UNDERWATER WEARABLE DEVICE, COMMUNICATION SYSTEM COMPRISING THE SAME AND COMMUNICATION METHOD

Abstract

A communication system (1) comprising a first and a second underwater wearable device to be worn by a swimmer at mutually distant locations is disclosed herein. The communication system is configured to derive information pertaining to the swimmer from properties of a version of the acoustic signal transmitted from a first communication module of the first underwater wearable device to a second communication module of the second underwater wearable device.

Claims

1. A communication system comprising: a first underwater wearable device provided with a first communication module that comprises: a signal source configured to provide an input signal to be transmitted; and an acoustic transmitter configured to convert the input signal into an acoustic signal suitable for transmission in water and to transmit the acoustic signal; and a second underwater wearable device provided with a second communication module that comprises: an acoustic receiver configured to receive a transmitted version of the acoustic signal and to convert the received transmitted version of the acoustic signal into a processable signal; and a signal processor configured to process the processable signal; wherein the first underwater wearable device with the first communication module and the second underwater wearable device with the second communication module are configured to be worn at mutually distant locations of a swimmer, wherein the communication system is configured to derive information pertaining to the swimmer from properties of the received transmitted version of the acoustic signal, and wherein the properties are due to behavior of the acoustic signal in water.

2. The communication system according to claim 1, wherein the mutually distant locations at which the underwater wearable device with the first communication module and the underwater wearable device with the second communication module are configured to be worn by the swimmer are axially different, wherein the signal processor of the second communication module is configured to determine a delay with which the received transmitted version of the acoustic signal is received by the second communication module from the first communication module, wherein the second communication module is configured to transmit an acoustic signal with an indication of the delay to the first communication module, and wherein the first communication module comprises a signal processor configured to determine a further delay with which a received transmitted version of the acoustic signal from the second communication module is received, wherein the signal processor of the first communication module is configured to provide an estimate as the information pertaining to the swimmer, a relative velocity of the swimmer relative to the water from the difference between the delay and the further delay.

3. The communication system according to claim 1, wherein the second communication module is configured to split the processable signal into a first signal component and a second signal component, wherein the first signal component is associated with a component of the received transmitted version of the acoustic signal that is directly transmitted to the second communication module, wherein the second signal component is associated with a component of the received transmitted version of the acoustic signal that is transmitted to the second communication module via a reflection, and wherein the signal processor of the first communication module is configured to determine a difference in a time of arrival of the first signal component and the second signal component and to estimate as the information a depth of the swimmer from the difference.

4. The communication system according to claim 1, wherein at least one of the first and second underwater wearable devices is configured to be worn at a wrist of a swimmer, and wherein the communication module of at least an other one of the first and second underwater wearable devices is configured to: determine a Doppler spread in the received transmitted version of the acoustic signal received from the communication module of the one of the first and second underwater wearable devices, and determine as, the information pertaining to the swimmer, a type of stroke used by the swimmer from the determined Doppler spread.

5. The communication system according to claim 1, further configured to provide a distance between the first communication module and the second communication module and a controller for accordingly controlling a frequency range used for acoustic transmission to optimize path loss versus delay spread.

6. The communication system according to claim 1, comprising a further communication module that is provided in a housing configured to be positioned or mounted at a predetermined location.

7. An underwater wearable device provided with a communication module comprising: a signal source configured to provide an input signal to be transmitted; an acoustic transmitter configured to convert the input signal into an acoustic signal suitable for transmission in water and to transmit the acoustic signal; an acoustic receiver configured to receive a transmitted version of an acoustic signal transmitted to the communication module of the underwater wearable device by a communication module of another species of the underwater wearable device worn by the swimmer at distance from the underwater wearable device and to convert the received transmitted version of the acoustic signal into a processable signal; a signal processor configured to process the processable signal by: deriving information pertaining to a swimmer wearing the underwater wearable device from the received transmitted version of the acoustic signal, wherein the deriving information pertaining to the swimmer is performed based on properties of the received transmitted version of the acoustic signal that are due to the behavior of the acoustic signal in water.

8. The underwater wearable device according to claim 7, wherein the acoustic transmitter comprises a plurality of acoustic transmission elements arranged at mutually different positions on an outer surface of the underwater wearable device.

9. The underwater wearable device according to claim 7, wherein the acoustic transmitter is configured to selectively assume one of at least a coherent transmission mode and a non-coherent transmission mode.

