Patent Description:
In communication technologies, in particular in wireless communication technologies, determining timing related information between devices is an important issue. For example in order to synchronize a clock of a first device to a clock of a second device, the first device needs to obtain timing information regarding the clock of the second device. Such timing information may be related, for example, to a frequency and/or an offset of the clock of the second device with regard to the first device. Hence, it is an object of the present disclosure to present a method, apparatus, system, computer program product and computer readable storage medium to enable communication of such timing information between devices.

<CIT> discloses a time control device that can synchronize time information with a master device in a network with high precision. The time control device includes: a calculating unit that calculates a time difference from the master device and a network delay based on the transmission times and the reception times of messages exchanged with the master device, the network delay indicating the period of time required for communicating the messages via the network; a PID processing unit that generates a feedback control value f1 based on the calculated time difference, the feedback control value f1 being used for performing feedback control on time information about the slave device; a f0 generating unit that generates a feedback control value f0 based on the generated feedback control value f1; and an adjusting unit that adjusts the time information about the slave device in accordance with the feedback control value f1 or the feedback control value f0, whichever is selected based on the calculated network delay.

<CIT> discloses a method, which includes, at a first node: transmitting a first calibration signal at a first time-of-departure measured by the first node; and transmitting a second calibration signal at a second time-of-departure measured by the first node. The method also includes, at a second node: receiving the first calibration signal at a first time-of-arrival measured by the second node; and receiving the second calibration signal at a second time-of-arrival measured by the second node. The method further includes: defining a first calibration point and a second calibration point in a set of calibration points, each calibration point comprising a time-of-departure and a time-of-arrival of each calibration signal; calculating a regression on the set of calibration points; and calculating a frequency offset between the first node and the second node based on the first regression.

The above-mentioned object is solved by the subject-matter of the attached independent claims. Further embodiments are disclosed in the attached dependent claims.

According to a first aspect of the disclosure, a method for determining a clock frequency offset between a first device having a first clock and at least one second device having at least one second clock comprises:.

An advantage of this method is that a clock frequency offset between different devices, each having its own free-running clock used for time measurements, may be determined based on only two messages, exchanged between the respective devices, and the corresponding times of departures/times of arrivals. This way, a determination of the frequency offset is achieved in an easy and resource saving manner, wherein only low overhead is produced.

Another advantage of this method is that the clock frequency offset between the devices may be determined without knowledge regarding a distance between the respective devices.

The messages exchanged between the devices may be furthermore used for any kind of communication between the devices. In other words, the method according to the first aspect may be implemented in devices exchanging any kind of communication messages, from which the corresponding times of departure may be obtained. Hence, for determining the frequency offset according to the first aspect, no additional messages are needed in that case, and an overhead in a corresponding communication network may be reduced.

In the context of this application, a time of departure of a message defines a time at which the corresponding message is sent from a sending device. A time of arrival of a message defines the time at which the corresponding message is received at a receiving device. The clock frequency offset between clocks in this application defines the difference between frequencies of clocks, which is an indicator for how much slower/faster one clock runs compared to another clock.

The clock frequency offset may be determined by the at least one second device itself, or may be determined by any other entity which, in that case, obtains the relevant information, i.e., the time of departure of the at least one first message, the time of arrival of the at least one first message, the time of departure of the at least one second message, and the time of arrival of the at least one second message.

According to at least one embodiment the method further comprises:.

An advantage thereof is that additionally to the clock frequency offset, also a clock phase offset may be determined based on only three messages, exchanged between the respective devices, and the corresponding times of departures/times of arrivals. This way, a determination of the phase offset is achieved in an easy and resource saving manner, wherein only low overhead is produced.

Another advantage herein is that also the clock phase offset between the devices may be determined without knowledge regarding a distance between the respective devices.

The clock phase offset may be determined by the at least one second device itself, or may be determined by any other entity which, in that case, obtains the relevant information, i.e., the time of arrival of the at least one first message, the time of departure of the at least one first message, the time of arrival of the at least one second message, the time of departure of the at least one second message, the time of departure of the at least one third message, and a time of arrival of the at least one third message.

