Double-sided two-way distance measurement protocol based on composite ultrasound tones

Abstract

Method for determining a distance between two units (1,2), wherein the method comprises: Executing a double-sided two-way distance measurement protocol (S200), wherein the latter causes the two units (1,2) to send and receive signals, each of the signals being a composite tone generated as a composition of two waveforms separated in time by a gap such that the composition forms a bipolar waveform, wherein: the two units (1,2) comprise a first unit and a second unit; the sent and received signals comprise a first signal and a second signal; and The execution of the double-sided two-way distance measurement protocol (S200) causes the first unit to perform a group of steps comprising: Sending the first signal to the second unit at a first time; Receiving the second signal, which was sent by the second unit after receiving the first signal, at a second time, and Correlating an expected signal with a first comparison element, the latter being obtained on the basis of the received second signal, to determine a first time interval between the first time and the second time and accordingly derive the distance between the two units (1,2), where the first unit further includes the following group of steps: Triggering an audio recording prior to the transmission of the first signal, wherein the first comparison element is obtained by recording both the transmitted first signal and the received second signal; and Correlating a representation of the first signal with the first comparison element to determine the first time point, wherein the expected signal is correlated with the first comparison element to determine the second time point, wherein the first unit determines the first time interval as a difference between the determined second time point and the determined first time point; wherein the representation of the first signal and the expected signal are correlated with the first comparison element to obtain corresponding cross-correlation signals, each exhibiting a triple-peak pattern showing two smaller peaks separated by a larger peak.

Description

BACKGROUND

[0001] The invention relates generally to computer-aided methods, units, and computer program products for determining distances between pairs of units using a double-sided two-way distance measurement protocol. In particular, it relates to methods for performing a two-way distance measurement based on (near) ultrasound tones, which are generated as a composition of core waveforms separated in time by a gap.

[0002] Two-way distance measurement techniques are used to measure distances between two wireless units. In radio technology, this technique utilizes delays that naturally occur during signal transmission (also known as time of flight) to determine the distance between two units, such as stations. For example, a first signal is sent from a first node A to a second node B. The time difference (referred to as the signal propagation delay) between sending from the first node and receiving it at the second node can be calculated. Next, the first node A receives an acknowledgment (i.e., a second signal) from the second node B. This acknowledgment is sent after a certain processing time (referred to as the processing delay).The time required for the second node B to process the incoming first signal, generate the acknowledgment, and prepare for transmission is crucial. Therefore, after receiving the acknowledgment signal from node B, the first node A can calculate a further signal propagation delay. These signal propagation delays are then used to determine the distance between the two nodes.

[0003] This method is two-sided because only two nodes are involved in measuring the distance between them. It is a two-way method in that a first signal (also called a test signal, e.g., a test packet) is sent by one of the two units, followed by a second signal (the acknowledgment) sent by the other of the two units.

[0004] The above steps are typically mirrored at the second node. In this case, the procedure is symmetrical because the measurements from node A to node B are mirrored at the second node; this procedure is called a symmetrical double-sided two-way distance measurement.

[0005] One can attempt to implement a two-way distance measurement method based on tones (i.e., sounds) exchanged between two units. As with ordinary two-way distance measurement techniques, a challenge lies in being able to capture the precise moment when a sound is sent and received, both by the sender and the receiver. Ideally, sub-millisecond accuracy is desirable. Prior art methods typically attempt to achieve this by recording the sound before and during the invocation of a protocol, as described above, and subsequently locating transmit fingerprints in the recording. Such methods seek (temporal) peaks in the cross-correlation measurement obtained between the known waveform and the recorded signal.In practice, however, many false peaks appear in cross-correlation measurements due to factors such as noise and interference, e.g., multiple echoes to the transmitted signal (also known as the 'multipath' problem). Being able to capture the correct peak in the cross-correlation signal is essential for the accuracy of the measurements.

[0006] US 6 272 073 B1 describes an electronic device for recreational divers that uses ultrasonic signals to measure distances, indicate relative direction, and maintain contact between at least two diving partners during a dive.

[0007] US 5 784 339 A describes a communication and positioning device for use in an underwater communication and locating system, which transmits data words over multiple frequency bands in parallel, uses fade-resistant waveforms to reliably distinguish binary data, and determines the relative position between two devices based on signal direction and propagation time. SUMMARY

[0008] According to a first embodiment, the present invention is embodied as a method for determining a distance between two units, i.e., a first unit and a second unit, which may be, for example, wireless units such as handheld devices. The method essentially revolves around the implementation of a two-way distance measurement protocol on the two units. This protocol causes the two units to send and receive signals to determine the distance between them, as in known two-way distance measurement methods. In the present case, however, each of the signals is a composite tone generated as a combination of two waveforms separated in time by a gap, such that the combination forms a bipolar waveform.The gap is typically a zero-amplitude signal or a low-amplitude signal that contrasts sharply (in terms of amplitude) with the two waveforms. This creates a specific pattern in the correlation profile, significantly simplifying the determination of the time of flight. Such a method can be advantageously applied to measure social distancing between individuals (or social contacts).

[0009] According to another embodiment, the invention is embodied as a unit comprising processing means as well as a tone generator, a transmitter, and a receiver, each connected to the processing means. The processing means are configured to organize operations of the tone generator, the transmitter, and the receiver so that the unit performs a two-way, two-way distance measurement protocol with a remotely located unit according to the method described above.

[0010] In operation, the first unit sends a first signal to the remote unit at a first time. The second unit then receives a second signal sent by the remote unit at a second time. The processing unit is configured to obtain a reference element based on the received second signal. It then correlates an expected signal with this reference element to determine a second time, based on which a time interval between the first and second time is calculated. Note that this reference element can advantageously be obtained based on both the first signal (as sent) and the second signal (as received). Subsequently, a further correlation operation can be performed to precisely determine the first time, i.e., the time at which the first signal was sent.Finally, the distance between the two units is determined according to this time interval. Both the first and second signals are composite tones, generated as a combination of two waveforms separated in time by a gap, so that the combination forms a bipolar waveform.

