Synchronizing clock signals in wireless networks

• US 6,542,754 B1
• Filed: 05/12/1999
• Issued: 04/01/2003
• Est. Priority Date: 05/12/1999
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Systems, Inc.
• Inventors: Sayers, Ian Leslie, Long, Paul Jan, Cripps, Peter Kendall
• Primary Examiner(s): Trost, William
• Assistant Examiner(s): Nguyen, Simon
• Application Number:
  US09/310,566
• Time in Patent Office: 1,420 Days
• Field of Search: 455/422, 455/502, 455/503, 455/507, 455/524, 455/561, 455/442, 370/350, 370/503, 370/518
• US Class Current: 455/502
• CPC Class Codes: H04B 7/269 Master/slave synchronisation
• CPC Class Codes: H04B 7/2693 Centralised synchronisation, i.e. using external universal time reference, e.g. by using a global positioning system [GPS] or by distributing time reference over the wireline network
• CPC Class Codes: H04W 56/0015 one node acting as a reference for the others
• CPC Class Codes: H04W 56/0035 detecting errors in frequency or phase

Abstract

A communications system extends over a cellular region formed of a plurality of wireless cells where each cell covers a portion of the cellular region. Each cell has a base station clock unit that facilitates reliable and accurate synchronization with other clocks units in other cells using a master clock signal from a global positioning system (GPS) or other accurate frequency source. The clock unit in each cell includes a local clock providing a local clock signal for clocking the cell. A master clock source provides one or more master clock signals and a clock synchronizer receives the master clock signals and forms a synchronizing master clock signal for synchronizing the local clock signal. In one or more of the wireless cells of the communication system, the clock unit is a master clock unit which includes a clock distributor for distributing a distributed master clock signal to one or more other wireless cells having slave clock units.

Claims

1. A communication system extending over a cellular region and formed of a plurality of wireless cells having a plurality of base stations, respectively, where each cell covers a portion of the cellular region and where each particular cell has a particular base station comprising:

a clock unit for the particular base station wherein the clock unit comprises,

a local clock providing a local clock signal for clocking the particular base station,

a master clock source for providing one or more master clock signals, and

a clock synchronizer receiving said one or more master clock signals to form a synchronizing master clock signal for synchronizing said local clock signal; and

wherein one of said particular base stations is a master base station and one or more others of said base stations are slave base stations, and wherein only the clock unit for the master base station includes a clock distributor for distributing a distributed master clock signal to the slave base stations, and where one or more of clock units of the one or more slave base stations synchronize one or more local clock signals of the one or more slave base stations;

the master base station additionally broadcasting a master identification message identifying the master base station as a sole distributor of the distributed master clock signal.

2. The communication system of claim 1 wherein, for one or more of said wireless cells, said clock unit is a master clock unit which includes a clock distributor for distributing the synchronizing master clock signal, as a distributed master clock signal, and wherein, for one or more other ones of said wireless cells, in response to receipt of a master identification message, said clock unit is connected to be a slave clock unit receiving said distributed master clock signal only from the master clock unit.

3. The communication system of claim 2 wherein said clock unit includes a clock synchronizer receiving said distributed master clock signal whereby said distributed master clock signal is one of said one or more master clock signals distributed only by the master clock unit.

4. The communication system of claim 1 wherein said master clock source means includes a GPS receiver for receiving a GPS signal as one of said one or more master clock signals.

5. The communication system of claim 1 wherein said master clock source means includes a high accuracy crystal controlled oscillator for generating an oscillator signal as one of said one or more master clock signals.

6. The communication system of claim 1 wherein said particular base station includes a processor for processing clock information for controlling said clock unit.

7. The communication system of claim 1 wherein said clock distributor distributes the distributed master clock signal as an RF carrier.

8. The communication system of claim 7 operating with a GSM protocol and wherein only said clock distributor in the master base station distributes the distributed master clock signal as an SCH channel on a BCCH carrier.

9. The communication system of claim 1 wherein a BCCH carrier signal carries the master identification message as a message identifying the master base station transmitting that BCCH carrier as a sole distributor of the distributed master clock signal.