According to at least one embodiment, the information regarding the time of departure of the at least one first message and/or of the at least one second message and/or of the at least one third message is in the at least one first and/or second and/or third message embedded in form of a timestamp in the at least one first and/or second and/or third message or derivable from a predetermined scheduling of the at least one first and/or second and/or third message.

An advantage of embedding the information regarding the time of departure in the respective messages in form of a timestamp is that no additional information, for example regarding a scheduling of messages, is required by the respective devices. An advantage of the information regarding the time of departure being in the respective messages in form of a predetermined scheduling of the respective messages and therefore being derivable from said predetermined scheduling is that no further information needs to be added to the transmitted messages in order to inform the receiving device of the time of departure of the received message, thereby further reducing overhead. For example, when certain messages are sent by a transmitting device at certain points in time, known to the receiving device, the receiving device immediately knows the time of departure when the respective message is received.

According to at least one embodiment, the method further comprises the step: synchronizing the at least one second clock to the first clock based on the determined clock frequency offset and/or based on the determined clock phase offset, or synchronizing the first clock to the at least one second clock based on the determined clock frequency offset and/or based on the determined clock phase offset.

Synchronizing the at least one second clock to the first clock based on the determined clock frequency offset may be done by adapting a clock frequency of the at least one second clock to a clock frequency of the first clock, i.e., minimizing the clock frequency offset between the respective clocks. Synchronizing the at least one second clock to the first clock based on the determined clock phase offset may be done by adapting a current time of the at least one second clock to the first clock, i.e., minimizing a timing offset between the respective clocks. In particular, also the clock frequency offset and the timing offset between respective clocks may be minimized by synchronizing the respective clocks based on both the determined clock frequency offset and the determined clock phase offset.

Alternatively, the first clock may be synchronized, analogously to the above, to the at least one second clock. In case more than one second device, and correspondingly more than one second clock, is present, this way, a synchronization of the first device to multiple second devices is made possible. An advantage herein is that a one-to-many synchronization is achieved in a resource saving and easy implementable manner. Further, in case multiple second devices are present, analogously also one of the corresponding second clocks may be synchronized to the first clock and/or to second clocks of the other second devices according to the above-described manner.

According to at least one embodiment, the synchronizing of the at least one second or the first clock comprises offsetting timing values of the at least one second or the first clock.

An advantage thereof is that an easy way of synchronizing the respective clocks is provided. For example, such synchronizing may be implemented in software, according to which timing values, such as for example certain timestamps generated by the respective device, are corrected based on the determined clock phase offset and/or the determined clock frequency offset. Clock synchronization may be performed based on clock model parameters, i.e. clock phase offset and clock frequency offset, rather than by physically steering the clock in this embodiment.

According to at least one embodiment, in case the at least one first and the at least one second message are received by at least two second devices and at least two third messages are sent by the at least two second devices: the at least one second clock and/or the first clock are synchronized with respect to the other clocks of the at least two second clocks and/or the first clock based on a weighted zero mean error; or the at least one second clock and/or the first clock are synchronized to one of the other clocks of the at least two second clock and/or the first clock, which serves as a master clock.

The other clocks of the at least two second clocks and/or the first clock, in this context, describe the clocks, except the one which is to be synchronized. For example, in case a first one of two second clocks is to be synchronized, the other clocks, in this case, are the second of the two second clocks and the first clock. In case the first clock is to be synchronized, all second clocks are the other clocks in this context.

An advantage of the above embodiments is that an easily implementable possibility of synchronizing clocks of multiple devices is provided. By synchronizing clocks to a weighted zero mean error, an average system time of multiple clocks comprised in such system is determined, to which single clocks then can be synchronized. This is advantageous in that for synchronizing the clocks, not only one single master clock needs to be trusted, and hence a more reliable synchronization is possible, for example in case such single master clock is erroneous or completely fails.