[0011] According to a final embodiment, the invention is embodied as a computer program product. The computer program product comprises a computer-readable storage medium which contains program instructions embodied therein, wherein the program instructions are executable by processing means of a unit to cause the latter to execute a double-sided two-way distance measurement protocol according to the method described above. Brief description of the different views of the drawings

[0012] These and other tasks, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which should be read in conjunction with the accompanying drawings. The illustrations serve to provide a clear presentation, enabling the person skilled in the art to understand the invention in conjunction with the detailed description. In the drawings: presents Fig. Figure 1A schematically depicts two units (here smartphones) which send and receive signals, as is the case in embodiments of the invention; is Fig. 1B a diagram which schematically illustrates selected components of such units as in embodiments; illustrative Fig. 2 a sequence of operations which are performed on two units, as in Fig. 1A shown, as part of a double-sided two-way distance measurement protocol according to embodiments; is Fig. 3A a graphic representation which depicts a composite tone as generated on each unit; this tone is generated as a composition of two waveforms separated by a gap, whereby, according to embodiments, a bipolar waveform is formed; is Fig. 3B a graphical representation showing a typical correlation measurement showing a triple-peak pattern as obtained from a unit after correlating signals, as in embodiments; is Fig. 4 a flowchart illustrating the steps of a method for performing a double-sided two-way distance measurement based on compound tones generated at the unit according to embodiments; is Fig. 5. A flowchart illustrating the general steps of a procedure for operating units with respect to measuring the distance between persons between users of such units. In this further procedure, units in the vicinity of a given unit are scanned to implement a two-way distance measurement procedure, as described in Fig. 2 or Fig. 4 is shown, illustrating how to implement various embodiments; and is Fig. 6. A flowchart illustrating how repeated attempts can be made to determine a distance between any two units, using a procedure as described in Fig. 2 or Fig. 4 shown to successfully determine a distance between units according to embodiments.

[0013] The accompanying drawings show simplified representations of units or parts thereof as they appear in embodiments. Similar or functionally similar elements in the figures have been assigned the same numerical reference numerals unless otherwise indicated.

[0014] Now, by means of non-limiting examples, units, computer-aided methods and computer program products that embody the present invention are described. DETAILED DESCRIPTION

[0015] In relation to Fig. Figures 1 to 4 describe an embodiment of the invention, which relates to a method for determining a distance between two units 1, 2. It should be noted that this method and its variants are collectively referred to as the "present methods". All reference drawings Sn refer to process steps of the sequence and flowcharts of the Fig. 2, Fig. 4, Fig. 5 and Fig. 6, while numerical reference symbols refer to components of units 1, 2, which are in Fig. 1 and Fig. Figure 2 is shown. For illustrative purposes, the following description generally assumes that units 1 and 2 are hand units, such as smartphones. The two-way distance measurement step is described in the processes of... Fig. 5 and Fig. 6 is generally referred to as step S200. Fig. Figure 4 shows detailed steps of the two-way distance measurement method as in preferred embodiments. Fig. Figure 2 illustrates the main operations performed to determine a distance.

[0016] This method is based primarily on the implementation of a double-sided, two-way distance measurement protocol S200 on two units 1 and 2. As is common in the field, this protocol causes the two units to send and receive signals. However, in the present invention, each of these signals is a composite tone generated as a combination of two waveforms separated in time by a gap, such that the combination forms a bipolar waveform. This makes it possible to significantly improve the measurement accuracy, as will be explained in detail below.

[0017] Several embodiments of the above protocol may be foreseen, in which signals are exchanged between pairs of units and then processed by the units to determine a distance value. In a preferred embodiment, one of the two units (let's say the first unit 1) sends a first signal to the second unit 2 at a first time (step S203 in Fig. 2 and Fig. 4). The first unit then receives 1 (step S204 in Fig. 2 and Fig. 4) A second signal, i.e., a signal sent by the second unit 2 after receiving the first signal (step S203, S204a). Then, the first unit 1 attempts to correlate signals (step S206) to determine the second time and, accordingly, a time interval between the first and second time, with a view to deriving the distance between the two units (S207 to S209). This time interval is referred to in this document as a first time interval for reasons that will become apparent later.

[0018] It is important that both the first and second signals are composite tones (i.e., sounds) generated at the first unit 1 and the second unit 2, respectively (steps S202, S202a). This tone is generated as a composite of two waveforms w1 and w2, separated (temporally) by a gap g, as shown in Fig. Figure 3A illustrates this. For reasons that will be explained later, any waveform can be, for example, an ultrasonic or near-ultrasound waveform. That is, every tone is generated as a temporal composition, i.e., as successive signal segments comprising a waveform w1, a gap g, and another waveform w2. The resulting tone is thus a time-dependent bipolar signal, i.e., a signal exhibiting a time-dependent bipolar wave structure.

[0019] The gap g ideally corresponds to a zero-amplitude signal, although it can also be low- or very-low-amplitude noise. That is, the amplitude of the signal in the gap g is much lower than, if not negligible in comparison to, the amplitude of the signals corresponding to the two waveforms w1 and w2. The signal-to-noise ratio (SNR) should typically be at least greater than 10, but is preferably greater than 20 or even greater, e.g., 30. The SNR can also be expressed in decibels as SNR ≤ 0. dB = 10 log 10 (SNR). The SNR dBFor example, it can be chosen to ensure a difference of at least 15 dB between the waveforms and the gap, which corresponds to an SNR of approximately 31.6. If, for example, the waveforms each have an average value of -60 dB, then the noise level should typically be lower than -75 dB on average. Thus, the low or very low signal amplitude at the gap level g results in a temporal gap between the two waveforms w1 and w2 in the generated tone.

[0020] The preferred embodiment described above focuses on the first unit 1, which performs steps S203 to S209 in the sequence of Fig. 4. However, on the second unit 2, mirrored steps are typically implemented, with the second unit 2 receiving the first signal S203a, transmitting the second signal S204a, and attempting to establish correlations between the time of reception and the time of transmission with regard to determining a second time interval S206a, also referred to in this document as the third and fourth time intervals, respectively. Thanks to the determined first and second time intervals, units 1 and 2 can derive the Time of Flight and, consequently, the distance between the two units S209 and S209a. Due to the symmetry of the operations performed on the two units, such an embodiment can be considered a symmetrical double-sided two-way distance measurement method.

[0021] Each unit 1, 2 may be able to determine the exact time at which it sends its own signal. In this case, it is sufficient for units 1, 2 to correlate an expected signal (i.e., the signal they expect to receive from the other unit) with a reference element obtained by recording the received signal in order to determine the exact time of reception.

