Timing Synchronization in Wireless Mesh Networks

• US 20100202436A1
• Filed: 04/17/2010
• Published: 08/12/2010
• Est. Priority Date: 04/22/2005
• Status: Active Grant

• Current Assignee: Zenner Performance Meters, Inc. (Zenner International GmbH & Co. KG)
• Original Assignee: Axiometric, LLC
• Inventors: Martinez, John V., Albert, David A.
• Granted Patent: US 8,351,409 B2
• Time in Patent Office: Days
• US Class Current: 370/350
• CPC Class Codes: H04J 3/0655 using timestamps
• CPC Class Codes: H04W 56/002 Mutual synchronization
• CPC Class Codes: H04W 56/0075 based on arrival time vs. expected arrival time
• CPC Class Codes: H04W 84/18 Self-organising networks, e.g. ad-hoc networks or sensor networks
• CPC Class Codes: H04W 92/18 between terminal devices

Abstract

A system for synchronizing time amongst a plurality of wireless network devices in a wireless network is provided, where a message is exchanged between a transmitting wireless device and a receiving wireless device. Such a system includes: a controller that inserts synchronized time information in the message, prior to transmitting the message, the synchronized time information comprising a current time value and a synchronization time value; and a processor that determines a local elapsed time value since last receiving a last synchronized time.

Claims

1. A system of coordinating communication across a wireless network comprising a plurality of electronic devices operatively connected to said wireless network, each electronic device of said plurality of electronic devices comprising:

a wireless transceiver transmitting and receiving messages;

a central processing unit coupled and operatively in communication with said wireless transceiver;

a memory unit coupled and operatively in communication with said central processing unit; and

a timer coupled and operatively in communication with said central processing unit, said timer comprising an oscillator measuring an amount of elapsed time;

wherein a first electronic device of said electronic devices periodically transmits a master timing message comprising a current time as measured and maintained by a first timer,

wherein upon receiving said master timing message, a second electronic device of said electronic devices stores said received current time as synchronization time in a second memory unit and synchronizes a second timer with said received current time,

wherein said second electronic device transmits a synchronized timing message comprising said stored synchronization time, and a second current time, as determined by said second timer,

wherein upon receiving said synchronized timing message, a third electronic device of said electronic devices compares said received synchronization time with a third synchronization time stored in a third memory unit and replaces said stored third synchronization time with said received synchronization time and synchronizes a third timer with said received current time only when said received synchronization time is newer than said stored third synchronization time.

2. The system of claim 1,

wherein a fourth electronic device of said plurality of electronic devices, upon receiving a first message comprising at least one of a synchronized timing message containing a newer synchronization time and a master timing message, and subsequently receiving a second message comprising at least one of a synchronized timing message comprising a newer synchronization time and a master timing message, said fourth electronic device calculates an elapsed time as being the time elapsed between receiving the first message and the second message,

wherein said fourth electronic device calculates a timing error as being a difference between the current time in said received second message and the current time as maintained by a fourth timer prior to synchronization with the current time in said second message,

wherein said fourth electronic device calculates a timer drift rate as being a ratio of said timing error divided by said elapsed time and stores said drift rate in a fourth memory unit, and

wherein said fourth electronic device periodically adjusts said fourth timer by advancing or retracting said fourth timer by an amount equal to said stored drift rate multiplied by the time elapsed since said fourth timer was last adjusted or synchronized.

3. The system of claim 2, wherein said fourth electronic device stores a drift rate threshold and a lock switch in said memory unit and when said calculated drift rate is less than said drift rate threshold, said fourth electronic device sets said lock switch in said memory unit and applies said calculated drift rate towards adjustment of said fourth timer, otherwise when said calculated drift rate is at least as great as said drift rate threshold, said fourth electronic device resets said lock switch in said memory unit and applies a fraction of said calculated drift rate to the adjustment of said fourth timer.

4. The system of claim 1, wherein a fifth electronic device of said plurality of electronic devices further comprises an accuracy timer coupled and operatively in communication with said central processing unit, said accuracy timer comprising an accuracy oscillator measuring an accuracy elapsed time.

5. The system of claim 4, wherein said fifth electronic device calculates a drift rate of a fifth timer relative to said accuracy timer and stores said drift rate as an estimated drift rate in a fifth memory unit.

6. The system of claim 5, wherein said fifth electronic device periodically advances or retards said fifth timer to compensate for said drift rate stored in said fifth memory unit.

7. The system of claim 2, wherein a fifth electronic device of said electronic devices further comprises a temperature sensor coupled and operatively in communication with said central processing unit, said temperature sensor measuring an ambient temperature, and

wherein said fifth electronic device periodically retrieves a temperature drift rate, corresponding to said ambient temperature as measured by said temperature sensor, from a drift rate table stored in a fifth memory unit and advances or retards said timer by said temperature drift rate.

8. The system of claim 7,

wherein said drift rate table comprises initial values and said initial values are calculated using at least one of a heuristic value, said ambient temperature measurement, and a difference between a first frequency and a second frequency, and

wherein said heuristic value comprises a predetermined drift rate calculation of said timer at said ambient temperature measurement.