Alternatively, clocks may be synchronized to one master clock. In case, for example, one device has a more reliable clock, it is advantageous to synchronize other clocks to that device's clock being used as master clock. Also in case one device has a clock that is being synchronized to another entity, such as, for example, a first device serving as a gateway between an indoor navigation system having multiple second devices and a global navigation satellite system (GNSS) satellite, it is possible to synchronize the first device to the GNSS satellite and then synchronize the second devices to the first device according to the above-described method, wherein the first device provides the master clock, to which each of the second devices may be synchronized.

According to a second aspect, an apparatus comprises a processor, a receiver and a second clock, wherein:.

According to at least one embodiment, the apparatus further comprises a transmitter, wherein:.

According to at least one embodiment, the clock frequency offset and/or the clock phase offset is determined by the processor of the apparatus or by an external device.

An advantage of determining the respective offsets by the processor of the apparatus is that overhead is kept low, since it is directly the apparatus itself which may determine those offsets. An advantage of determining the respective offsets by an external device is that processing resources of the apparatus are saved and, in particular in case multiple apparatuses like the one described above are present, a central entity may be used to determine the respective offsets for multiple apparatuses.

According to at least one embodiment, the processor is further arranged to synchronize the second clock to the first clock based on the determined clock frequency offset and/or the determined clock phase offset by offsetting timing values of the second clock.

According to a third aspect, a system comprises a first device and at least one apparatus according to the second aspect.

According to a fourth aspect, a computer program product comprises instructions which, when executed on a computer, cause the computer to perform the method according to the first aspect.

According to a fifth aspect, a computer readable storage medium comprises the computer program product according to the fourth aspect.

Advantages and embodiments of the second to fifth aspect may correspondingly apply and be combined with those described with respect to the first aspect and vice versa.

The above-mentioned aspects of the disclosure and their embodiments will be explained in more detail in the following with the aid of the drawings. Elements and functional blocks having the same or similar function bear the same reference numerals throughout the drawings. Hence their description is not necessarily repeated in following drawings.

<FIG> shows a schematic flowchart of a method <NUM> for determining a clock frequency offset and a clock phase offset according to one embodiment of the disclosure. The flowchart of the method <NUM> shows messages being exchanged between a first device <NUM> and a second device <NUM> in order to determine a clock frequency offset and a clock phase offset between a first clock, not shown herein, of the first device <NUM> and a second clock, not shown herein, of the second device <NUM>.

In a step <NUM>, the first device <NUM> sends a first message M1 to the second device <NUM>. The first message M1 is sent by the first device <NUM> at a certain time instance, which is defined as the time of departure td<NUM>A of the first message M1 from the first device <NUM>. The time of departure td<NUM>A is related to the first clock of the first device. The time of departure td<NUM>A of the first message M1 is in the first message M1. In this example, the time of departure td<NUM>A is embedded in form of a timestamp, included in and transmitted together with the first message M1, for example in a header of the first message M1. Alternatively, the time of departure td<NUM>A could be implicit in the first message M1, not in form of an explicit timestamp, but based on a scheduling of the first message, known to the second device <NUM>, such that the second device <NUM> would know from the schedule, when receiving the first message M1, that the first message M1 was sent by the first device <NUM> at such certain time instance.

In a further step <NUM>, the first message M1 is received by the second device <NUM>. The first message M1 is received by the second device <NUM> at a certain time instance, which is defined as the time of arrival ta<NUM>B of the first message M1 at the second device <NUM>. The second device <NUM> detects the time of arrival ta<NUM>B of the first message M1, which is measured by the second clock of the second device <NUM>, and stores the time of arrival ta<NUM>B. The second device <NUM> further obtains the time of departure td<NUM>A of the first message M1 from the first message M1 and stores it.

In a further step <NUM>, the second device <NUM> sends a third message M3 to the first device <NUM>. The third message M3 is sent by the second device <NUM> at a certain time instance, which is defined as the time of departure td<NUM>B of the third message M3 from the second device <NUM>. The time of departure td<NUM>B is measured and stored by the second device <NUM>, in this embodiment. Alternatively, as described with reference to the first message M1, also the time of departure td<NUM>B could be determined by the second device <NUM> based on a scheduling of the third message M3. In either case, the time of departure td<NUM>B relates to the second clock of the second device <NUM>.