[0022] If it is not possible for the unit to determine the transmission time precisely, then the comparator can further include a recording of the transmitted signal (in addition to the recording of the received signal), thus requiring the recording S201, S201a to begin early enough. In this case, the comparator includes a recording of both the transmitted and received signals. In this way, each unit 1, 2 can further correlate a representation of its own transmitted signal with the comparator to determine the exact time (t1 or t). 2') to determine the time at which it sent the signal. Thus, two correlation operations are performed: one aims to determine the time of reception, the other serves to determine the time of transmission. The comparison elements used by the two units 1 and 2 are called the first comparison element and the second comparison element, respectively. Finally, the first unit 1 determines a first time interval (Δ1 in Fig. 2) between the transmission time (t1 or t 2' in Fig. 2) and the time of reception (t2 or t1). Fig. 2), while the second unit 2 is a second time interval (Δ2 in Fig. 2) determined between the time of reception and the time of transmission.

[0023] In the embodiment above, S206, S206a each correlate a comparison element formed according to its own recording with a representation of its own transmit signal to mark a corresponding transmit time in the recording, and S206, S206a further correlates the comparison element with a representation of the transmit signal of the counterpart to mark a corresponding receive time, as in Fig. Assuming 4. Then S207, S207a calculate the time difference between these two points in time for each unit 1, 2. Subsequently, they exchange the time differences thus obtained (S208, S208a), which allows them to determine the Time of Flight (S209, S209a). The Time of Flight is equal to half the absolute difference between the two time intervals thus obtained. That is, the Time of Flight is simply calculated as |Δ1 - Δ2| / 2, where Δ1 = t2 - t1 and Δ2 = t 2' - t 1'The distance between the two units can then be easily derived from the time of flight based on the laws of physics. This last operation is usually simply a multiplication of the speed of sound in air by the time of flight.

[0024] The present methods can be used in particular for measuring distance compliance between people. For example, the above steps can be carried out on the two units 1 and 2 as soon as it is determined that they are likely to be within range. For this purpose, a handshake can be performed beforehand (S160). The handshake also makes it possible to determine which unit takes the lead, as described in the embodiments below.

[0025] Of particular advantage is the structure of the generated tones (i.e., waveform w1 - gap g - waveform w2), which enables correct identification of a corresponding peak in cross-correlation measurements. As in Fig. As illustrated in Figure 3B, this peak enables, in practice, a clearer, more precise, and more accurate recording. Unlike other waveforms typically used in prior art methods to generate a single central peak, here the composite tone ( Fig. 3A), which is generated by each of the two units 1, 2, a triple-peak pattern after correlating the signals, as in Fig. Figure 3B illustrates this. A triple-peak pattern refers to a large peak p2 between two smaller peaks p1 and p3, with gaps in between. These gaps have known durations. The gaps between p1 and p2, and between p2 and p3, are symmetrical and therefore have the same duration. The latter is determined by the gap g, which is predefined. This helps in detecting the occurrence of a peak in the resulting cross-correlation measurement (in the time domain), thus enabling a more accurate measurement of the time of flight as a basis for proximity calculations.

[0026] In other words, the present approach proposes implementing a two-way distance measurement method based on the exchange of special composite waveforms, which are received and transmitted as tones (i.e., sounds). This approach can be advantageously used to precisely determine (i.e., with sub-millisecond accuracy) the time at which the signals are sent and received, and conversely, to more accurately estimate the distance between the two units.

[0027] As previously mentioned, prior art methods typically attempt to achieve this by searching for significant peaks in the cross-correlation signal. However, this typically leads to the detection of false peaks due to noise and interference. In contrast, the present approach makes it easier to detect the "correct" peak in the cross-correlation profile, thus enabling a more accurate measurement.

[0028] All of this will now be described in detail with reference to specific embodiments of the invention. First, each of the two units 1, 2 is preferably a handheld unit, e.g., a wireless unit such as a mobile phone (a smartphone), a smartwatch, a tablet, or a personal assistant. The units 1, 2 can also be dedicated units that, for example, form part of an employee identification badge (a pager) or an ID card, or that incorporate such a badge, particularly where the present methods are carried out to measure social contacts. In variants, however, the present methods can be implemented in larger units, such as automobiles or other vehicles.

[0029] Each of the two units 1, 2 can, for example, comprise a loudspeaker 15 and a microphone 16 for transmitting and recording the signals, respectively. Furthermore, each unit 1, 2 can comprise processing equipment 11 (e.g., a central processing unit or CPU), main memory 12 (connected to the processing equipment 11), long-term storage (not shown), and various controls necessary to operate the loudspeaker and microphone as is known per se. That is to say, the processing equipment 11 is suitably connected to the loudspeaker 15 and the microphone 16 via a system bus to enable the signals to be transmitted and received as required to carry out the present methods.

[0030] In particular, the first unit 1 can send the first signal via its loudspeaker 15 S203 so that the second unit 2 can capture the first signal via its corresponding microphone 16 S203a. Next, the second signal is sent via the loudspeaker 15 of the second unit 2 S204a and received at the first unit 1 via the latter's microphone 16 S204. Finally, the first unit 1 can attempt to correlate its recording (which forms a first comparison element) with (i) a representation of the first signal and (ii) an expected signal, with a view to deriving a first time interval thanks to its corresponding processing means 11. Similar steps can be carried out at the second unit 2 to derive a second time interval, as already explained.

[0031] As in Fig. As illustrated in Figure 3A, each waveform w1, w2 extends over predefined time segments ts1 and ts3, respectively. These time segments ts1 and ts3 preferably have the same duration, i.e., ts1 = ts3, as shown in Figure 3A. Fig. 3A is assumed. Furthermore, the duration ts2 of the gap g is preferably longer than ts = ts1 = ts3. Making this gap g longer than the time duration ts of the waveforms w1, w2 results in a pleasantly uncorrelated triple-peak pattern, where the central peak does not overlap with the side peaks, as in Fig. Figure 3B illustrates this. Each of the two waveforms w1, w2 can, for example, extend over a given time segment ts = ts1 = ts3, while the gap ts2 can sum to a given number a of such time segments ts. That is, ts2 = a ts, where a > 1. Furthermore, the first waveform w1 can be preceded by another gap (not shown). Similarly, the second waveform w2 can be followed by another gap (not shown). For example, the gap signal generated at each unit 1, 2 can be augmented with any zero-amplitude segments preceding or following each of the core waveforms w1, w2. In all cases, cross-correlation of signals corresponding to the tones thus obtained produces a triple-peak pattern that can be advantageously used to accurately identify the timing of wave transmission and reception by both units 1, 2.

