Synchronization of media access control (MAC) superframes

• US 20090010245A1
• Filed: 09/04/2008
• Published: 01/08/2009
• Est. Priority Date: 09/29/2005
• Status: Active Grant

• Current Assignee: Novanta Inc.
• Original Assignee: TZero Technologies Incorporated (Novanta Inc.)
• Inventors: Zhou, Xiaoming, Worfolk, Patrick
• Granted Patent: US 7,826,860 B2
• Time in Patent Office: Days
• US Class Current: 370/350
• CPC Class Codes: H04W 56/002 Mutual synchronization

Abstract

A device and method for of synchronizing a MAC superframe of a wireless device is disclosed. The wireless device can be located within a chain of a plurality of other wireless devices. The method includes receiving beacons from at least one other device during a superframe of the wireless device, determining a superframe offset for each of the other wireless devices based on timing of the received beacons, determining a corrective delay based on the superframe offsets, inserting the corrective delay within a current superframe of the wireless device, and inserting a predictive delay within the current superframe, the predictive delay being determined by an estimate of a difference between a frequency a clock of the wireless device and a frequency of a slowest clock of the other wireless devices within the chain.

Claims

1. A method of synchronizing a MAC superframe of a wireless device to a MAC superframe of at least one neighboring device, the wireless device being located within a plurality of other wireless devices, the method comprising:

receiving beacons from at least one other device during a superframe of the wireless device;

determining a corrective delay based on a time difference between an actual beacon reception time and an expected beacon reception time;

inserting the corrective delay within a current superframe of the wireless device;

determining a predictive delay by estimating a frequency difference between a frequency of a clock of the wireless device and a frequency of a slowest clock of the other wireless devices within the plurality of devices;

inserting the predictive delay within the current superframe.

2. The method of claim 1, further comprising the wireless device transmitting a beacon within the current superframe.

3. The method of claim 1, wherein a superframe offset of each other device is determined by an amount of time a beacon of the other device arrives at the wireless device later than expected.

4. The method of claim 1, further comprising the corrective delay being inserted within the current superframe as early as possible but after a beaconing period of the current superframe.

5. The method of claim 1, further comprising the predictive delay of a current superframe occurring prior to beacon transmission of a following superframe.

6. The method of claim 1, wherein the estimate of the frequency of the slowest clock comprises computing an average over time of the corrective delay.

7. The method of claim 6, wherein if the average exceeds a predetermined high-watermark threshold, then the predictive delay is increased.

8. The method of claim 7, wherein the high-water threshold is determined by a clock resolution of the wireless device.

9. The method of claim 7, wherein the predictive delay is incrementally increased by a clock resolution of the wireless device.

10. The method of claim 6, wherein if the average falls below a predetermined low-watermark threshold, then the predictive delay is decreased.

11. The method of claim 1, wherein if a beacon from another wireless device is missed, the wireless device determines a superframe offset of the other wireless device by averaging superframe offset of previous superframes of the wireless device.

12. A method of synchronizing MAC superframes of wireless devices located within a plurality of wireless devices, the method comprising:

each device transmitting a beacon;

each device receiving beacons from at least one other device during a superframe of the wireless device;

each device determining a superframe offset for each of the other wireless devices based on timing of the received beacons, wherein the superframe offset comprises a difference between an actual beacon reception time and an expected beacon reception time;

each device determining a corrective delay based on the superframe offsets;

each device inserting the corrective delay within a current superframe of the wireless device;

each device determining a predictive delay by estimating a difference between a frequency of a clock of the wireless device and a frequency of a slowest clock of the other wireless devices within the plurality of devices;

each device inserting the predictive delay within the current superframe.

13. A method of synchronizing a MAC superframe of a wireless device, the wireless device being located within a plurality of other wireless devices, the method comprising:

receiving beacons from at least one other device during a superframe of the wireless device;

determining a superframe offset for each of the other wireless devices based on timing of the received beacons;

determining a corrective delay based on the superframe offsets;

determining a predictive delay by estimating a difference between a frequency of a clock of the wireless device and a frequency of a slowest clock of the other wireless devices within the plurality of devices, wherein the estimate of the frequency of the slowest clock comprises computing an average over time of the corrective delay;

inserting the predictive delay within the current superframe.

14. The method of claim 13, further comprising inserting the predictive delay of a current superframe prior to beacon transmission of a following superframe.

15. The method of claim 13, wherein the estimate of the frequency of the slowest clock comprises computing an average over time of the corrective delay.

16. The method of claim 15, wherein if the average exceeds a predetermined high-watermark threshold, then the predictive delay is increased.

17. The method of claim 16, wherein the predetermined high-water threshold is determined by a clock resolution of the wireless device.

18. The method of claim 16, wherein the predictive delay is incrementally increased by a clock resolution of the wireless device.

19. The method of claim 15, wherein if the average falls below a predetermined low-watermark threshold, then the predictive delay is decreased.

20. The method of claim 13, wherein if a beacon from another wireless device is missed, the wireless device determines the superframe offset of the other wireless device by averaging superframe offset of previous superframes of the wireless device.