Method for synchronizing reference clock time in wireless network

Abstract

The invention relates to a method of reference clock time synchronization in a wireless network, comprising: performing a connection time synchronization process to synchronize with a central device reference clock based on an established connection to a central device; receiving a callback message for time synchronization from the central device at the start and / or end of the connection interval, including a central device reference time corresponding to a local reference time of the central device at a point in time at which the callback message is sent to the peripheral device; calculating the number of past connection intervals between the transmission of the callback message by the central device and the reception of the callback message by the peripheral device, the reception of the callback message by the peripheral device being represented by the recorded local peripheral device reference time; determining a current time drift between receiving the reference time of the central device and recording the reference time of the local peripheral device based on the calculated number of connection intervals; the peripheral device reference time is adjusted to synchronize with the central device local reference clock based on the time offset according to the connected time synchronization process and based on the determined time drift.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for reference clock time synchronization in a wireless network. Furthermore, the invention relates to a peripheral device, a central device, a system and a computer program for this purpose. BACKGROUND

[0002] Bluetooth Low Energy (BLE) is a wireless communication technology introduced as part of the Bluetooth 4.0 standard. It was developed to enable low-power connections between devices at short range. BLE is particularly well suited for applications that require intermittent data exchange and have strict power consumption requirements. BLE works in the same 2.4 GHz band as traditional Bluetooth, but consumes significantly less energy. This makes it ideal for a wide range of devices, including wearable fitness trackers, smartwatches, medical devices, and IoT (Internet of Things) devices. BLE has a simpler and more streamlined communication protocol than traditional Bluetooth, enabling faster and more reliable data transfer. It also supports the concept of low-latency connections, meaning that devices can quickly establish and maintain connections, minimizing delay and ensuring a seamless user experience.

[0003] One inherent limitation of the BLE protocol is that the transmission time of a data packet is usually unknown. This is a result of the software implementation itself, but also due to timing differences of individual devices caused by technical limitations such as crystal oscillator precision. BLE uses the 2.4 GHz band, which is divided into 40 individual channels. When many BLE (or Wi-Fi) devices communicate at the same time, in order to avoid congestion and packet delays, frequency hopping is applied, i.e. the frequency band is continuously changed. If the frequency hopping is not sufficient or other influences, such as too low signal strength, cause the data packet not to be sent, the BLE protocol applies so-called retries, then it will try to resend the packet repeatedly until it has been successfully received. Since the number of retries is unknown and due to the aforementioned limitations, the propagation time is also unknown. The transmission of a packet occurs during a specified transmission window (connection interval), in which the connected devices can listen or send packets, followed by a transmission-free period. If the packet transmission requires too many retries and the end of the current transmission window is reached, it can happen that the transmission of said packet will be stopped until the beginning of the next transmission window, which increases the uncertainty of the propagation time. The current state of the art does not provide any stable, long-term time synchronization possibility based on the standard BLE protocol. SUMMARY

[0004] According to an aspect of the present application, there is provided a method having the features of claim 1, a computer program having the features of claim 7, a peripheral device having the features of claim 8, a further method having the features of claim 9 and a central device having the features of claim 10. Further features and details of the present application are disclosed in the respective dependent claims, the description and the drawings. Features and details described in the context of the method also correspond to the computer program of the present application, the data processing apparatus of the present application and the computer-readable storage medium of the present application, and vice versa in each case.

[0005] According to an aspect of the present application, there is provided a method for reference clock time synchronization in a wireless network, wherein the wireless network is a Bluetooth Low Energy wireless network. The method comprises the following steps performed by a peripheral device of the wireless network:

[0006] In a first step, a connection or initial time synchronization procedure is performed based on an established connection to a central device for synchronizing the peripheral device reference clock with the central device reference clock. This results in an initial synchronization of the peripheral device reference clock with the central device reference clock. The differences in the crystal oscillators of each device will advantageously degrade this initial synchronization quickly.