10. The underwater wearable device according to claim 9, wherein the acoustic transmitter is configured to select a transmission mode dependent on a detected type of swimming stroke of the swimmer.

11. The underwater wearable device according to claim 7, further comprising one or more sensors that are communicatively coupled to the signal source, to enable acoustic transmission of sensor signals.

12. A communication method carried out by a first underwater wearable device and a second underwater wearable device on a swimmer, the method comprising: attaching the first underwater wearable device and the second underwater wearable device at mutually distant body positions of the swimmer; providing, by the first underwater wearable device, an input signal to be transmitted; converting, in the first underwater wearable device, the input signal into an acoustic signal; acoustically transmitting through water, by the first underwater wearable device, the acoustic signal; remotely receiving through the water, by the second underwater wearable device, a transmitted version of the acoustic signal; and converting, in the second underwater wearable device, the remotely received transmitted version of the acoustic signal into a processable signal; processing, by the second underwater wearable device, the processable signal; deriving information pertaining to the swimmer from properties of the remotely received transmitted version of the acoustic signal due to behavior of the acoustic signal in the water.

13. The method of claim 12, wherein the mutually distant body positions are axially different; wherein the second underwater wearable device determines a delay with which it receives the transmitted version of the acoustic signal; wherein the second underwater wearable device transmits an acoustic signal with an indication of the determined delay to the first device; and wherein the first underwater wearable device determines a further delay with which it receives the transmitted version of the acoustic signal from the second device, and estimates as the information a relative velocity of the swimmer relative to the water from the difference between the delay and the further delay.

14. The method of claim 12, wherein the second underwater wearable device splits the processable signal into a first signal component and a second signal component, wherein the first signal component is associated with a component of the received transmitted version of the acoustic signal that is directly transmitted to the second underwater wearable device, wherein the second signal component is associated with a component of the received transmitted version of the acoustic signal that is transmitted to the second device via a reflection, and wherein the second device determines a difference in a time of arrival of the first signal component and the second signal component and estimates as the information a depth (d) of the swimmer from the difference.

15. The method of claim 12, wherein at least one of the first and second underwater wearable devices is worn at a wrist of the swimmer, and wherein at least an other one of the first and second underwater wearable devices: determines a Doppler spread in the received transmitted version of the acoustic signal received from the at least a first one of the underwater wearable devices, and determines, as the information pertaining to the swimmer, a type of stroke used by the swimmer from the determined Doppler spread.

16. The communication system according to claim 2, further configured to provide a distance between the first communication module and the second communication module and a controller for accordingly controlling a frequency range used for acoustic transmission to optimize path loss versus delay spread.

17. The communication system according to claim 3, further configured to provide a distance between the first communication module and the second communication module and a controller for accordingly controlling a frequency range used for acoustic transmission to optimize path loss versus delay spread.

18. The communication system according to claim 4, further configured to provide a distance between the first communication module and the second communication module and a controller for accordingly controlling a frequency range used for acoustic transmission to optimize path loss versus delay spread.

19. The communication system according to claim 6, further configured to provide a distance between the first communication module and the second communication module and a controller for accordingly controlling a frequency range used for acoustic transmission to optimize path loss versus delay spread.

20. The communication system according to claim 1, further comprising one or more sensors that are communicatively coupled to the signal source, to enable acoustic transmission of sensor signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] These and other aspects are described in more detail with reference to the drawings, therein:

[0026] FIG. 1 schematically shows an exemplary underwater wearable device.

[0027] FIG. 2A, 2B shows examples of a communication system with a first and a second communication module;

[0028] FIG. 3 shows a further examples of a communication system with a first and a second communication module;

[0029] FIG. 4 shows an exemplary communication system with five underwater wearable devices;

[0030] FIG. 5 shows two communication modules therein in more detail;

[0031] FIG. 6 shows an application of a communication system with a pair of underwater wearable devices to estimate a velocity of the swimmer;

[0032] FIG. 6A shows details of the communication modules used therein;

[0033] FIGS. 7, 7A and 7B shows an application of a communication system with a pair of underwater wearable devices to estimate a depth d of a swimmer;

[0034] FIG. 8, 8A-8E illustrate an application of a communication system with a pair of underwater wearable devices as well as a device accommodated at a fixed position inside the swimming pool, to determine properties of a swimming stroke used by the swimmer; Therein FIGS. 8D and 8E show a respective spectrogram of a received acoustic signal while the swimmer was at rest and while swimming breast stroke respectively;

[0035] FIG. 9A schematically illustrates the dampening of an acoustic signal;

[0036] FIG. 9B shows a communication system with a pair of communication modules that adapt the acoustic frequency in accordance with an estimation of the distance;

[0037] FIG. 10 shows the acoustic impulse response evolution obtained of an acoustic signal transmitted between devices attached to the wrists of a swimmer while swimming breast stroke.