[0032] It should be noted that the correlation operation performed could potentially be executed as the cross-correlation of two continuous functions representing the signals being compared. In practice, however, the algorithm preferably uses discrete time series, which can be computed in particular as C(s, t) = corr(X(s), Y(t)) for two variables X(s) and Y(t) and for all time delays s and t. Thus, the correlation is performed on the basis of two vectors (two matrices of values), which yield another vector as output. The maxima of the correlation values ​​obtained in the output represent the relevant times, i.e., the times corresponding to the indices of the maxima. This correlation is most practically performed as a convolution, i.e., as corr(X(s), Y(t)) = convolve(X(s), reverse(Y(t))), where reverse(Y(t)) corresponds to the inverse values ​​of Y(t).Values ​​that are inverted in the form of indices (i.e., also inverted in time).

[0033] One possible implementation (in Python) is the following (the correlation function is defined as "xcorr2"): a. static public double[] xcorr2(double[] a, double[] b, String type) { b. double[] revb = ArrayUtils.clone(b); c. ArrayUtils.reverse(revb); d. double[] c = new double[]{}; e. switch (type.toLowerCase()) { f. case (“full”): G. c = MathArrays.convolve(a, revb); h. break; i. case (“valid”): j. c = convolveValid(a, revb); k. break; l. case (“same”): m.c = MathArrays.convolve(a, revb); n. break; o.} p. return c; q.}

[0034] The convolution function (“MathArrays.convolve”) can be similar to the function in the Apache Commons Math 3.6 API.

[0035] In embodiments, each of the two waveforms w1, w2 (as used to generate the composite tone at each unit 1, 2 S202, S202a) has an inaudible sound frequency. In this way, the resulting tone is inaudible to humans. This is particularly advantageous when the present methods are used for maintaining distance between people; in this case, units 1, 2 are typically hand units. In particular, this frequency can be an ultrasonic or near-ultrasound frequency. Each of the two waveforms w1, w2 is thus an inaudible signal, typically generated in the low inaudible frequency range (± 18.5 - 20 kHz), which can be achieved with any current conventional loudspeaker hardware, but is inaudible to most people.

[0036] The two waveforms w1, w2 can be generated, in particular, by randomizing two ultrasound frequencies based on a given seed. That is, each unit 1, 2 randomizes two ultrasound frequencies based on a given seed. The two frequencies can initially be the same. Then, each unit generates two corresponding tones for a certain predetermined duration and combines the first waveform, a gap g containing a zero-amplitude signal or a low-amplitude signal, and the second waveform to create a complete waveform.

[0037] It should be noted that instead of microphones and loudspeakers, units 1 and 2 may rely on other types of signal transmitters, particularly when using ultrasound frequencies. In this case, the receiver instrumentation may include a gyroscope or another type of signal transmitter capable of detecting ultrasound sounds.

[0038] In two-way distance measurement methods based on the exchange of data packets, the relevant signal parameters can be included as part of the exchanged messages. However, such an approach is difficult to implement when the exchanged signals are audio signals. Such parameters can still be conveniently exchanged via a wireless communication channel established between the two units 1 and 2 S150, as shown in Fig. 5 accepted.

[0039] In this regard, and as already mentioned, the present procedures may further include performing S160 a preparatory handshake between the two units 1, 2 over the wireless communication channel established in step S150, as described in Fig. 5. This communication channel is preferably set up according to a wireless communication standard such as Wi-Fi, Bluetooth, and Bluetooth Low Energy (BLE) S150. The latter is assumed in the accompanying drawings.

[0040] This handshake is typically implemented automatically as part of the two-way distance measurement protocol S160, although it is carried out over a separate channel. That is, the two-way signals are sounds that propagate as acoustic waves, while messages and data exchanged between the two units 1 and 2 propagate as radio waves. As a result of this handshake, one of the two units is designated as the leading unit, while the other unit is designated as the following unit.

[0041] It is assumed that the first unit 1 is designated as a command unit, as in Fig. 2 and Fig. 4. As such, the first unit 1 can set certain signal parameters and transmit them to the second unit 2 during the preparatory handshake via the established communication channel S160. Such parameters may include, in particular, the seed (which is needed to randomize the ultrasonic frequencies), the durations of the waveforms and the gap g, and the time delay after which the second signal is to be sent S204a. This time delay may, for example, be approximately 250 ms. It must be sufficiently longer than an expected single propagation time. It should be noted that this time delay may be randomly selected by the first unit 1 to reduce the risk of collisions with signals received by other units, noting that unit 1 may interact with several remotely located units simultaneously.More precisely, this time delay can be the sum of a minimum preset duration plus a random duration. For example, an initial time delay of 50 ms can be applied before sending the first message tm1, and a second time delay of 250 ms can be set before sending the response signal tm2. The recording duration is typically on the order of one second for each unit 1, 2 to ensure that each recording captures both the sent signal and the response signal.

[0042] In the embodiment described directly above, the lead unit 1 sets the signal parameters and forwards them to the successor unit 2. In variants, some or all of these parameters can be predefined and specified as part of the protocol requirements. In other variants, the preparatory handshake S160 can cause the two units 1 and 2 to exchange their respective seeds and the durations of the waveforms and the gap g, noting that this gap need not be the same for the transmitted and received signals.

[0043] The following will again refer to Fig. 2. Referring to: it is assumed that the first unit 1 is designated as a lead unit and, accordingly, after completion of the handshake (S160), sends the first signal to the second unit 2 (S203). Simultaneously, each unit 1, 2 begins listening (S201, S201a) (i.e., recording) immediately after completion of the handshake. In this way, the recording performed by each unit 1, 2 includes a fingerprint of both the sent and received signals, enabling the units to accurately determine the relevant times and time intervals. It should be noted that the sending of the first signal (S203) may begin immediately after completion of the handshake. In variants, a timeout period precedes the sending of the first signal (S203) to allow sufficient time for the first unit 1 to generate the first signal (S202), if necessary.

[0044] As in Fig. As shown in section 2, S202, the first unit 1, generates a first signal (“Write(msg1)”), creating an audio signal tm1, which is then transmitted as a sound wave by the first unit 1 at time t1 (also referred to in this document as the “first time point”), S203. This signal is received by the second unit 2 at time t1 (also referred to in this document as the “third time point”), S203a. More precisely, the second unit 2 records the signal tm1 via its microphone to obtain an audio signal rm1. Once the time delay (as previously set and transmitted by the first unit 1) has elapsed, S204a transmits the second unit 2 at time t 2'(also referred to in this document as the fourth time point) the second signal. This signal is generated in time at step S202a (“Write(msg2)”). Subsequently, at a time point t2 (also referred to in this document as the “second time point”), the audio signal tm2 is received by the first unit 1 (S204). That is, the first unit 1 records the audio signal tm2 via its microphone and accordingly receives an audio signal rm2.

