Methods and apparatus for interfrequency handoff in a wireless communication system

• US 6,252,861 B1
• Filed: 03/26/1998
• Issued: 06/26/2001
• Est. Priority Date: 03/26/1998
• Status: Expired due to Term

• Current Assignee: RPX Corporation
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Wang, Xiao, Weaver, Carl F., Meyers, Martin H., Kuo, Wen-Yi, Bernstein, Neil E., Bernstein, Xiao C.
• Primary Examiner(s): Kizou, Hassan
• Assistant Examiner(s): Elallam, Ahmed
• Application Number:
  US09/048,626
• Time in Patent Office: 1,188 Days
• Field of Search: 370/32, 370/329, 370/331, 370/332, 370/335, 370/342, 370/343, 455/422, 455/436, 455/438, 455/440, 455/443
• US Class Current: 370/331
• CPC Class Codes: H04W 36/14 Reselecting a network or an air interface
• CPC Class Codes: H04W 36/30 by measured or perceived connection quality data
• CPC Class Codes: H04W 36/302 due to low signal strength

Abstract

Inter-frequency handoffs in a CDMA or other wireless communication system are controlled using a noise-limited coverage trigger metric which is able to distinguish between same-frequency cell boundaries and other-frequency cell boundaries in the system. The trigger metric may be generated as a function of an average signal-to-noise measure for pilot signals received at a mobile station of the system and a linear sum of the signal-to-noise measures. The signal-to-noise measures may be generated in the mobile station and included in messages transmitted from the mobile station to one or more base stations of the system. The trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call. The trigger metric may alternatively be based on a measure of mobile receive power alone. Other aspects of the invention reduce unnecessary searching for a new frequency and decrease the likelihood of “ping-ponging” from a current frequency to a new frequency by providing additional checks in the handoff process. For example, receive power and pilot signal-to-noise measures may be generated at a mobile station for both current and new frequencies. The mobile station then continues to operate at the current frequency as long as certain predetermined threshold conditions based on the measures are satisfied.

Claims

1. A method of controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the method comprising the steps of:

generating a trigger metric as a function of an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of the signal-to-noise measures for at least a subset of the plurality of pilot signals; and

utilizing the trigger metric to control a handoff from a current frequency to a new frequency in an ongoing call.

2. The method of claim 1 wherein the generating step includes generating the trigger metric as the difference between an average transmit signal-to-noise measure for the plurality of pilot signals and a linear sum of pilot signal-to-noise measures for the plurality of pilot signals.

3. The method of claim 1 wherein the signal-to-noise measures are generated in the mobile station and included in messages transmitted from the mobile station.

4. The method of claim 3 wherein the system is an IS-95 CDMA system and the messages transmitted from the mobile station include at least one of a Power Measurement Report Message (PMRM) and a Pilot Strength Measurement Message (PSMM).

5. The method of claim 1 wherein the trigger metric is configured so as to distinguish same-frequency cell boundaries from other-frequency cell boundaries.

6. An apparatus for use in controlling frequency handoffs in a wireless communication system in which a mobile station communicates with one or more base stations, the apparatus comprising:

a processor for generating a trigger metric as a function of an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of the signal-to-noise measures for at least a subset of the plurality of pilot signals; and

a memory for at least temporarily storing a representation of the trigger metric, such that the trigger metric is used to control a handoff from a current frequency to a new frequency in an ongoing call in the system.

7. The apparatus of claim 6 wherein the trigger metric is generated as the difference between an average transmit signal-to-noise measure for the plurality of pilot signals and a linear sum of pilot signal-to-noise measures for the plurality of pilot signals.

8. The apparatus of claim 6 wherein the signal-to-noise measures are generated in the mobile station and included in messages transmitted from the mobile station to the one or more base stations.

9. The apparatus of claim 8 wherein the system is an IS-95 CDMA system and the messages transmitted from the mobile station include at least one of a Power Measurement Report Message (PMRM) and a Pilot Strength Measurement Message (PSMM).

10. The apparatus of claim 6 wherein the trigger metric is configured so as to distinguish same-frequency cell boundaries from other-frequency cell boundaries.

11. A method of controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the method comprising the steps of:

generating measures of at least one of receive power and pilot signal-to-noise for each of a current frequency and a new frequency; and

utilizing at least a subset of the measures to obtain a trigger metric configured so as to distinguish same-frequency boundaries of the system from other-frequency boundaries of the system, the trigger metric being generated utilizing an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of signal-to-noise measures for at least a subset of the plurality of pilot signals, the trigger metric being used to determine if an ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency.

12. The method of claim 11 wherein the generating step includes generating a measure of receive power at the current frequency, a measure of receive power at the new frequency, a sum of pilot signal-to-noise measures at the current frequency, and a sum of pilot signal-to-noise measures at the new frequency.

13. The method of claim 12 wherein the system is a CDMA system and the receive power measures and the sums of pilot signal-to-noise measures are generated in the mobile station and transmitted in an Other Frequency Report Message (OFRM) to the one or more base stations.

14. The method of claim 11 further including the step of continuing to operate at the current frequency if the receive power at the current frequency is greater than a threshold.

15. The method of claim 11 further including the step of continuing to operate at the current frequency if a sum of signal-to-noise measures at the current frequency is greater than a threshold.

16. The method of claim 11 further including the step of continuing to operate at the current frequency if a difference between the receive power at the new frequency and the receive power at the current frequency is greater than a threshold.

17. The method of claim 11 further including the step of sending one or more threshold values to the mobile station in a message transmitted from one of the base stations, wherein the threshold values are used to determine if the ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency.

18. An apparatus for controlling a frequency handoff in a wireless communication system in which a mobile station communicates with one or more base stations, the apparatus comprising:

a processor which is operative (i) to obtain measures of at least one of receive power and pilot signal-to-noise for each of a current frequency and a new frequency, and (ii) to utilize at least a subset of the measures to obtain a trigger metric configured so as to distinguish same-frequency boundaries of the system from other-frequency boundaries of the system, the trigger metric being generated utilizing an average transmit signal-to-noise measure for a plurality of pilot signals and a sum of signal-to-noise measures for at least a subset of the plurality of pilot signals, the trigger metric being used to determine if an ongoing call involving the mobile station should continue to operate at the current frequency or be handed off to the new frequency; and

a memory for at least temporarily storing one or more thresholds for use by the processor in determining if the ongoing call should continue at the current frequency or be handed off to the new frequency.

19. The apparatus of claim 18 wherein the measures obtained by the processor include a measure of receive power at the current frequency, a measure of receive power at the new frequency, a sum of pilot signal-to-noise measures at the current frequency, and a sum of pilot signal-to-noise measures at the new frequency.

20. The apparatus of claim 19 wherein the system is a CDMA system and the receive power measures and the sums of pilot signal-to-noise measures are generated in the mobile station and transmitted in an Other Frequency Report Message (OFRM) to the one or more base stations.

21. The apparatus of claim 18 wherein the ongoing call continues to operate at the current frequency if the receive power at the current frequency is greater than a threshold.

22. The apparatus of claim 18 wherein the ongoing call continues to operate at the current frequency if a sum of signal-to-noise measures at the current frequency is greater than a threshold.

23. The apparatus of claim 18 wherein the ongoing call continues to operate at the current frequency if a difference between the receive power at the new frequency and the receive power at the current frequency is greater than a threshold.

24. The apparatus of claim 18 wherein the one or more threshold values are supplied to the mobile station in a message transmitted from one of the base stations.