DETAILED DESCRIPTION OF EMBODIMENTS

[0038] FIG. 1 schematically shows an exemplary underwater wearable device 9. The exemplary device is provided with a brace 90 with which a swimmer can wear the device 9 around the wrist or the ankle for example. The brace 90 is attached to a waterproof housing 91 which may have a display 92, for example a touch sensitive display with which the swimmer can control an operation. As further schematically shown in FIG. 2A, the housing 91 is provided with a first communication module 10 that comprises a signal source 11 configured to provide an input signal S.sub.11 to be transmitted and further an acoustic transmitter 12 configured to convert the input signal S.sub.11 into an acoustic transmission signal A.sub.T10 to be transmitted through water. As shown in FIG. 1, the acoustic transmitter 12 may have a plurality of transmitter elements 121, 122, . . . , 125 distributed on the surface of the housing 91 for a large spatial transmission angle.

[0039] In the example shown in FIG. 2A, the first communication module 10 of the underwater wearable device 9 cooperates with a second communication module 20 as part of a communication system 1. The second communication module 20 includes an acoustic receiver 25 configured to receive an external acoustic signal A.sub.R20, here being a modified version of the transmitted acoustic transmission signal A.sub.T10, and to convert the received external acoustic signal into a processable signal S.sub.25. The second communication module 20 also includes a signal processor 26 that is configured to process the processable signal S.sub.25. It is noted that a signal processor such as signal processor 16, 26 may be provided in various embodiments. In one embodiment the signal processor is a general purpose processor, which performs its computational tasks according to a program stored in a memory, for example a rewritable memory. The latter embodiment has the advantage that the product can be upgraded with new features. Alternatively the signal processor may be provided as dedicated hardware. Typically this has the advantage of a lower power consumption and a higher processing speed as compared to that of a programmable processor. As a still further alternative signal processing may be implemented by optical components. Still further, various components, programmable, non-programmable, electronic and optical may be combined for optimizing functionality, computation speed and power consumption.

[0040] In the example shown in FIG. 2A, not claimed herein, the second communication module 20 is arranged at a fixed position, e.g. at the side of a swimming pool. In other examples a second communication module may be accommodated in another underwater wearable device. It will be clear that typically also a power supply utility will be present in the communication modules, E.g. in the form of a battery, which may be replaceable or rechargeable, for example by an external power source or by an internal power source, e.g. a solar cell provided on the housing or a motion to electric energy conversion device. For a communication module arranged at a fixed position also a mains connection may be contemplated.

[0041] As shown in FIG. 2B, a first and a second acoustic communication module 10a, 10b, are both accommodated in a housing 91a, 91b of an underwater wearable device. Parts of the acoustic communication module 10a in the housing 91a corresponding to those of the module 10 in FIG. 2A have an additional suffix a. Parts of the acoustic communication module 10b in the housing 91b corresponding to those of the module 20 in FIG. 2A have a reference wherein the first digit “2” is replaced with a “1”, and have an additional suffix b.

[0042] As schematically shown in FIG. 3 the communication modules may be configured for full-duplex communication. To that end the first communication module 10 further includes an acoustic receiver 15 configured to receive an external acoustic signal A.sub.R10 and to convert the received external acoustic signal into a processable signal S.sub.15. The first communication module 10 also includes a signal processor 16 configured to process the processable signal S.sub.15, S.sub.15a. Conversely, the second communication module 20 further includes a signal source 21 configured to provide an input signal S.sub.21 to be transmitted and acoustic transmitter 22 configured to convert the input signal S.sub.21 into an acoustic transmission signal A.sub.T20 to be transmitted in the water.