[0045] Thus, both units record audio throughout the entire process. For this purpose, each unit 1, 2 triggers audio recording in a timely manner via its corresponding microphone 16 (S201, S201a) in order to be able to receive the signal sent by the other unit 2, 1. In the Fig. 2 and Fig. In the embodiments shown in Figure 4, it is assumed that each unit 1, 2 begins recording immediately after the completion of the handshake S160. Such recordings are typically triggered for a limited duration; for example, an upper limit is tied to an expected transmission cycle. In variants, units 1, 2 can attempt to record only the counterpart's signal, assuming they are able to know precisely when they transmitted their own signals.

[0046] The in Fig. The process shown in point 2 can be implemented in particular as follows. First, a handshake is performed (S160). Then, after the handshake is complete (S201), the first unit (S203) begins recording and transmits its signal after 50 ms (S203). Similarly, the second unit (S201a) begins recording after the handshake is complete (S201a), but transmits its signal only after 250 ms (S204a), according to parameters transmitted by the first unit (S204a) during the handshake. The recording of the first unit (S1) includes its own transmission, followed by the signal transmitted by the second unit (S2). Similarly, the recording of the second unit (S2) includes the transmission from unit (S1) followed by the transmission from unit (S2).

[0047] As mentioned, each unit 1, 2 can reconstruct a signal from its corresponding audio recording in order to subsequently cross-correlate the reconstructed signal with representations of the expected counterpart signal and its own transmission. For this purpose, it can use the seeds and time durations as set during the handshake. It should be noted that signal reconstruction may involve trimming, filtering (of noise), and / or amplifying the obtained audio sample. For example, the audio samples may first be trimmed (using a suitable decision rule to identify the expected patterns), then denoised, amplified, and, if necessary, clipped, flattened, etc., applying any suitable audio signal processing technique known in the field.The reconstructed signals provide comparison elements, also referred to in this document as the first comparison element (for the first unit 1) and the second comparison element (for the second unit 2). Finally, S206, S206a cross-correlates each unit 1, 2 its reconstructed signal (the comparison element) with a representation of both (i) its own transmission and an expected signal (the signal it expects to receive, according to the handshake). Each cross-correlation results in a triple-peak pattern, as shown, for example, in . Fig. Figure 3B illustrates this. The central peak can be precisely located in time (with negligible error). This makes it possible to determine the relevant time points (on each side) and then the corresponding time intervals (on each side). Thus, the corresponding Time of Flight can be derived to determine the distance between the two units.

[0048] This, in turn, makes it possible to determine the time of occurrence in the triple-peak pattern obtained from the correlation. The temporal recording of the triple-peak pattern is then used to determine the time interval S207, S207a, i.e., the delta time delay between the transmission time and the reception time, denoted as Δ1 and Δ2 in Fig. 2. The time delay calculated by each party 1, 2 can then be transmitted to the other party via the enveloping communication channel S208, S208a. In this way, each unit 1, 2 is able to calculate the distance. In one variant, one of the units 1, 2 can send the time delay it has calculated to the other party so that it can calculate the distance and then send the calculated distance back so that the counterpart unit can derive a time delay and then a distance.

[0049] Fig. Figure 4 shows a preferred sequence of operations, which will now be described in detail. After the S160 handshake ( Fig. 5), whereby the first unit 1 has been designated as a lead unit, the first unit 1 begins recording (possibly after a timeout period) and generates a first signal as a composite tone, as previously explained, in step S201. In step S203, it transmits the first signal via its loudspeaker. This signal is recorded by the second unit 2. The first unit 1 then receives a response signal in step S204, i.e., it records a corresponding waveform via its microphone. In step S205, a signal is reconstructed from this audio recording; the reconstructed signal provides a first comparison element. Finally, in step S206, unit 1 correlates this comparison element with representations of both the transmitted signal and the expected reception. Accordingly, in step S207, a time interval is determined and transmitted to the second unit 2 via the previously opened communication channel, e.g., via BLE, in step S208.Similarly, the first unit 1 receives a time interval determined by the second unit 2, which is used to determine the distance value S209.

[0050] A simultaneous process is implemented at the second unit, which was designated as a successor unit during handshake S160. After the handshake is complete, unit 2 begins listening (S201a), i.e., it initiates an audio recording. It receives the first signal (S203a) via its microphone and records it. At this point, unit 2 may have already generated a second signal (S202a) (again as a composite tone), which it transmits via its loudspeaker in step S204a. This signal is recorded by unit 2. Subsequently, a suitable signal is reconstructed from the audio recording (S205a) to obtain a second comparison element. This latter element is then correlated by unit 2 with both the received and transmitted signals (S206a) to determine a time interval (S207a).The latter is passed on to the first unit 1 in step S208a, while the time delay calculated by the first unit 1 is passed on to the second unit 2 so that it can calculate the distance value S209a.

[0051] Several cycles may be required to successfully derive a distance value, for example, to overcome potential conflicts with other pairs located within the same space, or to improve this value, as is now the case with regard to Fig. 6 described. This means that the two-way distance measurement protocol can have one or more additional cycles of operations S200. During each additional cycle, S203, the first unit 1 (the master unit), sends another test signal (usually the same signal as previously sent) to the second unit 2, S204 receives a response signal (usually the same signal as previously sent by the second unit 2), and S206 correlates the signals, as explained above, to determine the distance between the two units 1 and 2 (S207 to S209). Again, mirrored steps can be performed on the second unit 2.

