WO2009024945A2

WO2009024945A2 - Synchronisation method

Info

Publication number: WO2009024945A2
Application number: PCT/IB2008/053361
Authority: WO
Inventors: Hans-Juergen Reumerman; Yunpeng Zang; Lothar Stibor; Bernhard Walke
Original assignee: Philips Intellectual Property & Standard Gmbh; Koninklijke Philips Electronics N.V
Priority date: 2007-08-22
Filing date: 2008-08-21
Publication date: 2009-02-26
Also published as: TW200926660A; WO2009024945A3

Abstract

The present invention relates to a method of synchronising a first communication device with a second communication device in a radio communication system, where the first and second communication devices comprise local clocks for obtaining local time signals. In the method the first communication device first receives (509) periodically a beacon signal from the second communication device, the beacon signal comprising a time signal of the second communication device and the quality of the time signal. Then the first communication device determines (512) quality of the local clock time signal of the first communication device, and determines (513) whether the received time signal is of higher quality than the local time signal of the first communication device, and in case the received time signal is of higher quality, then corrects the accuracy of the local clock of the first communication device.

Description

SYNCHRONISATION METHOD

TECHNICAL FIELD

The present invention relates to a method of time synchronisation in communication systems. More specifically the invention relates to a method of providing a distributed method of obtaining a global time synchronisation in a communication device by utilising a beacon signal from another communication device. The invention also relates to a corresponding computer program product and communication device.

BACKGROUND OF THE INVENTION Many services running on modern digital communication networks require accurate synchronisation for correct operation. Communication networks rely on the use of highly accurate primary reference clocks which are distributed network wide using synchronisation links and synchronisation supply units. Modern communication networks use highly accurate primary reference clocks that must meet the international standards requirement for long term frequency accuracy better than 1 part in 10¹¹. To achieve this performance, systems based on atomic clocks or receivers using global positioning system (GPS) are normally used. Thus, time synchronisation is the basis of any time division multiple access (TDMA) system, which is recognised to be more efficient than the unsynchronised system in terms of efficiency and quality of service (QoS) support.

An example of a communication system, where time synchronisation is particularly difficult to implement properly is vehicular ad-hoc network (VANET) defined by the IEEE 1609 standard. VANET is a wireless ad-hoc communication network designed for stations with high mobility. In VANET it is required that each station has a globally synchronised time basis for coordinating the channel access. The globally synchronised time basis is important for vehicular communication systems, which are intended for enhancing the human safety during driving, as every station in the network has to know when it shall listen to the safety channel for critical danger warning messages and when it may leave the safety channel for other non- safety services. Failed time synchronisation in such systems may prevent the station from receiving the important danger warning messages in time and cause dangerous situations. For vehicular communication systems using the TDMA channel scheme, the global time synchronisation, e.g. synchronised to the coordinated universal time (UTC) time, is important.

Time synchronisation in communication systems is a task that needs to be considered carefully, especially if precise time synchronisation is required. This is especially true in high mobility ad-hoc networks, such as VANET, where no locally centralised solution is considered to be feasible for providing the required global time synchronisation in vehicular environment.

The synchronisation problem in wireless ad-hoc networks has been addressed by previous works. For instance, the WiMedia system described in ECMA International, High Rate Ultra Wideband PHY and MAC Standard, Standard ECMA-368, 1 st Edition, December 2005 achieves and maintains the local time synchronisation by periodically exchanging beacons among neighbouring stations in a distributed way. However, the solution in WiMedia system can only provide local synchronisation, but not global time synchronisation. The time synchronisation functions from GPS and Galileo systems are the preferred solutions for synchronising the local clock to the global time reference, e.g. UTC, with sufficient precision among all vehicle stations. The GPS system can provide 1 pulse per second (pps) time synchronisation signal with a precision of 200ns, which is sufficient for current communication systems. However, the availability of GPS signal can not be guaranteed in all locations. For instance vehicle stations may lose the time synchronisation when they drive into a tunnel or a building, where the satellite signal can not be received anymore. Without frequently receiving the global time reference information, the local clock at each station may drift from the global time reference as time goes by. The offset of the local clock from the global time reference depends on the elapsed time since last synchronisation and the local clock skew, which describes the frequency shift of the local clock. Therefore, a scheme is needed to maintain the global synchronisation achieved through the precise time reference in case this precise time reference is not available for some or all of the stations.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is proposed a time synchronisation method as recited in claim 1.

Thus, the present invention provides a method of maintaining the global time synchronisation achieved based on the global time references from satellite positioning systems, such as GPS or Galileo, when the global time reference is no longer available for some or all of the devices.