[0007] In a next step, a callback message for time synchronization is received from the central device at the beginning and / or end of the connection interval, wherein the callback message comprises a central device reference time corresponding to the local reference time of the central device at the point in time when the callback message is sent to the peripheral device. This has the effect that the propagation time of the callback message between the central device and the at least one peripheral device is synchronized with the connection interval (CI), which advantageously enables the calculation of the time of the time synchronization. The callback message can be a notification or response sent by the peripheral device to the central device (like a smartphone, tablet or computer) to inform it about a certain event or state change. For example, a callback message can be utilized in the case of a certain event that can occur on the peripheral device and needs to be notified to the central device. The event can be anything from a sensor reading reaching a certain threshold to a button press on a remote control device. The callback message can contain information about the event such as the type of event and any relevant data.

[0008] The connection interval defines the duration between the start of two consecutive connection events between the central device and the peripheral device. During each connection event, data packets can be exchanged between the devices. Within each connection interval, one or more connection events occur. A connection event consists of specific phases, including the connection event itself, data exchange, and any idle time. The number of connection events per connection interval depends on the connection parameters negotiated between the devices. Furthermore, the choice of connection interval can depend on the specific requirements of the application. For real-time applications requiring low latency, a shorter connection interval can be preferred to ensure timely data exchange. In contrast, an application prioritizing energy efficiency can choose a longer connection interval to minimize energy consumption.

[0009] In a next step, the number of connection intervals elapsed between the central device sending the call-back message and the peripheral device receiving the call-back message is calculated. Here, the point in time of receiving the call-back message is represented by the logged local peripheral device reference time.

[0010] In a next step, based on the calculated number of connection intervals, a current time drift between the received central device reference time and the logged local peripheral device reference time is determined.

[0011] This has the advantage that the correct propagation time of the call-back message between the peripheral device and the central device reference clock can be determined with high precision, thereby effectively compensating the drift.

[0012] In a next step, based on the time offset according to the connection time synchronization procedure and based on the determined time drift, the peripheral device reference time is adjusted to synchronize with the central device local reference clock. Adjusting the peripheral device reference time preferably comprises synchronizing the peripheral device reference time with the central device local reference clock.

[0013] The time synchronization procedure according to the present application allows to ensure more accurate and consistent time stamps on data transactions or events on a wireless network. Furthermore, this advantageously enables better data integrity and ensures reliability of data operations.

[0014] In a further example, the method comprises the following steps:

[0015] - upon receiving the call-back message, logging, in particular storing, the local peripheral device reference time in order to be able to calculate the number of connection intervals.

[0016] This allows to perform the necessary calculations regarding possible time differences and / or possible drifts between the peripheral device and the central device reference time much more efficiently.

[0017] In an alternative example, the method comprises the following steps:

[0018] - establishing a connection to a central device of the wireless network.

[0019] This allows the peripheral device to exchange data with the central device after the connection has been established.

[0020] In an alternative example, the call-back message is received periodically, wherein the time period in which the call-back message is configured to be received is configured such that the maximum time drift is less than half the connection interval.

[0021] This has the advantage that the central device can more efficiently synchronize its operation with the peripheral device. Furthermore, the periodic call-back message allows a consistent communication rhythm to be maintained between the central and peripheral devices. This consistency improves the reliability of the data exchange by reducing the likelihood of lost or delayed messages, resulting in a more robust communication link. Furthermore, configuring the time period in which the call-back message is configured to be received to minimize the time drift ensures efficient power management. By aligning the call-back message interval with the connection interval, both devices can optimize their power usage, maximizing battery life without sacrificing responsiveness.

[0022] In an alternative example, during reception of the call-back message, the method comprises the further steps of:

[0023] receiving a call-back message sent by the central device in response to receiving the call-back from the peripheral device to obtain a propagation time of the call-back message synchronized with the connection interval.

[0024] In particular, the received call-back message is sent by the central device in response to receiving the central data reception call-back to obtain a propagation time of the call-back message synchronized with the connection interval. Here, the call-back message is sent directly or immediately or without delay in response to receiving the central data reception call-back.

[0025] This has the advantage that the propagation time of the sent call-back message can be determined more easily and more quickly due to the synchronization with the connection interval.

[0026] In an alternative example, during the execution of the connection time synchronization procedure, the method comprises the further steps of:

[0027] - recording a local connection peripheral device reference time when the connection to the central device has been established,

[0028] - receiving a call-back message for time synchronization from the central device, wherein the call-back message comprises a connection central device reference time, which corresponds to a local reference time of the central device when the connection to the at least one peripheral device has been established,

[0029] - calculating a time difference between the received connection central device reference time and the recorded local connection peripheral device reference time as a basic time offset between the central and peripheral device clocks to synchronize the central and peripheral device reference times.