9. The system of claim 7, wherein said temperature drift rate stored in said memory is periodically updated by replacing an existing value with a new value calculated as a function of said stored temperature drift rate and said calculated timer drift rate.

10. The system of claim 2, wherein said second electronic device further comprises:

an auxiliary oscillator,

wherein when said adjusted drift rate of said second device synchronized hardware timer is determined to be less than a drift tolerance of said auxiliary oscillator, a measured frequency of said auxiliary oscillator is measured by comparing said frequency of said auxiliary oscillator with said second device synchronized hardware timer, and

wherein any difference between a nominal and said measured frequency of said auxiliary oscillator is corrected by adjusting or retracting said auxiliary oscillator by an appropriate amount as to compensate for said measured difference.

11. The system of claim 10, wherein said auxiliary oscillator comprises a radio frequency oscillator and said radio frequency oscillator generates a radio frequency signal.

12. A system for synchronizing time amongst a plurality of wireless network devices in a wireless network where a message is exchanged between a transmitting wireless device and a receiving wireless device, said system comprising:

a controller that inserts synchronized time information in said message, prior to transmitting said message, said synchronized time information comprising a current time value and a synchronization time value; and

a processor that determines a local elapsed time value since last receiving a last synchronized time.

13. The system of claim 12, further comprising:

a gateway device comprising;

a gateway receiver;

a gateway transmitter that transmits said message that includes gateway time synchronization information;

a gateway hardware timer comprising an oscillator;

a gateway central processing unit coupled and operatively in communication with said gateway receiver, said gateway transmitter, and said gateway hardware timer; and

a gateway memory unit coupled and operatively in communication with said gateway central processing unit, said gateway memory unit comprising a drift rate table;

at least one remote device comprising;

a remote receiver that receives time synchronization information that includes a current time value and a synchronization time value;

a remote transmitter that transmits time synchronization information;

a remote hardware timer comprising an oscillator;

a remote micro-controller coupled and operatively in communication with said remote receiver, said remote transmitter, and said remote hardware timer; and

a remote memory unit coupled and operatively in communication with said remote micro-controller, wherein said remote memory unit comprises;

a current time value;

a synchronization time value; and

a drift rate value.

14. The system of claim 13, wherein said remote device updates said remote hardware timer with said received current time and replaces said stored synchronization time with said received synchronization time when said received synchronization time is newer than said stored synchronization time value.

15. The system of claim 13, wherein said remote device further comprises a memory manager that stores a difference between a nominal frequency value and a measured actual frequency value of said remote hardware timer, and wherein said remote micro-controller of said remote device accumulates said difference between said nominal frequency value and said actual frequency value for a cycle of said remote hardware timer and compares said accumulated difference until said accumulated difference exceeds an approximated time of transmitting one bit of information, wherein:

when said accumulated difference is as least as great as said approximated time of transmitting one bit of information, adjusting said remote hardware timer to compensate for said difference;

otherwise,

waiting until next cycle of said remote hardware timer.

16. The system of claim 13, wherein said remote device further comprises a temperature sensor measuring an ambient, and

wherein said remote device periodically retrieves a temperature drift rate, corresponding to said ambient temperature measured by said temperature sensor, from a drift rate table stored in said remote memory unit and advances or retards said timer according to said temperature drift rate.

17. The system of claim 13,

wherein a second electronic device of said at least one remote device synchronizes a second timer with said current time received in said master timing message and subsequently transmits a synchronized timing message containing a second device current time as measured and maintained by said remote hardware timer,

wherein upon receiving said synchronized timing message with a newer synchronization time or receiving said master timing message, a third electronic device of said at least one remote device calculates a timing error as being a difference between said received current time and said current time maintained by third electronic device remote hardware timer,

wherein said third electronic device calculates a drift rate as a ratio of said timing error divided by an elapsed time that has elapsed since said third electronic device remote hardware timer was last synchronized according to said synchronized timing message, and

wherein said third electronic device periodically adjusts said third electronic device remote hardware timer by advancing or retracting said third electronic device remote hardware timer by an amount equal to said drift rate multiplied by said elapsed time since the last adjustment.

18. The system of claim 17,

wherein said drift rate table comprises initial values and said initial values are calculated using at least one of a heuristic value, said ambient temperature measurement, and a difference between a first frequency and a second frequency, and

wherein said heuristic value comprises a predetermined drift rate calculation of said timer at said ambient temperature measurement.

19. The system of claim 17, wherein said temperature drift rate stored in said remote memory unit is periodically updated by replacing the initialized value with a new value calculated as a function of said temperature drift rate and a timer drift rate.

20. The system of claim 13, further comprising a radio frequency (RF) oscillator,

wherein said radio frequency oscillator generates a radio frequency signal,

wherein an adjusted drift rate of a synchronized hardware timer is determined to be less than a drift tolerance of said RF oscillator, a measured frequency of said RF oscillator is measured by comparing said frequency of said RF oscillator with said synchronized hardware timer, and

wherein any difference between a nominal and said measured frequency of said RF oscillator is corrected by adjusting or retracting said RF oscillator by an appropriate amount as to compensate for said measured difference.