[0043] As noted above, the second acoustic communication module is accommodated in another underwater wearable device. In fact an acoustic communication system may be provided with a plurality of acoustic communication modules, each being provided as part of an underwater wearable device and each being capable of duplex communication with each of the other communication modules. An example thereof is shown in FIG. 4. Therein the swimmer Sw wears various underwater wearable devices 9, 9a, . . . , 9e, each with a respective acoustic communication module 10, 10a, . . . , 10e. A first swimmer wearable device 9 with a first acoustic communication module 10 is provided at the right wrist. A second underwater wearable device 9a with a second acoustic communication module 10a is provided at the right ankle. A third underwater wearable device 9b with a third acoustic communication module 10b is provided at the left ankle. A fourth underwater wearable device 9c with a fourth acoustic communication module 10c is provided at the left wrist. A fifth underwater wearable device 9d with a fifth acoustic communication module 10d is provided at the chest. Furthermore it may be contemplated to provide a sixth underwater wearable device 9e with a sixth acoustic communication module 10e around the head, for example, provided in a hear ribbon. In practice, however, such a head-mounted device is less useful for swimmers as it would me mostly above water level. It may however have an additional value for divers. In this connection it is noted that a diver is a specific instance of a swimmer. For clarity, the details of the underwater wearable devices 9, 9a, . . . 9e are not shown in detail in FIG. 4. Only the location of the acoustic communication modules 10, 10a, 10b, 10c, 10d, 10e is indicated therein. The acoustic communication modules may communicate with each of the others. For example acoustic communication modules 10a, 10b may communicate with each other as shown in more detail in FIG. 5. The communication modules in the underwater wearable devices may further be configured to communicate with a communication module arranged at a fixed position. For example one of the first communication modules, e.g. the first communication module 10 may be dedicated for communication at a larger distance, and other communication modules 10a, 10b, 10c, 10d, 10e worn by the swimmer Sw may communicate with the remote communication module via the first communication module 10.

[0044] Underwater wearable devices 9, 9a, . . . , 9e equipped with the communication modules 10, 10a, . . . , 10e may have additional functionalities, dependent on their location on the human body. For example underwater wearable devices 9 and/or 9c may be provided as a smartwatch, which in addition to the communication module 10, 10c further comprises a touch screen a clock and programmable processing facilities for various user applications. As another example, underwater wearable device 9d, provided with a chest band may include in addition to the communication module 10d a heart monitor.

[0045] By way of example, the pair of acoustic communication modules 10a, 10b is shown in more detail in FIG. 5. Parts of the acoustic communication modules 10a, 10b therein corresponding to those in FIG. 3 are provided with the suffix ‘a’, and ‘b’ respectively. Whereas the acoustic transmitter 12, 12a, 12b is schematically indicated as a single block the acoustic transmitter may in practice (schematically shown in FIG. 1) comprise a plurality of acoustic transmission elements arranged at mutually different positions on an outer surface of the underwater wearable device to improve the spatial distribution of the acoustic transmission signal. In the example shown, the acoustic communication modules 10a, 10b are communicatively coupled with a proper touch screen display 92a, 92b that is incorporated in the underwater wearable device.

[0046] FIG. 6 shows an example, wherein an acoustic communication between a pair of acoustic communication modules 10a, 10e worn as part of a underwater wearable device 9, 9a, . . . , 9e by the swimmer Sw are used to estimate a velocity with which the swimmer moves through the water. To that end the communication modules involved therein are worn at mutually axially distant locations of the human body. In this case communication modules 10a is attached as part of a underwater wearable device 9a to an ankle and the other communication module 10d is attached to the breast of the swimmer as part of another underwater wearable device 9d. An embodiment of the communication modules 10a, 10d is shown in FIG. 6A.

[0047] Parts of the acoustic communication modules 10a, 10d therein corresponding to those in FIG. 3 are provided with the suffix ‘a’, and ‘11’ respectively. The acoustic communication modules 10a, 10d further include a clock unit 13a, 13d respectively.

[0048] In the embodiment shown, the signal processor 16d of the communication module 10d is configured to determine a delay L.sub.ad with which an acoustic signal is received from the communication module 10a. The signal processor 16d can therewith estimate a relative velocity V.sub.rel of the swimmer Sw relative to the water using equation (1):

[00001]$\begin{array}{cc}{V}_{\mathrm{rel}}=V_{ac}-\frac{D_{ad}}{L_{ad}}& \left(1\right)\end{array}$

Therein V.sub.ac is the velocity of sound in water (V.sub.ac≈1500 m/s) and Dad is the distance between the communication modules 10a, 10d.