[0030] This allows for a more accurate way of transmitting timestamps on data transactions or events over a wireless network in an advantageous manner. Furthermore, the time synchronization, in particular the initial synchronization based on an established connection between devices, simplifies the scalability of the system by providing a standardized time reference which can be easily maintained and scaled on a growing number of devices in the wireless network.

[0031] In another aspect of the present application, a computer program, in particular a computer program product, comprising instructions which, when the computer program is executed by a computer of a peripheral device according to the present application, cause the computer to carry out the method according to the present application can be provided. Thus, the computer program according to the present application can have the same advantages as already described in detail with reference to the method according to the present application.

[0032] In another aspect of the present application, a peripheral device for a wireless network, preferably for a Bluetooth Low Energy wireless network, can be provided, wherein the peripheral device comprises means for carrying out the method according to the present application. As a peripheral device, for example, a computer executing the computer program according to the present application can be provided. The computer can comprise at least one processor which can be used to execute the computer program. Furthermore, a non-volatile data storage can be provided, wherein the computer program can be stored and from which the processor can read the computer program for execution. Thus, the peripheral device according to the present application can have the same advantages as already described in detail with reference to the method according to the present application.

[0033] According to another aspect of the present application, a further method for reference clock time synchronization in a wireless network is provided, which is carried out by a central device of the wireless network. The wireless network is preferably a Bluetooth Low Energy (BLE) wireless network. The method comprises the following steps:

[0034] - establishing a connection to at least one peripheral device in the wireless network,

[0035] - sending a callback message for time synchronization to the at least one peripheral device, wherein the callback message comprises a central device reference time, which corresponds to a local reference time of the central device at the time of establishing the connection to the at least one peripheral device,

[0036] - sending a further callback message for time synchronization from the central device at the beginning and / or at the end of a connection interval in dependence on receiving a callback message from the at least one peripheral device, wherein each further callback message comprises a further central device reference time, which corresponds to a local reference time of the central device at the point in time of sending the further callback message to the at least one peripheral device.

[0037] Hence, the further method according to the application can have the same advantages as already described in detail with reference to the method according to the application.

[0038] According to a further aspect of the application, a central device is provided which is connectable to at least one peripheral device in a wireless network, preferably a Bluetooth Low Energy wireless network. The central device comprises means for performing the method according to the application. Hence, the central device according to the application can have the same advantages as already described in detail with reference to the method according to the application.

[0039] According to a further aspect of the application, a system is provided which comprises a peripheral device and a central device which is connectable or connected to the peripheral device in a wireless network, preferably a Bluetooth Low Energy wireless network.

[0040] According to a further aspect of the application, a computer readable storage medium can be provided which comprises a computer program and / or instructions according to the application which, when executed by a computer, cause the computer to perform the steps of the method according to the application. The storage medium can be formed as a data storage device such as a hard disk and / or a non-volatile memory and / or a memory card and / or a solid state drive. For example, the storage medium can be integrated into the computer.

[0041] Further, the method according to the application can be implemented as a computer implemented method. BRIEF DESCRIPTION OF DRAWINGS

[0042] Further advantages, features and details of the application will become apparent from the following description, in which embodiments of the application are described in more detail with reference to the drawings. In this case, features mentioned in the claims and the description can be essential to the application, both individually and in any combination. Shown are:

[0043] Figure 1 is a method, a computer program, a storage medium and a peripheral device according to embodiments of the application;

[0044] Figure 2 is a schematic overview of a wireless network comprising devices according to embodiments of the application,

[0045] Figure 3 is a schematic process flow of time synchronization according to embodiments of the embodiments, and

[0046] Figure 4 is a schematic diagram regarding drift of a reference time TC, TP according to embodiments of the application. DETAILED DESCRIPTION

[0047] The core of the present invention is based on the use of a Bluetooth Low Energy standard (BLE) connection callback method to create a high-precision time synchronization of about 1 millisecond between a central device and at least one peripheral device in a wireless BLE network. Possible drifts between such devices can be adjusted by sending a callback message (connection message) and / or a periodic callback message (ping message) from the central device 10 to the at least one peripheral device 20, 20a, 20b, 20c, 20n. The drift can be a direct consequence of the inaccuracy of the internal clock of each peripheral device 20, 20a, 20b, 20c, 20n, usually expressed in ppm. Moreover, the method according to the present invention is able to calculate a propagation time which allows to maintain the initial time synchronization. Furthermore, two different types of callback messages (connection and ping messages) can be used to perform the time synchronization. Each peripheral device 20, 20a, 20b, 20c, 20n will be time synchronized with the central device reference clock.