In a further step <NUM>, the third message M3 is received by the first device <NUM>. The third message M3 is received by the first device <NUM> at a certain time instance, which is defined as the time of arrival ta<NUM>A of the third message M3 at the first device <NUM>. The first device <NUM> detects the time of arrival ta<NUM>A of the third message M3, which is measured by the first clock of the first device <NUM>.

In a further step <NUM>, the first device <NUM> sends a second message M2 to the second device <NUM>. The second message M2 is sent by the first device <NUM> at a certain time instance, which is defined as the time of departure td<NUM>A of the second message M2 from the first device <NUM>. The time of departure td<NUM>A is related to the first clock of the first device. The time of departure td<NUM>A of the second message M2 is embedded in the second message M2. Additionally, also the time of arrival ta<NUM>A of the third message M3 is in the second message M2. In this example, the time of departure td<NUM>A and the time of arrival ta<NUM>A are embedded in form of a timestamp, as described above with regard to the time of departure td<NUM>A.

However, as also described above, also different implementations of communicating the respective time instances are possible.

In this embodiment, as shown in <FIG>, the third message M3 is sent and received before the second message M2 is sent and received. However, this sequence may also be changed, in particular in case the time of arrival of the third message ta<NUM>A is not transmitted in form of a timestamp in the second message M2, but, for example, in form of the above-described scheduling.

In a further step <NUM>, the second message M2 is received by the second device <NUM>. The second message M2 is received by the second device <NUM> at a certain time instance, which is defined as the time of arrival ta<NUM>B of the second message M2 at the second device <NUM>. The second device <NUM> detects the time of arrival ta<NUM>B of the second message M2, which is measured by the second clock of the second device <NUM>, and stores the time of arrival ta<NUM>B. The second device further obtains the time of departure td<NUM>A of the second message M2 and the time of arrival ta<NUM>A of the third message M3 from the second message M2 and stores those.

In a further step <NUM>, the second device <NUM> then determines a clock frequency offset f<NUM>BA and a clock phase offset τ<NUM>BA between the first clock of the first device <NUM> and the second clock of the second device <NUM> from the above-mentioned times of arrivals and times of departures.

The clock frequency offset f<NUM>BA between the first and the second device is determined, according to this application, according to the following formula: <MAT>.

The clock phase offset τ<NUM>BA between the first and the second device may be determined, for example, according to the following formula: <MAT>.

The clock frequency offset f<NUM>BA and the clock phase offset τ<NUM>BA may therefore be determined based only on the times of arrival and times of departure of the first, second and third message.

In a further step <NUM>, the second device <NUM> then synchronizes its second clock to the first clock according to the determined clock frequency offset f<NUM>BA and the determined clock phase offset τ<NUM>BA. The synchronizing may be done, for example, using a frequency and phase locked loop, which is a second order tracking loop. This synchronization provides a consistent clock alignment, which can be reliably interpolated over long time intervals. Alternatively, however, also only synchronizing the second clock based on the determined clock frequency offset f<NUM>BA or the determined clock phase offset τ<NUM>BA is possible.

With the method <NUM> disclosed herein, as well as in the following embodiments described below, the second order clock error is canceled out when calculating the offset values as described above. Therefore, the interval between clock synchronization message exchanges can be much longer, for example, of a magnitude of <NUM> milliseconds or even up to <NUM> second, for mainstream low cost temperature compensated crystal oscillators, TCXOs. In case of high accuracy, high precision clocks being used, such as rubidium, caesium clocks, hydrogen maser clocks, etc., even longer time intervals may be achieved.

<FIG> shows a schematic flowchart of a method <NUM> for determining a clock frequency offset and a clock phase offset according to another embodiment of the disclosure. The flowchart of the method <NUM> shows messages being exchanged between a first device <NUM>, a second device <NUM> and a third device <NUM> in order to determine clock frequency offsets and clock phase offsets between those devices <NUM>, <NUM>, <NUM>.