[0049] The delay may for example be determined in that the communication module 10a transmits its acoustic signal A.sub.T10a with a time-stamp t1 provided by its clock unit 13a, and that the communication module 10d determines the difference with a time of arrival determined by clock unit 13d. Alternatively it may be contemplated that communication module 10a transmits the acoustic signal A.sub.T10a at predetermined points in time, known to the communication module 10d, and that the latter determines the delay with which it receives the acoustic signal with respect to these predetermined points in time. In that case, it is not necessary to transmit a time stamp.

[0050] In another embodiment, the communication modules 10a, 10d are both configured with transmission and receiving utilities, as shown in FIG. 5 for the communication modules 10a, 10b. Therewith a more accurate estimation of the relative velocity is possible using the following computation (equation 2).

[00002]$\begin{array}{cc}{V}_{\mathrm{rel}}=\frac{D_{ae}}{2}\left(\frac{1}{L_{da}}-\frac{1}{L_{ad}}\right)& \left(2\right)\end{array}$

[0051] Therein communication module 10d computes the delay L.sub.ad with which it receives the acoustic signal from communication module 10a in the same way as described above, and likewise communication module 10a computes the delay L.sub.da with which it receives the acoustic signal from communication module 10d. One of the communication modules then transmits a message indicative for the measured delay to the other one, and the other one estimates the relative velocity V.sub.rel from the both delay values L.sub.ad, L.sub.da with equation (2).

[0052] As an example the distance Dad between the communication modules 10a, 10d may be 1.7 m. Therewith the measured delay would be 1.1333 ms in both directions if the swimmer Sw is at rest. When the swimmer moves forward with a speed of 1 m/s the delay L.sub.ad is increased to 1.1341 ms and the delay L.sub.da us decreased to 1.1325 ms.

[0053] It is an advantage of this other embodiment that the equation (2) used to estimate the relative velocity V.sub.rel is independent of the velocity V.sub.ac of sound in water. It may further be contemplated to estimate the velocity V.sub.ac from the delay values L.sub.ae, L.sub.ea with the following equation (2a).

[00003]$\begin{array}{cc}{V}_{ac}=\frac{D_{ad}}{2}\left(\frac{1}{L_{da}}+\frac{1}{L_{ad}}\right)& \left(2a\right)\end{array}$

[0054] Furthermore, the water temperature T.sub.W can be estimated from the value estimated for V.sub.ac as a substantially linear relationship may be presumed in normal conditions. For example in fresh water the speed of sound increases substantially linearly from 1403 m/s to 1481 m/s in the temperature interval from 0 to 20° C. In the same temperature interval the speed of sound increases substantially linearly from 1449 m/s to 1522 m/s in seawater.

[0055] Alternatively a dedicated sensor, e.g. thermistor based, may be used for temperature measurements.

[0056] FIGS. 7 and 7A illustrate a second application of a pair of underwater wearable communication modules 10a, 10b wherein a depth d of a diver Dv with respect to the water surface Ws is estimated on the basis of a measured time of arrival between a directly transmitted acoustic wave and an acoustic wave received indirectly via refection at the water surface.

[0057] As shown schematically in FIG. 7, the diver Dv wears a first and a second acoustic communication module 10a, 10b, at the right and the left wrist respectively. The acoustic communication modules 10a, 10b are accommodated in a respective underwater wearable device, for example the underwater wearable device 9 shown FIG. 1. The communication modules 10a, 10b may be provided as shown in FIG. 2B or as in FIG. 5 for example.

[0058] As shown in FIG. 7, the pathlength for the directly transmitted acoustic wave is the distance w between the acoustic communication modules 10a, 10b. The path length for the reflected component is equal to:

[00004]$\begin{array}{cc}r1+r2=2\sqrt{{\left(\frac{w}{2}\right)}^2+d^2}& \left(3\right)\end{array}$

The difference (Δt) in time of arrival is related to the depth as follows.

[00005]$\begin{array}{cc}\Delta t=\frac{2\sqrt{{\left(\frac{w}{2}\right)}^2+d^2}-w}{V_{ac}}& \left(4\right)\end{array}$

Accordingly, the depth can be estimated with equation (5)

[00006]$\begin{array}{cc}d=\sqrt{{\left(\frac{\Delta t.Math.V_{ac}}{2}\right)}^2+\Delta t.Math.V_{ac}.Math.w}& \left(5\right)\end{array}$

By way of example, the diver Dv may be at a depth of 50 cm below the surface of the water W, and the distance w at the time of measurement is 1 m.
In that case the path length of the reflected wave is 1.41 m. Hence the difference in path length (0.41 m) corresponds to corresponds to difference of arrival time of 0.273 ms.

