receiving unit), and an estimation unit 59. The estimation unit 59 may include a sonar device (not shown).

[0071] The estimation unit 59 estimates the position of the transmitting device 2 mounted on the mobile body at a predetermined period. For example, if the estimation unit 59 is equipped with a sonar device (not shown), the estimation unit 59 transmits sound waves toward the mobile body (not shown) on which the transmitting device 2 is mounted. The estimation unit 59 estimates the position of the transmitting device 2 at a predetermined period based on the round-trip time of the sound waves reflected from the mobile body (not shown), the speed of sound, and the direction of arrival of the sound waves. For example, if the transmitting device 2 is equipped with a positioning information system (not shown) such as an inertial navigation system, the estimation unit 59 may receive frames including position information of the transmitting device 2 from the transmitting device 2 using sound waves.

[0072] In the second embodiment, the “maximum propagation delay amount” is expressed as in equation (3): where “D” represents the estimation error of the position of the transmitter 2 .

[0073]

[0074] For example, if the estimation error of the position of the transmitter 2 is 5 m, the symbol rate is 25 kBaud, and the frame length is 100 symbols, the maximum propagation delay is determined to be approximately 83 symbols using equation (3). Furthermore, the number "L" is determined to be 2 or more based on equation (1) above.

[0075] In the first embodiment described above, when the longest distance "d" between the reception points 3 exceeds 6 m, the number "L" is greater than 2. For example, when the longest distance "d" is 100 m, the number "L" is 34 or greater. In this way, in the second embodiment, the number "L" of preamble detection units 51 is reduced compared to the first embodiment.

[0076] As described above, the estimation unit 59 estimates the position of the transmitter 2. The maximum propagation delay is calculated by multiplying the estimation error "D" of the position of the transmitter 2 by the symbol rate "f ｓｙｍ " is multiplied by the velocity "v" of sound waves in water, which is the propagation delay amount.

[0077] As a result, when time-series frames transmitted from the transmitting device 2 using sound waves underwater are acquired at two or more receiving points 3, the receiving device 5b with a simple configuration can correctly synchronize the time-series frames acquired at each receiving point 3. In addition, multi-point reception can be effectively performed.

[0078] 6 is a diagram showing an example of the hardware configuration of the communication device 200 in each embodiment. The example of the hardware configuration of the communication device 200 corresponds to the example of the hardware configuration of the transmission device 2 in each embodiment, the example of the hardware configuration of the reception device 5a in the first embodiment, and the example of the hardware configuration of the reception device 5b in the second embodiment.

[0079] The communication device 200 is realized as software by a processor 201, such as a CPU (Central Processing Unit), executing a program stored in a storage device 203 having a non-volatile recording medium (non-transitory recording medium) and a memory 202. The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid state drive (SSD) built into a computer system. The communication unit 204 executes predetermined communication processing.

[0080] The communication device 200 may be realized using hardware including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0081] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0082] The present invention is applicable to underwater acoustic communication systems (communication systems).

[0083] 1...Underwater acoustic communication system, 2...Transmitting device, 3...Receiving point, 4...Backhaul line, 5a, 5b...Receiving device, 6...Cooperative reception area, 31...Receiver, 51...Preamble detection unit, 52...Selecting unit, 53...Reception time determination unit, 54...Reference time determination unit, 55...Difference determination unit, 56...Frequency synchronization unit, 57...Timing synchronization unit, 58...Multi-channel receiving unit, 59...Estimating unit, 101...Preamble, 102...Payload, 200...Communication device, 201...Processor, 202...Memory, 203...Storage device, 204...Communication unit

Claims

1. A transmitting device that stores a different known sequence for each of a plurality of predetermined frames in the preamble of each frame and transmits the plurality of frames in time series using sound waves; a receiver that acquires the plurality of frames transmitted in time series for each of two or more receiving points; a plurality of preamble detection units that detect, in time series for each of the receiving points, the preambles in which a different known sequence is stored for each of the frames; a selection unit that selects, for each of the receiving points, frames in which the same known sequence is stored for the two or more receiving points; a reception time determination unit that determines, for each of the receiving points, the reception time of the selected frames; a reference time determination unit that determines, for each of the receiving points, the reference time common to the two or more receiving points, based on the reception time of the frame determined for each of the receiving points; a difference determination unit that determines, for each of the receiving points, the difference between the reception time of the frame determined for each of the receiving points and the determined reference time; and two or more timing synchronization units that offset the timing of the acquired plurality of frames for each of the receiving points based on the difference determined for each of the receiving points. An underwater acoustic communication system comprising:

2. The underwater acoustic communication system of claim 1, wherein the number of the plurality of frames is predetermined to be an integer greater than the result of dividing a propagation delay amount equivalent to twice the maximum propagation delay amount of the frame at the two or more receiving points by the symbol length of the frame.

3. The underwater acoustic communication system of claim 2, wherein the maximum propagation delay is a propagation delay equivalent to the product of the longest distance between the receiving points and the symbol rate of the frame, divided by the speed of the sound wave.

4. An underwater acoustic communication system as described in claim 2, further comprising an estimation unit that estimates the position of the transmitting device, wherein the maximum propagation delay amount is a propagation delay amount equivalent to the result of multiplying the position estimation error by the symbol rate of the frame and dividing the result by the speed of the sound wave.

5. An underwater acoustic communication system according to claim 1, wherein the known sequence is a pseudo-noise code.

6. A synchronization method executed by an underwater acoustic communication system, comprising: a transmitting step of storing a different known sequence for each of a plurality of predetermined frames in the preamble of each frame, and transmitting the plurality of frames in time series using sound waves; a receiving step of acquiring the plurality of frames transmitted in time series for each of two or more receiving points; a plurality of preamble detecting step of detecting the preambles storing a different known sequence for each of the frames in time series for each of the receiving points; a selecting step of selecting, for each of the receiving points, frames of the preambles storing the same known sequence for the two or more receiving points; a receiving time determining step of determining, for each of the receiving points, the reception time of the selected frame; a reference time determining step of determining a reference time common to the two or more receiving points based on the reception time of the frame determined for each of the receiving points; and a difference determining step of determining, for each of the receiving points, the difference between the reception time of the frame determined for each of the receiving points and the determined reference time. two or more timing synchronization steps of offsetting the timing of the acquired frames for each of the reception points based on the difference determined for each of the reception points.

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