The first and second device <NUM>, <NUM> may correspond to those described with reference to <FIG>. The third device has a third clock, not shown herein. Moreover, a sending of a first message M1 in a step <NUM>, a receiving of the first message M1 in a step <NUM>, a sending of a third message M3 in a step <NUM>, a receiving of the third message M3 in a step <NUM>, a sending of a second message M2 in a step <NUM> and a receiving of the second message M2 in a step <NUM> may be performed according to the corresponding steps as described with respect to <FIG> and is not repeated herein.

Additionally to the sending of the first message M1 from the first device <NUM> to the second device <NUM>, the first message M1 is also sent from the first device <NUM> to the third device <NUM>. In this embodiment, the first message M1 is broadcast and received by both, the second and third device <NUM>, <NUM>. The first message M1 is received by the third device <NUM> in a step <NUM>. Alternatively, however, the first message M1 could also be sent as two separate messages to the second and third device <NUM>, <NUM>, at the same or different moments in time. This may apply to any of the messages discussed herein and is not repeated in the following.

Analogously to the receiving of the first message M1 by the second device <NUM> in step <NUM>, the third device <NUM> detects a time of arrival ta<NUM>C of the first message M1 at the third device <NUM>, which is measured using the third clock of the third device <NUM>, and stores the time of arrival ta<NUM>C. The third device <NUM> further obtains the time of departure td<NUM>A of the first message M1 from the first message M1 and stores it.

Analogously to the above, in step <NUM>, the third message M3 having the time of departure td<NUM>B is broadcast by the second device <NUM> and received, in addition to the receiving of the third message M3 by the first device <NUM>, by the third device <NUM> in a step <NUM>. Thereby, a time of arrival ta<NUM>C of the third message M3 at the third device <NUM> is measured and stored by the third device <NUM>, and the time of departure td<NUM>B is obtained from the third message M3 and stored by the third device <NUM>.

At a step <NUM>, the third device <NUM> broadcasts a fourth message M4. The fourth message M4 has embedded a time of departure td<NUM>C of the fourth message M4. Moreover, the fourth message M4 may contain the time of arrival ta<NUM>C of the first message M1 at the third device <NUM> and/or the fourth message M4 may contain the time of arrival ta<NUM>C of the third message M3 at the third device <NUM>. The fourth message M4 is received, by the first and second device <NUM>, <NUM>, respectively, at steps <NUM> and <NUM>.

Analogously to the first message M1, in step <NUM> also the second message M2 is broadcast, by the first device <NUM>. The second message M2 is received in a step <NUM>, in addition to the receiving of the second message M2 by the second device <NUM>, by the third device <NUM> at a time of arrival ta<NUM>C of the second message M2 at the third device <NUM>, which is measured by the third clock of the third device <NUM> and stored by the third device <NUM>.

In a step <NUM>, the second device <NUM> determines a clock frequency offset f<NUM>BA and a clock phase offset τ<NUM>BA between the first clock of the first device <NUM> and the second clock of the second device <NUM>. This corresponds to the step <NUM> as described with reference to <FIG> and may be determined, for example, according to the equations discussed with regard to <FIG>.

Moreover, in a step <NUM>, a clock frequency offset f<NUM>CA and a clock phase offset τ<NUM>CA between the first clock of the first device <NUM> and the third clock of the third device <NUM> may be determined by the third device <NUM>.

The clock frequency offset f<NUM>CA between the first and the third device may be determined, for example, according to the following formula: <MAT>.

The clock phase offset τ<NUM>CA between the first and the third device may be determined, for example, according to the following formula: <MAT>.

The clock frequency offset f<NUM>CA and the clock phase offset τ<NUM>CA may therefore be determined based only on the times of arrival and times of departure of the first, second and fourth message.