[0052] As further in Fig. As shown in step 6, each subsequent cycle is preferably triggered (S220: No, S230: No) after a timeout has elapsed, see step S240. This timeout is preferably randomly selected by the first unit 1 (S240). More precisely, the timeout can include a pre-set portion and a random portion. This timeout is then transmitted to the second unit 2 via the communication channel, e.g., as previously configured (S150), to enable the preparatory handshake (S160), or as necessary, reconfigured. That is, the two units 1 and 2 can agree to use random timeout durations before attempting to determine the distance again to prevent potential collisions with signals sent by other units, if necessary, provided that each unit 1 and 2 can interact with multiple remotely located units simultaneously. Such cycles can be repeated as needed to successfully determine distance values.Once it is determined that the distance has been successfully determined (step S220: Yes), the algorithm can return to a standard mode, where, for example, one unit searches for the presence of other units (i.e., back to step S100 in ). Fig. 5) Preferably, the protocol provides a maximum number of attempts (step S230). If this number is exceeded (S230: Yes), the cycles are interrupted and the unit returns to a standard mode (e.g., back to step S100 in...). Fig. 5).

[0053] Performing multiple acquisition cycles, as described above, can be useful for arriving at sensitive distance values. It can also be applied to improve robustness and suitability in situations where more than two units interact. In this regard, it should be noted that the present procedures can include a self-verification mechanism in step S220, using any suitable decision rule. For example, each unit 1, 2 can check the calculated time intervals against expected values. The units can at least verify whether positive time delays were obtained, although it should be noted that audio communication interference can lead to incorrect acquisitions and thus incorrect time delays.

[0054] As already mentioned, the present procedures can be advantageously implemented to measure compliance with distance between persons (i.e., social contacts), whereby distance values ​​are determined for each unit 1, 2 with regard to creating and updating a history of contacts (S207 to S209, S207a to S209a). For this purpose, each unit 1, 2 can continuously contact other units (S150), for which it is determined (S100 to S150) that they are at a sufficiently close distance, as now detailed in relation to Fig. 5 described.

[0055] In principle, measurements of social distancing between persons can be obtained by continuously performing a two-way, two-sided distance measurement procedure (S200), i.e., for each pair of units expected to be at a sufficiently close distance (S120: Yes). The S200 distance values ​​thus determined are then recorded in a history for each unit (1, 2). The history can then be used to issue warnings, notifications, etc., to the users of the units and / or authorities. It should be noted that such procedures described herein are assumed to be carried out with the consent of the users and in accordance with applicable regulations.

[0056] A preferred sequence is as follows. It is assumed that this sequence is executed on the first unit 1, although it should be noted that similar sequences can be executed on any unit compatible with this protocol. First, S100, unit 1 searches for the presence of units in the environment, using, for example, a suitable BLE sampling protocol. Accordingly, several remotely located units may be discovered. In step S110, unit 1 selects the next unit, as listed in the sample result. Unit 1 may first search its own history S130 to check if the currently selected (remotely located) unit is found in its current history S140. If so (S140: Yes), it is not necessary to proceed further, and unit 1 selects the next unit from the sample list.If not (S140: No), unit 1 establishes a communication channel with the currently selected unit (e.g., a BLE channel) on S150 to perform a handshake on S160, as previously described. Subsequently, a two-way distance measurement is performed on S200, and the determined distance value is stored in the history of unit 1.

[0057] As further in Fig. As shown in Figure 5, Unit 1 can first check whether the Receive Signal Strength Indicator (RSSI) shows a proximity that might require testing before starting interaction with the currently selected unit. If the radio signal strength indicates sufficient proximity (S120: Yes), then Unit 1 proceeds (steps S130, S140, etc.). If not (S120: No), it selects another unit from the sample list.

[0058] The above embodiments have been briefly described with reference to the accompanying drawings and can include a number of variants. Various combinations of the above features are possible. For example, in particularly preferred embodiments, each unit 1, 2 is a smartphone comprising a speaker 15 and an audio microphone 16. Furthermore, each unit is configured to generate a unique composite tone (as already described) for measuring a time of flight to another similarly configured unit. That is, two core waveforms are combined to form a unique tone comprising a first frequency segment, a second segment with zero amplitude (or very low-level noise), and a third frequency segment, with both the first and third frequencies being generated using certain predefined seeds.These frequencies are ultrasound or near-ultrasound frequencies, which makes the transmissions inaudible to humans. The three segments mentioned above have predetermined durations, with the gap having the longest duration, as already explained.

[0059] With more specific reference to Fig. 1A and Fig. Section 1B now describes another embodiment of the invention, which relates to a unit 1, 2. Embodiments of this unit have already been described in relation to the present methods. They are only briefly described below.

[0060] Each unit 1, 2 has processing means 11, e.g., a CPU. Various components are connected to the CPU, e.g., via a system bus and controllers (not shown). In particular, the unit 1, 2 has a tone generator 14, a transmitter 15 (e.g., a loudspeaker), and a receiver 16 (e.g., an audio microphone). A memory 12 is also typically connected to the CPU, as is common in this field. All these components are known as such, but they must be correctly configured, programmed, etc., to be able to execute the procedures described.The memory 12 can include computer-aided procedures stored in the form of program code instructions (software) which, when loaded into main memory and executed, cause the CPU to organize operations of the tone generator 14, the transmitter 15 and the receiver 16 to enable the unit 1, 2 to perform a double-sided two-way distance measurement protocol with a remotely arranged unit 2, 1 S200, as already described in relation to the present procedures.

[0061] In particular, the tone generator 14 can be configured to generate the two waveforms w1, w2 as ultrasonic frequency tones by randomizing two ultrasonic frequencies accordingly based on a predetermined seed. This generator 14 can further be configured to combine the two waveforms w1, w2 with a zero-amplitude gap (or noise-level gap) g to produce a composite tone. As previously explained, each bipolar waveform w1, w2 preferably extends over a time segment shorter than the duration of the gap g. In this way, the unit 1 is able to send and receive signals (and reconstruct suitable signals) to determine a distance between this unit and a remotely located unit.

[0062] A final manifestation concerns a computer program product for determining a distance between two units. This computer program product has a computer-readable storage medium containing program instructions. Such instructions can, for example, be stored in a long-term memory of a unit 1, 2, as described above, with regard to executing procedures as described herein. That is, such instructions are executable by processing means 11 of a unit 1, 2 to cause the latter to execute a two-way, two-sided distance measurement protocol, as already described in relation to Fig. 2 and 4 to 6 described.

[0063] It is understood that the present invention may be a method, a unit, and / or a computer program product. The computer program product may comprise a computer-readable storage medium (or media) containing computer-readable program instructions for inducing a processor to execute manifestations of the present invention.