10. The communication system of claim 1 wherein said particular base station includes:

a local area network connection for connecting to other ones of said base stations to exchange clock information among the slave base stations in the cellular region.

11. The communication system of claim 1 wherein said base station includes a radio frequency analyzer for analyzing received radio frequency signals to detect a distributed master clock signal.

12. The communication system of claim 11 wherein said radio frequency analyzer includes comparator means for comparing the local clock signal with the distributed master clock signal to provide an error signal for indicating whether the local clock signal has a frequency greater than or less than a frequency of the distributed master clock signal.

13. The communication system of claim 12 wherein said particular base station includes a processor responsive to said error signal for synchronizing one of said one or more master clock signals.

14. The communication system of claim 1 wherein said particular base station includes:

a local area network connection for connecting to other ones of said base stations to exchange clock information among the base stations in the cellular region, and the local area network connection is used to broadcast the master identification message.

15. The communication system of claim 14 wherein said particular base station includes a processor for using said clock information to facilitate the recovery of the distributed master clock signal.

16. The communication system of claim 1 wherein said radio frequency analyzer includes comparator means for comparing the local clock signal with the distributed master clock signal to provide an error signal for indicating whether the local clock signal has a frequency greater than or less than a frequency of the distributed master clock signal.

17. The communication system of claim 1 wherein a plurality of said base stations are master base stations each associated with one or more others of said base stations acting as slave base stations and wherein only the clock unit for each of the master base stations includes a clock distributor for distributing a distributed master clock signal and where one or more of clock units of the associated one or more slave base stations synchronizes one or more local clock signals of the associated one or more slave base stations.

18. The communication system of claim 17 operating with a GSM protocol and wherein each clock distributor in each master base station distributes the distributed master clock signal as an SCH channel on a BCCH carrier.

19. The communication system of claim 17 wherein a BCCH carrier carries the master identification message as a data message identifying the master base station associated with the BCCH carrier as a sole distributor of the distributed master clock signal.

20. The communication system of claim 17 wherein each said particular base station includes:

a local area network connection for connecting to other ones of said base stations to exchange clock information among the base stations in the cellular region, and the local area network connection is used to broadcast the master identification messages.

21. The communication system of claim 1 wherein said particular base station includes:

a local area network connection for connecting the master base station to the slave base stations, wherein the slave base stations receive the distributed master clock signal and the master identification message only from the master base station over the local area network connection.

22. In a communication system extending over a cellular region and formed of a plurality of wireless cells having a plurality of base stations, respectively, where each cell covers a portion of the cellular region and where each particular cell has a particular master base station including a master clock unit for the particular master base station a method for operating the master clock unit comprising the steps of:

providing a local clock signal for clocking the particular master base station,

providing one or more master clock signals,

forming a synchronizing master clock signal from said one or more master clock signals for synchronizing said local clock signal; and

broadcasting an identification message identifying the particular master base station as a master base station to which a plurality of slave base stations may synchronize.

23. A communication system extending over a cellular region and formed of a plurality of wireless cells having a plurality of base stations, respectively, where each cell covers a portion of the cellular region and where each particular cell has a particular base station comprising:

a clock unit for the particular base station wherein the clock unit comprises,

a local clock providing a local clock signal for clocking the particular base station,

a master clock source for providing one or more master clock signals, and

a clock synchronizer receiving said one or more master clock signals to form a synchronizing master clock signal for synchronizing said local clock signal;

wherein one of said particular base stations is a master base station and one or more others of said base stations are slave base stations, and wherein only the clock unit for the master base station includes a clock distributor for distributing a distributed master clock signal to the slave base stations, and where one or more of clock units of the one or more slave base stations synchronize one or more local clock signals of the one or more slave base stations;

the master base station additionally broadcasting a master identification message identifying the master base station as a sole distributor of the distributed master clock signal; and

a local area network connection for connecting the master base station to the slave base stations, wherein the slave base stations receive the distributed master clock signal and the master identification message only from the master base station over the local area network connection.