In a further step <NUM>, the second device <NUM> then synchronizes its second clock to the first clock according to the determined clock frequency offset f<NUM>BA and the determined clock phase offset τ<NUM>BA. Analogously, in a step <NUM>, the third device <NUM> synchronizes its third clock to the first clock according to the determined clock frequency offset f<NUM>CA and the determined clock phase offset τ<NUM>CA.

In this embodiment, the sending of the first, third and fourth message M1, M3, M4 is part of a periodic sending of those respective messages. Accordingly, the sending of the second message M2 is, in fact, a periodic repetition of the sending of the first message M1. Furthermore, according to said periodic sending, the second device <NUM> may further broadcast a fifth message M5, corresponding to the sending of the third message M3, at a time of departure td<NUM>B at a step <NUM>, the fifth message M5 having the time of departure td<NUM>B embedded in the fifth message M5. The third device <NUM> may receive the fifth message M5 at a time of arrival ta<NUM>C at a step <NUM>, the first device <NUM> may receive the fifth message M5 at a time of arrival ta<NUM>A at a step <NUM>.

Furthermore, analogously to the broadcasting of the fourth message M3, the third device <NUM> also broadcasts a sixth message M6 at a step <NUM> according to said periodicity. The sixth message M6 is received by the first and second device <NUM>, <NUM>, respectively, at steps <NUM> and <NUM>. This generally corresponds to the sending and receiving of the previous messages, and details are not repeated herein.

Analogously to the above, the third device <NUM> may also determine a clock frequency offset f<NUM>CB and a clock phase offset τ<NUM>CB between the second device <NUM> and the third device <NUM>. The third device <NUM> may then, alternatively to synchronizing its third clock to the first clock of the first device <NUM>, synchronize its third clock to the second clock of the second device <NUM>.

Further alternatively, however, the third device <NUM> may also determine a weighted zero mean error of its third clock based on the clock frequency offsets f<NUM>CA and f<NUM>CB and the clock phase offsets τ<NUM>CA and τ<NUM>CB. In that case, instead of synchronizing its third clock to a single other device, the third device may correct its timing values based on said weighted zero mean error, i.e., based on a weighted average of the offsets from the first and second devices <NUM>, <NUM>.

Due to the broadcasting of the messages described herein, the method <NUM> is scalable with respect to the number of devices determining the respective clock phase offsets and clock frequency offsets and the scope of the disclosure shall not be limited with regard to the three devices shown in <FIG>.

Further, due to the periodic sending of the respective messages described above, the respective clocks of the devices <NUM>, <NUM>, <NUM> may be tracked and synchronized continuously. The respective clock frequency offsets and clock phase offsets may be determined periodically and the synchronizing of the clocks may be performed periodically. This is in particular advantageous in mobile and fluid networks that are not fixed with continually changing connectivities because fully redundant clock tracking is made possible.

In the embodiment shown in <FIG>, the determining of the respective clock phase offsets and clock frequency offsets and the corresponding synchronization of the respective clocks is shown after two cycles of the periodic sending of the messages M1 to M3, i.e., M1 to M6 in total. However, this is merely to be understood exemplary and the periodic determining of those offsets and the periodic synchronizing may be performed once the required messages and respective times of arrival and times of departure for determining the offsets according to the above-described method have been sent and received.

<FIG> shows a system <NUM> according to one embodiment of the disclosure. The system <NUM> may be, for example, a <NUM> small cell or an indoor positioning system. The system <NUM> comprises a first apparatus <NUM> and a second apparatus <NUM>. The first and second apparatus <NUM>, <NUM> may be, for example, anchors of the aforementioned indoor positioning system. To provide an easily understandable description of the concept of the disclosure, only two apparatuses <NUM>, <NUM> are shown herein. However, the system <NUM> may, of course, also comprise further apparatuses corresponding to those described herein.