[0064] The computer-readable storage medium can be a physical unit capable of retaining and storing instructions for use by an executing unit. For example, the computer-readable storage medium can be an electronic storage unit, a magnetic storage unit, an optical storage unit, an electromagnetic storage unit, a semiconductor storage unit, or any suitable combination thereof, without limitation.A non-exhaustive list of more specific examples of computer-readable storage media includes the following: a removable computer floppy disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), removable compact disc read-only memory (CD-ROM), a digital versatile disc (DVD), a memory stick, a floppy disk, a mechanically coded unit such as punched cards or raised structures in a groove on which instructions are stored, and any suitable combination thereof. For the purposes of this text, a computer-readable storage medium shall not be understood as volatile signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission medium (e.g., a wireless communication device).Optical fiber cables (transmitting light pulses) or electrical signals transmitted through a wire.

[0065] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to individual data processing units (DPUs) or, via a network such as the internet, a local area network (LAN), a wide area network (WAN), and / or a wireless network, to an external computer or storage device. The network may include copper cables, fiber optic cables, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each DPU receives computer-readable program instructions from the network and forwards them for storage on a computer-readable storage medium within the respective DPU.

[0066] The computer-readable program instructions for performing operations of the present invention may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state-setting data, configuration data for integrated circuits, or either source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., as well as conventional procedural programming languages ​​such as the programming language "C" or similar programming languages.The computer-readable program instructions can be executed entirely on the user's computer, partially on the user's computer as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on the remote computer or server. In the latter case, the remote computer can be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection can be made to an external computer (for example, via the internet using an internet service provider).In some embodiments, electronic circuits, including, for example, programmable logic circuits, field programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), can execute the computer-readable program instructions by using state information from the computer-readable program instructions to personalize the electronic circuits to perform manifestations of the present invention.

[0067] Manifestations of the present invention are described herein with reference to flowcharts and / or block diagrams of processes, systems, and computer program products according to embodiments of the invention. It is understood that each block of the flowcharts and / or block diagrams, as well as combinations of blocks in the flowcharts and / or block diagrams, can be implemented by means of computer-readable program instructions.

[0068] These computer-readable program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or any other programmable data processing device to create a machine such that the instructions executed by the processor of the computer or other programmable data processing device generate means for realizing the functions / actions specified in the block or blocks of the flowcharts and / or block diagrams.These computer-readable program instructions may also be stored on a computer-readable storage medium capable of controlling a computer, programmable data processing device and / or other units to function in a particular manner, such that the computer-readable storage medium on which instructions are stored has a manufactured product, including instructions that implement manifestations of the function / action specified in the block or blocks of the flowchart and / or diagrams.

[0069] The computer-readable program instructions can also be loaded onto a computer, other programmable data processing device, or other unit to cause the execution of a series of operational steps to be carried out on the computer, other programmable device, or other unit to produce a process realized on a computer, such that the instructions executed on the computer, other programmable device, or other unit realize the functions / actions specified in the block or blocks of the flowcharts and / or block diagrams.

[0070] The flowcharts and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, processes, and computer program products according to various embodiments of the present invention. In this context, each block in the flowcharts or block diagrams can represent a module, segment, or part of instructions that includes one or more executable instructions for realizing the specified logical function(s). In some alternative embodiments, the functions specified in the block may occur in a different order than shown in the figures. For example, two blocks shown consecutively may in reality be executed essentially simultaneously, or the blocks may sometimes be executed in reverse order depending on the corresponding functionality.It should also be noted that each block of the block diagrams and / or flowcharts, as well as combinations of blocks in the block diagrams and / or flowcharts, can be implemented by special hardware-based systems that perform the specified functions or actions, or by combinations of special hardware and computer instructions.

[0071] Although the present invention has been described with respect to a limited number of embodiments, variants, and the accompanying drawings, the person skilled in the art understands that various modifications can be made and substitutions with equivalents can take place without departing from the scope of the present invention. In particular, a feature (device feature or process feature) specified in a given embodiment, specified in a variant, or illustrated in a drawing can be combined with or replaced by another feature in another embodiment, variant, or drawing without departing from the scope of the present invention. Accordingly, various combinations of the features described with respect to any of the above embodiments or variants may be foreseeable, which remain within the scope of the appended claims.Furthermore, many minor modifications can be made to adapt a particular situation or material to the teachings of the present invention without deviating from its scope. Therefore, the present invention is not intended to be limited to the specific embodiments disclosed, but rather to encompass all embodiments within the scope of the appended claims. Moreover, many other variants may be foreseeable beyond those expressly mentioned above.