According to a second aspect of the invention there is provided a computer program product comprising instructions for implementing the method according the first aspect of the invention when loaded and run on computer means of the first communication device. According to a third aspect of the invention there is provided a communication device arranged for implementing the method according to a first aspect of the present invention as recited in claim 7.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which:

- FIG.1 shows the control and service channels of IEEE 1609 along a timeline; - FIG.2 is a simplified block diagram of the communication device in accordance with an embodiment of the present invention; - FIG.3 is a simplified block diagram of a time synchronisation block in accordance with an embodiment of the present invention;

- FIG.4 shows one beacon period, comprised of beacon slots along a timeline; and - FIG.5 shows a flow chart in accordance with an embodiment of the present invention.

DETAILED DESRIPTION OF EMBODIMENTS OF THE INVENTION

In the following description some non-limiting exemplary embodiments of the invention will be described in more detail in the context of VANET operating in accordance with the IEEE 1609 standard. However, it is to be understood that the invention is not restricted to this environment, but the teachings of the invention are equally applicable in other communication systems employing time division multiple access (TDMA) scheme.

In FIG.1 there are shown two channel intervals of the IEEE 1609 system, namely a control channel (CCH) interval and a service channel (SCH) interval. These intervals are globally synchronised. In the CCH interval every station has to stay on the control channel for the purpose of exchanging critical danger warning messages, while in the SCH interval stations can optionally switch to other service channels for performing non- safety applications, but before the starting time of the next CCH interval every station must switch back again to the control channel. Beacon signals are also transmitted on the control channel.

FIG.2 shows a simplified block diagram of a communication device 200, in this example a vehicular communication on board unit (OBU), where the teachings of the present invention can be applied. It is to be noted that FIG.2 only contains elements that are closely related to the present invention. Other conventional elements are not described in this context. The device 200 contains a transmission/reception antenna (TX/RX) 201 for transmitting and receiving data. A single antenna can be used for both the transmission and reception. In this case the antenna is used for vehicular communication. The TX/RX antenna 201 is connected to a transmission block 203 and a receiver block 205. These communication blocks take care of the transmission and reception of packets according to certain communication protocol, e.g., IEEE 802.1 1 p, which is used to implement the medium access (MAC) layer of the IEEE 1609 standard.

The device 200 further comprises a GPS antenna 207 and a GPS receiver 209, which is connected to the GPS antenna 207. These units are needed for receiving the GPS signal, which provides the global time reference. The GPS elements could equally be replaced with any other satellite positioning system elements. For instance, the system could be arranged to receive Galileo signals instead of GPS signals. It is also possible that the device 200 is arranged to receive signals from different satellite positioning systems. The device 200 also comprises a local clock 21 1 , which serves as a local oscillator providing the local time reference for all the blocks. Time offset and frequency skew are the main reason of system synchronisation problem. For correcting the clock skew of the local oscillator in local clock 21 1 , the device comprises a local clock skew correction block 213. Then there is also shown a time synchronisation block 215, which is the central block for implementing the synchronisation method in accordance with the present invention.

The structure of the synchronisation block 215 is shown in more detail in FIG.3. The time synchronisation block 215 comprises a beacon generator 301 for generating the beacon frame, which carries the information of local time quality. The beacon generator 301 is connected to the transmission block 203 through a transmission interface 303. The time synchronisation block 215 further contains a beacon analyser 305 for analysing the time information from the received beacon. The beacon analyser is connected to the receiver block through a receiver interface 307. The time synchronisation block 215 also contains a local time quality estimator 309 for tracking the time quality of the local clock. A synchronisation controller 31 1 makes the decision on adjusting the local clock according to the received beacon or GPS time information. The synchronisation controller 31 1 is connected to the GPS receiver 209, local clock skew correction 213, local clock 21 1 , local time quality estimator 309, beacon analyser 305 and beacon generator 301.

As a basis of the present invention, a distributed beaconing scheme is used. According to an embodiment of the present invention, all devices of the communication system follow the globally synchronised system structure, which includes a beacon period consisting of multiple beacon slots of equal length, as shown in FIG.4. A beacon is always transmitted at the start time of each beacon slot and carrying the slot number it used. Upon receiving a beacon frame one can derive the clock difference between the sender's and its own clocks by calculating and comparing a common time reference point, e.g. the beacon period start time. Propagation delay also affects timing uncertainty, but in a short-range network propagation delays are small and thus they can be ignored. The present invention provides a method for maintaining the global time synchronisation achieved by receiving the global time references from satellite positioning systems, such as GPS or Galileo systems, when the global time reference is no longer reachable by some or all of the devices. In accordance with the present invention, the idea is that in addition to the time synchronisation using the satellite positioning system signal, a "local" synchronisation algorithm based on the distributed beaconing scheme is employed in this method. The distributed synchronisation method forms a "local" time basis among the neighbouring devices and keeps the "local" time basis as close as possible to the global time reference. According to the present invention, the devices exchange the clock time quality information using a distributed beaconing scheme. A device 200 receiving a beacon, which has a higher clock time quality than its local one, adjusts its local clock and clock time quality according to the received beacon. The local clock time quality is determined by the time since last synchronisation action and the local clock skew. A higher time quality indicates a smaller offset from the global time reference, while a lower time quality indicates a bigger time offset from the global time reference. To further improve the time quality, each device applies the local clock skew correction depending on the received high precision global time reference. In accordance with this embodiment, all the devices in the mutual communication range will synchronise to the one who has the highest clock time quality, i.e. the closest clock to the global time reference, even if some of the devices cannot receive the satellite signal directly.