The system <NUM> further comprises a central controlling entity <NUM>. The central controlling entity may be, for example, a GNSS receiver of the aforementioned indoor positioning system, which may be used as a gateway to synchronize the first and second apparatus <NUM>, <NUM> of the indoor positioning system to a GNSS satellite, wherein the first and second apparatus <NUM>, <NUM> are out of sight of the GNSS satellite. The central controlling apparatus <NUM>, in this case, is synchronized to a GNSS satellite, not shown herein.

The first apparatus <NUM> comprises a first clock <NUM>, a first receiver <NUM>, a first transmitter <NUM> and a first processor <NUM>. The second apparatus <NUM> comprises a second clock <NUM>, a second receiver <NUM>, a second transmitter <NUM> and a second processor <NUM>. The central controlling entity <NUM> comprises a third receiver <NUM>, a third transmitter <NUM>, a third processor <NUM> and a third clock <NUM>. The third cock <NUM>, as mentioned above, is synchronized to a GNSS satellite, and therefore serves as a master clock for the system <NUM>. The central controlling entity <NUM>, the first apparatus <NUM> and the second apparatus <NUM> may communicate with each other via a wireless network <NUM>.

In this embodiment, the central controlling entity <NUM> broadcasts a first message using its third transmitter <NUM> via the wireless network <NUM> and the first message is received by the second apparatus <NUM> and the first apparatus <NUM> with their respective receivers <NUM>, <NUM>. The first message is broadcast by the central controlling entity <NUM> according to a predetermined schedule, which is known to the first and second apparatus <NUM>, <NUM>. The first message is broadcast, for example, every <NUM> milliseconds according to a Wireless-Fidelity, Wi-Fi, standard, i.e., a standard according to the IEEE <NUM> family of standards. Alternatively, the first message may also be broadcast, for example, every second, or even every <NUM> seconds.

When the first and second apparatus <NUM>, <NUM> receive the first message, the first and second apparatus <NUM>, <NUM>, according to the schedule, each know that the central controlling entity <NUM> passed the beginning of such <NUM> millisecond period, according to its third clock <NUM>, which in this case is used as a reference for a time of departure of the first message. This reference is unambiguous, since the apparatuses <NUM>, <NUM> and the central controlling entity <NUM> are located so close to each other in the system <NUM>, that a transmission time of the message is shorter than the period for the sending of the scheduled message.

Further, the first apparatus <NUM> broadcasts a third message using its first transmitter <NUM> via the wireless network <NUM>. The third message is also broadcast according to a scheduling, such that the central controlling entity <NUM> and the second apparatus <NUM> obtain, when receiving the third message, a time of departure of the third message according to the schedule, as discussed with reference to the first message above.

Further, the central controlling entity <NUM> is arranged to broadcast a second message, according to a schedule, having embedded a time of arrival of the third message at the central controlling entity <NUM> measured by the third clock <NUM>. When the first and second apparatus <NUM>, <NUM> receive the second message, the first and second apparatus <NUM>, <NUM> obtain the time of arrival of the third message at the central controlling entity <NUM> and the time of departure of the second message, analogously to the above. Correspondingly, the second apparatus <NUM> may also broadcast a fourth message, which may be received by the central controlling entity <NUM> and the first apparatus <NUM>.

The first processor <NUM> of the first apparatus <NUM> is then able to determine a clock phase offset and a clock frequency offset of the first clock <NUM> from the third clock <NUM> based on the obtained timing information from the first, second and third messages. Correspondingly, the second processor <NUM> of the second apparatus <NUM> is able to determine a clock phase offset and a clock frequency offset of the second clock <NUM> from the third clock <NUM> based on the obtained timing information from the first, second and fourth messages. This may be done according to the equations discussed with respect to <FIG> and <FIG>.

The first apparatus <NUM> may then, with its first processor <NUM>, correct timing values of the first clock <NUM> based on the obtained offsets. The second apparatus <NUM> may then, with its second processor <NUM>, correct timing values of the second clock <NUM> based on the obtained offsets.

Alternatively, it would also be possible for the central controlling entity <NUM>, to determine offsets of the clocks of the first and second apparatus <NUM>, <NUM> and to communicate those to the respective apparatus. Alternatively, also a weighted zero mean error may be determined, based on which of the clocks of the first and second apparatus <NUM>, <NUM> are corrected, as discussed above with regard to <FIG>.