Claims

[1] Method for determining a distance between two units (1,2) wherein the method comprises: Executing a double-sided two-way distance measurement protocol (S200), wherein the latter causes the two units (1,2) to send and receive signals, each of the signals being a composite tone generated as a composition of two waveforms separated in time by a gap such that the composition forms a bipolar waveform, wherein: the two units (1,2) comprise a first unit and a second unit; the sent and received signals comprise a first signal and a second signal; and The execution of the double-sided two-way distance measurement protocol (S200) causes the first unit to perform a group of steps comprising: Sending the first signal to the second unit at a first time; Receiving the second signal, which was sent by the second unit after receiving the first signal, at a second time, and Correlating an expected signal with a first comparison element, the latter being obtained on the basis of the received second signal, to determine a first time interval between the first time and the second time and accordingly derive the distance between the two units (1,2), where the first unit further includes the following group of steps: Triggering an audio recording prior to the transmission of the first signal, wherein the first comparison element is obtained by recording both the transmitted first signal and the received second signal; and Correlating a representation of the first signal with the first comparison element to determine the first time point, wherein the expected signal is correlated with the first comparison element to determine the second time point, wherein the first unit determines the first time interval as a difference between the determined second time point and the determined first time point; wherein the representation of the first signal and the expected signal are correlated with the first comparison element to obtain corresponding cross-correlation signals, each exhibiting a triple-peak pattern showing two smaller peaks separated by a larger peak. [2] Method according to claim 1, wherein the execution of the double-sided two-way distance measurement protocol (S200) further causes the second unit to: triggers an audio recording before receiving the first signal sent by the first unit in order to obtain a second comparison element by recording both the first signal received by the second unit and the second signal sent by the second unit; Both an expected signal and a representation of the second signal are correlated with the second comparison element to determine times at which the first signal was received by the second unit and the second signal was sent by the second unit, respectively, and a second time interval is determined accordingly as a difference between the determined times; and the second time interval is sent to the first unit. [3] The method of claim 2, wherein the first unit performs the group of steps, further comprising: Receiving the second time interval; and Subtracting the second time interval from the first time interval to derive a Time of Flight of the first signal and the second signal and accordingly to determine the distance between the two units (1,2). [4] Method according to claim 1, wherein the method further comprises as part of the protocol: Performing a preparatory handshake between the two units (1,2) via a wireless communication channel established between the two units (1,2), wherein the first unit is designated as a leading unit and accordingly sends the first signal to the second unit after completion of the handshake. [5] Method according to claim 4, wherein the execution of the two-way distance measurement protocol (S200) comprises executing one or more further cycles of operations, each of the further cycles comprising executing the group of steps, each of the cycles being triggered after a time-out has elapsed, the time-out being randomly selected by the first unit and transmitted to the second unit during the preparatory handshake via the established communication channel. [6] The method of claim 4, wherein the first unit carries out the method, further comprising: Determining one or more parameters of a group consisting of the seed, a duration of the time segment, a duration of the gap, and a time delay after which the second signal is to be sent, and Transmitting the specified one or several parameters to the second unit via the established communication channel during the preparatory handshake. [7] The method of claim 6, wherein the method further comprises: Determining the time delay caused by the first unit using a random principle. [8] Method according to claim 4, wherein the communication channel is set up according to a wireless technology standard which is one of Bluetooth, Bluetooth Low Energy and Wi-Fi. [9] Method according to claim 1, wherein each of the two units (1,2) is a hand unit; and Each of the two waveforms has an inaudible sound frequency, so that the resulting tone is inaudible to a human. [10] Method according to claim 9, wherein each of the two units (1,2) comprises a loudspeaker (15), a microphone (16) and processing means connected to the loudspeaker (15) and the microphone (16), wherein the first unit: the first signal is sent via its corresponding loudspeaker (15) so that the second unit can detect the first signal via its corresponding microphone (16); the second signal is received via its corresponding microphone (16), the second signal being transmitted via the loudspeaker (15) of the second unit; and the expected signal is correlated with the first comparison element, the first time interval is determined, and thanks to its corresponding processing means, the distance between the two units (1,2) is derived. [11] Method according to claim 1, wherein the method further comprises sending the second signal to the second unit only after a predetermined time delay has elapsed. [12] Method according to claim 1, wherein each of the two waveforms extends over a predetermined time segment and one gap duration is longer than the predetermined time segment. [13] Method according to claim 12, wherein the two waveforms are formed as ultrasonic frequency tones by randomizing two ultrasonic frequencies accordingly on the basis of a predetermined seed. [14] Method for measuring compliance with distance between persons, wherein the method according to claim 1 is carried out, the method comprising on the first unit: Continuous contact with every second unit that is determined to be at a sufficiently close distance; and For each second contacted unit, execute the double-sided two-way distance measurement protocol (S200). [15] unit, having processing aids, and, associated with the processing equipment: a tone generator (14); a transmitter; and a recipient; where the processing means are configured to organize operations of the tone generator (14), the transmitter and the receiver, so that the unit is able to perform a double-sided two-way distance measurement protocol (S200) with a remotely located unit, wherein in operation: the tone generator (14) generates a first signal, the transmitter sends the first signal to the remotely located unit at a first time; the receiver receives a second signal from the second unit at a second time, and the processing medium a first comparison element is obtained based on the received second signal, and an expected signal is correlated with the first comparison element to determine a time interval between the first time point and the second time point and accordingly to determine the distance between the two units (1,2), and whereby Both the first signal and the second signal are composite tones, generated as a combination of two waveforms separated in time by a gap, so that the combination forms a bipolar waveform, wherein: the two units (1,2) comprise a first unit and a second unit; the sent and received signals comprise a first signal and a second signal; and The execution of the double-sided two-way distance measurement protocol (S200) causes the first unit to perform a group of steps comprising: Sending the first signal to the second unit at a first time; Receiving the second signal, which was sent by the second unit after receiving the first signal, at a second time, and Correlating an expected signal with a first comparison element, the latter being obtained on the basis of the received second signal, to determine a first time interval between the first time and the second time and accordingly derive the distance between the two units (1,2), where the first unit further includes the following group of steps: Triggering an audio recording prior to the transmission of the first signal, wherein the first comparison element is obtained by recording both the transmitted first signal and the received second signal; and Correlating a representation of the first signal with the first comparison element to determine the first time point, wherein the expected signal is correlated with the first comparison element to determine the second time point, wherein the first unit determines the first time interval as a difference between the determined second time point and the determined first time point; wherein the representation of the first signal and the expected signal are correlated with the first comparison element to obtain corresponding cross-correlation signals, each exhibiting a triple-peak pattern showing two smaller peaks separated by a larger peak. [16] Unit according to claim 15, wherein the tone generator (14) is configured to: Forming the two waveforms as ultrasound frequency tones by randomizing two ultrasound frequencies accordingly based on a given seed; and Combining the two waveforms and the gap such that each of the two waveforms extends over a given time segment which is shorter than the duration of the gap. [17] Computer program product for determining a distance between two units (1,2), wherein the computer program product comprises a computer-readable storage medium with program instructions embodied therein, wherein the program instructions are executable by processing means of each of the two units (1,2) to cause the latter to execute a double-sided two-way distance measurement protocol (S200) which causes the two units (1,2) to send and receive signals, wherein each of the signals is a composite tone generated as a composition of two waveforms separated in time by a gap, such that the composition forms a bipolar waveform, wherein: the two units (1,2) comprise a first unit and a second unit; the sent and received signals comprise a first signal and a second signal; and The execution of the double-sided two-way distance measurement protocol (S200) causes the first unit to perform a group of steps comprising: Sending the first signal to the second unit at a first time; Receiving the second signal, which was sent by the second unit after receiving the first signal, at a second time, and Correlating an expected signal with a first comparison element, the latter being obtained on the basis of the received second signal, to determine a first time interval between the first time and the second time and accordingly derive the distance between the two units (1,2), where the first unit further includes the following group of steps: Triggering an audio recording prior to the transmission of the first signal, wherein the first comparison element is obtained by recording both the transmitted first signal and the received second signal; and Correlating a representation of the first signal with the first comparison element to determine the first time point, wherein the expected signal is correlated with the first comparison element to determine the second time point, wherein the first unit determines the first time interval as a difference between the determined second time point and the determined first time point; wherein the representation of the first signal and the expected signal are correlated with the first comparison element to obtain corresponding cross-correlation signals, each exhibiting a triple-peak pattern showing two smaller peaks separated by a larger peak.

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