An embodiment of the invention is described now with reference to the flow chart of FIG.5. In step 501 the device 200 is in a "starting up" state and determines whether it has received a GPS signal or a valid beacon signal. If no signal has been received during a certain period of time, there is a scanning time out. In this case the device 200 sets the local time quality to the worst value and transits to an unsynchronised state. The device 200 then waits until a valid timing signal has been received. If on the other hand a valid timing signal is received in step 501 , then in step 503 the device 200 adjusts its local clock to correspond to the received timing signal and the timing quality is updated. Now the device 200 operates in a synchronised state. If the received signal is from the GPS, then the device 200 adjusts its local clock 21 1 according to the time signal from the GPS and resets the local time quality to the best value. If on the other hand the received signal is a beacon signal then the device 200 adjusts the local clock according the sender's clock and sets the time quality to the same value as indicated in the received beacon.

Next in step 505 it is determined whether a GPS signal is received. The GPS signal may be received once in a second and it provides a precision of 200 ns. If the GPS signal is received, then in step 507 the device 200 adjusts its local clock according to the timing signal from the GPS and updates its timing quality by setting the timing quality to the best possible value. The local clock skew is also corrected. Thus the device 200 remains in the synchronised state.

If on the other hand no GPS signal is received, then the procedure continues in step 509 by determining whether a beacon is received. Also from step 507 the method continues directly in step 509. If in step 509 it is determined that no beacon signal is received, then in step 515 the local clock is again adjusted and timing quality updated. If on the other hand in step 509 it is determined that a beacon is received, then in step 51 1 the device 200 determines whether the timing quality of the received beacon is valid. In this example the beacon is received periodically. The device 200 calculates the local clock value of the beacon sender by analysing the received beacon. If the timing quality is valid, then in step 512 the quality of the local clock of the receiving device 200 is determined and once this is done, it is determined in step 513 whether the received timing quality is greater than the timing quality of the local clock, i.e. whether the timing accuracy of the received beacon is of higher precision than the accuracy of the local clock of the receiving device 200. If this is the case, then in step 515 the local clock of the receiving device 200 is adjusted and the timing quality is updated. Thus, the receiving device 200 has to continuously listen to beacons from the neighbouring devices and it also has to a transmit beacon at a chosen beacon slot in every beacon period. The local time quality is carried by the beacon signal.

In step 517 a cumulative timing quality calculation is calculated. Also if in steps 51 1 or 513 the answer is negative, the procedure continues directly in step 517. For obtaining the cumulative timing quality, the device 200 has to keep track on the local timing quality at each local clock tick. The timing quality of the local clock deteriorates by the step of T_tιΛ x ClockSkew till the worst clock quality is reached unless the local clock 21 1 is adjusted. T_t,_Ck denotes the clock resolution and the ClockSkew is the frequency skew of the local oscillator. Next in step 519 it is determined whether the cumulative timing quality is below a threshold value. If this is the case the procedure continues in step 501 and the device 200 moves to unsynchronised state. On the other hand if the cumulative timing quality is still larger than the threshold, then the procedure continues in step 505 and the device 200 remains in the synchronised state.

In this embodiment, the frequency skew of the local oscillator is a parameter set by the manufacturer of the system, and which is statistically estimated as the worst case. This means that even if the clock skew of a considered system is far lower than the worst case (i.e. the clock quality is better than estimated), the quality indicator will drop quickly, and will not represent well the quality of the local clock. Moreover, the frequency skew value can also vary with the time, because of ageing of the crystal for instance, thus the frequency skew value could become underestimated.

Consequently, in a variant of the invention, it is proposed to estimate and update the local clock frequency skew. This can be done for instance by comparison of the local clock and the global time reference. Thus, a more accurate skew value of the local clock can be used as ClockSkew and the quality indicator using this ClockSkew value is thus closer to the reality and not underestimated. Then, this clock skew can be estimated periodically.