In the embodiment of <FIG>, in case one of the apparatuses <NUM>, <NUM> loses connection to the central controlling entity <NUM>, said apparatus may still synchronize its respective clock indirectly to the third clock <NUM> of the central controlling entity <NUM>, which serves as a master clock, by synchronizing its clock to the clock of the other apparatus, which is synchronized to the third clock <NUM>.

For example in the embodiment of <FIG>, assuming that the second apparatus <NUM> cannot receive messages from the central controlling entity <NUM>. However, the second apparatus <NUM> still receives messages from the first apparatus <NUM> and vice versa, and the first apparatus <NUM> receives messages from the central controlling entity <NUM> and vice versa. The first apparatus <NUM> synchronizes the first clock <NUM> to the third clock <NUM> according to the exchange of messages described above. The second apparatus synchronizes the second clock <NUM> to the first clock <NUM> corresponding to the above, based on the third, fourth and a fifth message as described with respect to the method as shown in <FIG>. This way, the second apparatus <NUM> may still synchronize its second clock <NUM> to the third clock <NUM>, even when the second apparatus <NUM> cannot receive the first and second message broadcast by the central controlling entity <NUM>.

<FIG> shows a schematic flowchart of a method <NUM> for determining a clock frequency offset according to one embodiment of the disclosure.

In a step <NUM>, at least one second device receives at least one first message from a first device, wherein information regarding a time of departure of the at least one first message is in the at least one first message.

In a step <NUM>, the at least one second device determines a time of arrival of the at least one first message.

In a step <NUM>, the at least one second device receives at least one second message from the first device, wherein information regarding a time of departure of the at least one second message is in the at least one second message.

In a step <NUM>, the at least one second device determines a time of arrival of the at least one second message.

In a step <NUM>, a clock frequency offset between the first clock and the at least one second clock is determined based on the time of departure of the at least one first message, the time of arrival of the at least one first message, the time of departure of the at least one second message, and the time of arrival of the at least one second message.

<FIG> shows an apparatus <NUM> according to one embodiment of the disclosure. The apparatus <NUM> comprises a processor <NUM>, a receiver <NUM> and a second clock <NUM>.

The receiver <NUM> is arranged to receive a first message from a first device, wherein information regarding a time of departure of the first message is in the first message, and to receive a second message from the first device, wherein information regarding a time of departure of the second message is in the second message.

The processor <NUM> is arranged to determine a time of arrival of the first message and a time of arrival of the second message, wherein a clock frequency offset between a first clock of the first device and the second clock <NUM> is determined based on the time of departure of the first message, the time of arrival of the first message, the time of departure of the second message, and the time of arrival of the second message.

Claim 1:
Method (<NUM>) for determining a clock frequency offset between a first device having a first clock and at least one second device having at least one second clock, the method comprising:
- receiving (<NUM>), by the at least one second device, at least one first message from the first device, wherein information regarding a time of departure of the at least one first message is in the at least one first message;
- determining (<NUM>), by the at least one second device, a time of arrival of the at least one first message;
- receiving (<NUM>), by the at least one second device, at least one second message from the first device, wherein information regarding a time of departure of the at least one second message is in the at least one second message;
- determining (<NUM>), by the at least one second device, a time of arrival of the at least one second message; and
- determining (<NUM>) a clock frequency offset between the first clock and the at least one second clock based on the time of departure of the at least one first message, the time of arrival of the at least one first message, the time of departure of the at least one second message, and the time of arrival of the at least one second message, wherein the clock frequency offset between the first clock and the at least one second clock is determined according to the following formula: <MAT>
- wherein: <MAT> is the clock frequency offset between the first clock and the at least one second clock, <MAT> is the time of arrival of the at least one second message, <MAT> is the time of arrival of the at least one first message, <MAT> is the time of departure of the at least one second message, and <MAT> is the time of departure of the at least one first message.