[0059] It is noted that the distance w will not be constant, but is a function of time. To take this variation into account, the current distance w may for example be determined by a separate estimation, for example by determining a delay with which the acoustic wave is directly received. As the movement of the diver is typically of a periodical nature, the difference of arrival time measurement may for example be scheduled at the point in time where the distance has a predetermined value. The width w may be approximated by a constant value in the short time interval (e.g. less than a ms) required for a measurement.

[0060] FIG. 7A schematically shows an exemplary embodiment of the signal processor 16b for this purpose. In a first process module 161b, signal components S.sub.15d1, and S.sub.15d2 are extracted from the processable signal S.sub.15d. This may for example be achieved in that the acoustic signal transmitted by the first acoustic communication module 10a has a time-dependent pattern, for example as a frequency sweep from a first to a second acoustic frequency. Alternatively, the signal may be modulated with a time dependent signal. Therewith the first process module 161b can separate the components. Delay time evaluation modules 162a1 and 162a2 may then determine the delays t1, t2 with which the signal components S.sub.15d1, and S.sub.15d2 corresponding to the directly transmitted wave and the acoustic transmitted wave are received, and the computation module 163d may then estimate the depth d on the basis of the expression above. As the extracted signal component S.sub.15d1, is less disturbed by reflections than the original processable signal S.sub.15d it is very suitable for further processing wherein information from the signal is extracted. Such information is for example a timestamp or measurement data from a sensor conveyed by the acoustic signal.

[0061] It is noted that alternatively an autocorrelation function may be evaluated for the processable signal S.sub.15d, wherein the first peak in the autocorrelation function occurs at the position corresponding to the difference of arrival time t2-t1. In the example shown in FIG. 7A, the distance w and the velocity V of sound in water are provided as external data to the computation module 163d. Alternatively, the computation module 163d may estimate the distance w from the measured delay t1, using equation 1 or otherwise.

[0062] Whereas the strongest reflection of the acoustic signal will typically occur at the surface Ws of the water, the received acoustic signal may also include reflections at other surfaces, e.g. a wall of a swimming pool, a bottom of a swimming pool or of the sea, an object in the neighborhood etc. By including further communication modules, at mutually different positions of the human body it is possible to distinguish such reflections and therewith use a set of underwater wearable communication modules as a sonar. This is schematically illustrated in FIG. 7B. In this embodiment a plurality of communication modules in the set is configured to transmit an acoustic signal. The first process module 161b in this case separates the processable signal S.sub.15d into components attributed to each of the transmitting modules and delay time evaluation modules 162b1, 162b2, . . . , 162bn identify for each of the components a respective set of delay time differences t.sub.1x, t.sub.2x, . . . , t.sub.2n. The first set of delay time differences t.sub.1x, indicates the delays with which the various reflections of an acoustic wave from the first transmitting module is received. The second set of delay time differences t2), indicates the delays with which the various reflections of an acoustic wave from the second transmitting module is received. Each delay time difference is indicative for a distance to the point of reflection, and that distance can be determined with equation (5). With this information from the delay time evaluation modules 162b1, 162b2, . . . , 162bn the computation module 163d can construct a course 3D image of the reflecting entities.

[0063] It may alternatively be possible to use measurement data obtained with a single transmitting module at mutually different phases of a swimming stroke of the swimmer Sw. In the mutually different phases the single transmitting module assumes mutually different positions with respect to surrounding reflecting surfaces. Therewith the measurement data obtained with the single transmitting module at the mutually different positions corresponding to the phases of the swimming stroke are substantially equivalent to measurement data obtained with a plurality of transmitting modules arranged at those positions at a particular point in time, provided that the reflecting surfaces can be approximated as static during the measurement.

[0064] FIGS. 8 and 8A illustrate a further embodiment of a communication system comprising a first and a second communication module 10a, 10b worn as part of a respective swimmer wearable device 9a, 9b by a swimmer Sw. Therein FIG. 8 is an overview and FIG. 8A shows the communication modules 10a, 10b in more detail. The first communication module 10a is incorporated in a first underwater wearable device 9a worn at the right wrist of a swimmer Sw. The second communication module 10b is worn at the left wrist of a swimmer Sw. The signal processor 16b of the second communication module 10b is configured to determine a Doppler spread in the signal A.sub.R10b received from the first communication module 10a. The signal processor 16b is further configured to determine a type of stroke used by the swimmer Sw from the determined Doppler spread. By way example FIGS. 8B and 8C respectively show the Doppler spectrum for a swimmer using the breaststroke and using the frontcrawl respectively. It can be seen in FIG. 8B that the Doppler spectrum in case of the swimmer using the breaststroke is relatively narrow. I.e. the width defined for −10 dB is about 40 Hz. As shown in FIG. 8C, when the swimmer uses frontcrawl, the Doppler spectrum is substantially broader, having a width for −10 dB of about 250 Hz.