Another advantage of this estimated frequency skew value is that it can be used to create a compensation of the local clock skew, in order to correct the deviancy of the local clock. Indeed, the estimated frequency skew tells whether the local clock is slower or faster than the global time reference, and gives an estimation of the amount. Thus the local clock is periodically compensated with help of the estimated ClockSkew, towards the minimum frequency skew regarding the global time reference. When the global time signal is not available, the local clock is used, and its deviancy has been corrected with help of the estimated ClockSkew, simulating thus a more accurate local clock.

This can be implemented for example with help of a voltage controlled crystal oscillator (VCXO) connected to an error estimation circuit, which estimates the frequency skew value. Thus, the clock frequency can be adjusted with a voltage generated by a control circuit on the basis of a signal representative of the frequency skew value.

Above an embodiment of the invention was described. The invention thus provides a distributed beaconing algorithm by which each device periodically sends its beacon, each beacon carrying the local time quality describing the estimated local clock shift from the global time reference. Each device locally estimates its clock time quality according to the time elapsed since last synchronisation and the local clock skew. Alternatively the clock time quality is calculated by multiplying the time elapsed by the local clock skew.

The invention equally relates to a computer program product that is able to implement any of the method steps of the embodiments of the invention when loaded and run on computer means of the device 200. The method is implemented on each device and performs the synchronisation functionality in a distributed way. The invention equally relates to an integrated circuit that is arranged to perform any of the method steps in accordance with the embodiments of the invention.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not restricted to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

A computer program may be stored/distributed on a suitable medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope of the invention.

C L A I MS

1 . A method of synchronising a first communication device (200) with a second communication device in a radio communication system, where the first and second communication devices comprise local clocks (21 1 ) for obtaining local time signals, the method comprises the following steps performed by the first communication device (200):

- receiving (509) a beacon signal from the second communication device, the beacon signal comprising a time signal of the second communication device and the quality of the time signal;

- determining (512) quality of the local clock (21 1 ) time signal of the first communication device (200); and

- determining (513) whether the received time signal of the second communication device is of higher quality than the local time signal of the first communication device (200), and in case the received time signal is of higher quality, then correcting the accuracy of the local clock (21 1 ) of the first communication device (200).

2. The method according to claim 1 , wherein the method further comprises obtaining (505) a timing signal from a global reference and based on this signal, performing a synchronisation by correcting the accuracy of the local clock (21 1 ) of the first communication device (200) and updating the quality of the of local clock (21 1 ) signal of the first communication device (200).

3. The method according to claim 2, wherein the global reference is a satellite.

Claims

4. The method according to claim 2 or claim 3, wherein the method further comprises comparing the local clock with the timing signal from the global reference, and deducing an estimation of the local clock skew.

5. The method according to any of the preceding claims, wherein the method further comprises adjusting the local clock by means of the local clock skew.

6. The method according to any of the previous claims, wherein the quality of the time signal is determined based on the time elapsed since the last synchronisation and/or the clock skew.

7. The method according to any of the previous claims, wherein the method further comprises comparing (519) the quality of the time signal of the first communication device (200) to a predefined threshold value, and in case the quality of the time signal drops below the threshold value, the first communication device moves to an unsynchronised state.

8. The method according to any of the previous claims, wherein the method further comprises the first communication device (200) periodically transmits a beacon signal at a chosen beacon slot in every beacon period, wherein the beacon signal comprises a time signal of the first communication device (200) and the quality of the time signal.

9. A computer program product comprising instructions for implementing the steps of a method according to any one of claims 1 -8 when loaded and run on computer means of the first communication device (200).

10. A communication device (200) arranged to be synchronised with another communication device in a radio communication system, where the communication devices comprise local clocks (21 1 ) for obtaining local time signals, the communication device (200) comprises: - a receiver (201 , 205) for receiving a beacon signal from the other communication device, the beacon signal comprising a time signal of the other communication device and the quality of the time signal;

- a beacon analyser (305) for analysing the received beacon signal; - a local time quality estimator (309) for determining quality of the local clock time signal of the communication device (200); and

- a synchronisation controller (31 1 ) for determining whether the received time signal of the other communication device is of higher quality than the local time signal of the communication device (200), and in case the received time signal is of higher quality, then the synchronisation controller (31 1 ) is arranged to correct the accuracy of the local clock of the communication device (200).

1 1. The communication device (200) of claim 10, wherein the communication device (200) further comprises a beacon generator (301 ) and transmitter (201 , 203) for periodically transmitting a beacon signal at a chosen beacon slot in every beacon period, wherein the beacon signal comprises a time signal of the communication device (200) and the quality of the time signal.

12. The communication device (200) of any one of claims 10 or 1 1 , wherein the communication device (200) further comprises means (207, 209) for receiving satellite signals.