[0048] Figure 1 A schematic diagram of a method according to an embodiment of the present invention is depicted. In particular, Figure 1 A method 100 for reference clock time synchronization in a wireless network 1 is shown. The wireless network 1 can be a Bluetooth Low Energy, BLE, wireless network. The method 100 comprises the following steps performed by a peripheral device 20, 20a, 20b, 20c, 20n in the wireless network 1 :

[0049] In step 101, based on an established connection to the central device 10, a connection time synchronization procedure is performed to synchronize with the central device reference clock. In the next step 102, at the beginning and / or at the end of a connection interval, a callback message for time synchronization is received from the central device 10, wherein the callback message comprises a central device reference time which corresponds to the local reference time of the central device 10 at the point in time when the callback message is sent to the peripheral device 20, 20a, 20b, 20c, 20n.

[0050] In step 103, the number of connection intervals elapsed between the sending of the callback message by the central device 10 and the reception of the callback message by the peripheral device 20, 20a, 20b, 20c, 20n is calculated, wherein the point in time of the reception of the callback message is represented by the recorded local peripheral device reference time.

[0051] In step 104, based on the calculated number of connection intervals, a current time drift between the received central device reference time and the recorded local peripheral device reference time is determined. Then, in step 105, based on the time offset according to the connection time synchronization procedure and based on the determined time drift, the peripheral device reference time is adjusted to be synchronized with the central device local reference clock.

[0052] Furthermore, Figure 1A peripheral device 20 comprising a computer readable storage medium 25 is depicted. The storage medium 25 comprises a computer program 50.

[0053] Figure 2 A schematic overview of a wireless network comprising a device according to an embodiment of the application is depicted. The wireless network 1 can be a Bluetooth wireless network, in particular a Bluetooth Low Energy (BLE) wireless network 1. The wireless network 1 comprises a central device 10 and several peripheral devices 20, 20a, 20b, 20c, 20n. The central device 10 can be communicatively connected to one or more peripheral devices 20, 20a, 20b, 20c, 20n.

[0054] The peripheral devices 20, 20a, 20b, 20c, 20n for a Bluetooth Low Energy (BLE) wireless network can be, for example, devices that collect data or perform specific functions and transmit that data to the central device 10, such as a smartphone or a gateway. The peripheral devices can be fitness trackers, such as heart rate monitors, step counters, and activity trackers that collect health and fitness data, or medical devices, such as blood glucose monitors, blood pressure monitors, and pulse oximeters that collect and transmit health-related data, or smart home sensors for monitoring temperature, humidity, light levels, and other environmental factors in a smart home system, or beacons, which are small devices that broadcast signals to nearby smartphones or other devices for use in location-based services, such as indoor navigation or proximity marketing.

[0055] Figure 3 A schematic process flow of time synchronization according to embodiments of the embodiments is shown. In particular, a time synchronization process flow is depicted. The process can be split into an initial time synchronization process 310 and a continuous time synchronization process 315, 320 to compensate 305 for possible time drift in the peripheral device clock.

[0056] Figure 4A schematic diagram is shown with respect to the drift of the reference time TC, TP between the central device 10 and at least one peripheral device 20, 20a, 20b, 20c, 20n. When the central device 10 and one or more peripheral devices 20, 20a, 20b, 20c, 20n are connected 301 to each other, a respective callback message, such as e.g. a connection callback or connection message, will be generated from the BLE stack on each device 10, 20, 20a, 20b, 20c, 20n. In the connection callback message, the actual reference time TC, TP can be included and stored as a respective set value on the central device 10 and the one or more peripheral devices 20, 20a, 20b, 20c, 20n. The central device 10 sends 302 a callback message or connection message to each peripheral device 20, 20a, 20b, 20c, 20n, including its set value, which is the central reference set time TC. When each peripheral device 20, 20a, 20b, 20c, 20n receives this central reference time, an initial time synchronization 310 of high precision, e.g. about 1 ms, can be performed and this initial time synchronization 310 can be used, e.g. to time stamp transmitted device data.