[0065] With reference to FIG. 8D and FIG. 8E another approach is illustrated. FIG. 8D shows the spectrogram of the received acoustic signal A.sub.R10b that was obtained while the swimmer remained statically under water. FIG. 8E shows the spectrogram of the received acoustic signal obtained while the swimmer was swimming breast stroke. In FIGS. 8D and 8E, each communication message is visible as a combination of two vertical blocks. FIG. 8E shows that in this example seven communication messages were transmitted during a single cycle of breaststroke swimming. From a comparison of these spectrograms it becomes apparent that the nature of the intensity variations and the repetition period of these variations are indicative for the type of swimming stroke and the repetition period of the swimming stroke used.

[0066] As schematically shown in FIG. 9A, the dampening of an acoustic signal is strongly dependent on a main frequency f(Hz) of the acoustic signal. An indication for the dampening is:

TL.sub.dB(l,f)=20.Math.log.sub.10+l.Math.f.sup.2.Math.3.Math.10.sup.−7  (6)

Wherein l is the distance from the source in m.
By way of example an acoustic signal with a relatively low frequency of 100 kHz has relatively modest dampening of 12 dB at a distance l=4 m from the source, whereas a signal with a relatively high frequency of 2 MHz has a relatively high dampening of 17 dB.

[0067] In an embodiment shown in FIG. 9B distance estimation means are provided in the signal processors 16a, 16b for estimating a mutual distance between the communication modules 10a, 10b. The distance estimation means may for example estimate the mutual distance from a delay with which an acoustic signal is received. The communication modules 10a, 10b are further provided with a frequency controller 17a, 17b with which a transmission frequency is controlled in accordance with the estimated distance. A relatively high frequency may for example be selected if the estimated distance is small and a relatively low frequency may be selected if the distance is large. This may also applied if communication takes place with a communication module arranged at a fixed position, e.g. at the side of the swimming pool, in which case the distance is typically larger than a distance between two communication modules worn by the same swimmer Sw.

[0068] The transmission of an acoustic signal can be characterized by its impulse response h(τ), i.e. the value h of the response at a point in time, delay τ, relative to a reference time, i.e. the time of transmission. During a swimming stroke the impulse response function is itself a time-dependent function due to the time dependency of the relative position of the transmitter and the receiver. This time-dependent function is denoted the acoustic impulse response evolution h(t,τ), wherein t is an indication for the point in time at which the acoustic impulse response was measured. For example t is the time of transmission of the acoustic signal for which the impulse response is measured. FIG. 10 the right hand side illustrates the acoustic impulse response evolution h(T,τ) which was measured during 2 seconds of swimming breast stroke. Therein the vertical axis indicates the time T of transmission, the horizontal axis indicates the delay time T and the brightness is indicative for the logarithmic value of the measured response amplitude, i.e. for the value:

20 log.sub.10(|ĥ(T,τ)|)

[0069] The left side of FIG. 10 shows in vertical direction representative impulse responses for T=0.2 s, T=0.9 s, T=1.3 s and T=1.8 s respectively. The motion of the wrists in the vertical direction can be determined based on an analysis of this impulse response evolution. E.g., for depth estimation of the swimmer or diver. As can be seen in FIG. 10 for example, the impulse response shows clearly distinguishable peaks during certain phases of the swimming cycle. For example, as specifically shown for points in time T=0.2 s, and T=1.8 s, the impulse response has a first peak (a) which is due to a direct transmission of the acoustic signal and a second peak (b), which is delayed with respect thereto with about 25 ms. This second peak results from a reflection of the acoustic signal against the water surface. This implies that the difference in pathlength for the direct transmission and for the transmission by reflection is about 0.38 m. As illustrated by this example, the motion of the wrists in the vertical direction can be determined based on an analysis of this impulse response evolution. Therewith also a depth of the swimmer or diver can be estimated.