[0057] On each peripheral device 20, 20a, 20b, 20c, 20n, a synchronization-based all-time adjustment is performed to synchronize the peripheral device reference clock time with the central device reference clock time.

[0058] They can be calculated according to the following formula:

[0059] (1) tC= tP+ (Tc- TP) = tP+ AT.

[0060] Furthermore, the time stamping in the central device 10 can be done without any time adjustment. In an ideal case, the initial time synchronization 310 can be valid and can last forever. However, in practice, the reference clocks in the peripheral devices 20, 20a, 20b, 20c, 20n and the central device 10 can drift with respect to each other. Therefore, the precision of the time synchronization can deteriorate quickly.

[0061] Therefore, when calculating and synchronizing the drift, the following can be considered, as shown in Figure 3 and Figure 4

[0062] (2) tC= tP+ AT+ drift.

[0063] ​For continuous time synchronization 315, 320, the central device 10 can send a 303b callback message or verification message to each connected peripheral device 20, 20a, 20b, 20c, 20n. These messages should be sent periodically. The period of this message should be configured such that the maximum clock drift is much less than half a connection interval (CI) 303a. This means: drift << CI / 2 (Requirement 1). The callback message used for time synchronization may include the central reference time tCs at which the central device 10 sends the callback message or verification 303b. The callback message or verification should be sent directly from the central data receive callback from the connected peripheral devices 20, 20a, 20b, 20c, 20n to obtain the propagation time synchronized with the connection interval (CI) 303a. (Requirement 2) When this is taken into account, the propagation time tping of the verification or callback message will be n times the connection interval, for example, tping = n * CI milliseconds.

[0064] like Figure 3 and Figure 4 As shown, after an unknown propagation time tping, peripheral devices 20, 20a, 20b, 20c, and 20n can receive the 303b callback message at their local peripheral time tPr. Whenever peripheral devices 20, 20a, 20b, 20c, and 20n receive a callback message or verification message including the central reference time tcs from the central device 10, each peripheral device 20, 20a, 20b, 20c, and 20n records or stores the reception time (tPr) of the callback message or verification message. Then, the estimated peripheral time tPs for the central device 10 to send the 303 callback message is calculated. ～ Without drifting, such as

[0065] (3)tPs ～ =TP + (tCs - TC)

[0066] To update or adjust the settings TP and TC of time synchronization 320, calculate the actual peripheral verification time tPs, including drift, for 304 and 305 according to the following formula:

[0067] (4) tPs=tPs ～ +Drift

[0068] (5) tPs=tPr-tPing

[0069] like Figure 4 As shown, both the drift and propagation time tping are unknown. However, by considering requirements 1 and 2 above, the actual peripheral reference time can be calculated as follows, since tping = n * CI

[0070] (6) tPs=tPs ～ +Drift = tPr - n * CI

[0071] In case of requiring a drift < < CI / 2, the lower limit of n is known from

[0072] (7) n > (tPr-tPs ～ CI / 2) * 1 / CI where

[0073] Then tPs can be calculated 320, 304. Furthermore, the (local) central reference time tCs and the (local) peripheral reference time tPs can be used as new respective set values, TP = tPs and TC = tCs. Furthermore, the successive high-precision time adjustment (~1 ms) can be performed again according to equation (1) above, but using the updated set values TC and TP, until the next synchronization event, where the set values will be updated or adjusted again.

[0074] The above explanations of embodiments describe the application in the context of examples. Of course, the individual features of the embodiments can be freely combined with each other, as far as technically reasonable, without departing from the scope of the application.

Claims

1. A method (100) for reference clock time synchronization in a wireless network (1), wherein the wireless network (1) is preferably a Bluetooth Low Energy (BLE) wireless network, wherein the method (100) includes the following steps performed by peripheral devices (20, 20a, 20b, 20c, 20n) of the wireless network (1): - Based on the established connection to the central device (10), perform the (101) connection time synchronization process to synchronize with the central device reference clock. - At the beginning and / or end of the connection interval, receive (102) a callback message for time synchronization from the central device (10), wherein the callback message includes a central device reference time, which corresponds to the local reference time of the central device (10) at the time when the callback message is sent to the peripheral devices (20, 20a, 20b, 20c, 20n). -Calculate (103) the number of connection intervals that have elapsed between the central device (10) sending the callback message and the peripheral devices (20, 20a, 20b, 20c, 20n) receiving the callback message, wherein the reception of the callback message by the peripheral devices (20, 20a, 20b, 20c, 20n) is represented by the recorded local peripheral device reference time. -Based on the number of calculated connection intervals, determine the current time drift between the received central device reference time and the recorded local peripheral device reference time (104). - Adjust the peripheral device reference time (105) to synchronize with the central device local reference clock based on the time offset of the connection time synchronization process and the determined time drift.

2. The method (100) of claim 1, Its features are, The method includes the following steps: - When a callback message is received, log it, especially storing the local peripheral device reference time, so that the number of connection intervals can be calculated.

3. The method (100) according to any one of the preceding claims, Its features are, The method includes the following steps: - Establish a connection to the central device (10) of the wireless network.

4. The method (100) according to any one of the preceding claims, Its features are, Callback messages are received periodically, with a time period configured for receiving them such that the maximum time drift is less than half a connection interval.

5. The method (100) according to any one of the preceding claims, Its features are, During the receipt of the callback message, the method includes the following further steps: - Receive callback messages sent by the central device in response to receiving callbacks from peripheral devices, to obtain the propagation time of the callback messages synchronized with the connection interval.

6. The method (100) according to any one of the preceding claims, Its features are, During the connection time synchronization process, the method includes the following further steps: - Once a connection to the central device is established, record the reference time of the locally connected peripheral device. - Receive a callback message for time synchronization from the central device, wherein the callback message includes a connection central device reference time, which corresponds to the local reference time of the central device when a connection to at least one peripheral device has been established. - Calculate the time difference between the received connection central device reference time and the recorded local connection peripheral device reference time as the base time offset between the central and peripheral device clocks to synchronize the central and peripheral device reference times.

7. A computer program (50) comprising instructions that, when executed by a computer (21) of a peripheral device (20), cause the computer (21) to perform the method (100) of any of the preceding claims.

8. A peripheral device (20, 20a, 20b, 20c, 20n) for a wireless network, wherein the wireless network is preferably a Bluetooth Low Energy (BLE) wireless network, and wherein the peripheral device includes means for performing the method (100) of any one of claims 1 to 6.

9. A method (200) for reference clock time synchronization in a wireless network (1), wherein the wireless network (1) is preferably a Bluetooth Low Energy (BLE) wireless network, wherein the method (200) includes the following steps performed by a central device (10) of the wireless network (1): - Establish a connection to at least one peripheral device (20, 20a, 20b, 20c, 20n) in the wireless network (1), - A callback message for time synchronization is sent to the at least one peripheral device (20, 20a, 20b, 20c, 20n), wherein the callback message includes a central device reference time, which corresponds to the local reference time of the central device (10) at the time the connection to the at least one peripheral device (20, 20a, 20b, 20c, 20n) is established. - Based on the callback message received from at least one peripheral device (20, 20a, 20b, 20c, 20n), additional callback messages for time synchronization are sent from the central device (10) at the beginning and / or end of the connection interval, wherein each additional callback message includes an additional central device reference time corresponding to the local reference time of the central device (10) at the time when the additional callback message is sent to at least one peripheral device (20, 20a, 20b, 20c, 20n).

10. A central device (10) connected to at least one peripheral device (20, 20a, 20b, 20c, 20n) in a wireless network (1), wherein the wireless network (1) is preferably a Bluetooth Low Energy (BLE) wireless network, wherein the central device (10) includes means for performing the method (200) of claim 9.

11. A system comprising the peripheral devices (20, 20a, 20b, 20c, 20n) of claim 8 and the central device (10) of claim 10, the central device (10) being connectable to or connected to the peripheral devices (20, 20a, 20b, 20c, 20n) in a wireless network (1), wherein the wireless network (1) is preferably a Bluetooth Low Energy (BLE) wireless network.

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