Method for preventing a dropped call during a handoff in a radiotelephone system

• US 5,590,177 A
• Filed: 09/30/1994
• Issued: 12/31/1996
• Est. Priority Date: 09/30/1994
• Status: Expired due to Fees

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Vilmur, Richard J., Bruckert, Eugene J.
• Primary Examiner(s): Kuntz, Curtis
• Assistant Examiner(s): Chernoff, Michael B.
• Application Number:
  US08/316,697
• Time in Patent Office: 823 Days
• Field of Search: 379/58, 379/59, 379/60, 379/63, 455/33.1, 455/33.2, 455/33.4, 455/54.1, 455/54.2, 455/56.1, 375/200, 375/205
• US Class Current: 455/436
• CPC Class Codes: H04W 36/08 Reselecting an access point
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 36/30 by measured or perceived connection quality data

Abstract

A method, performed by a mobile station (119), prevents an active call from being dropped when the mobile station (119) suddenly transitions from a first coverage area (121) to a second coverage area (122) in a code division multiple access (CDMA) cellular radiotelephone system (100). The active call is determined (601) to be in danger of being dropped responsive to the mobile station (119) suddenly transitioning from the first coverage area (121) to the second coverage area (122). The first base station (115) is notified (602) that the active call is in danger of being dropped responsive to the step of determining (601). The active call with the first base station (115) is determined (603) to be in the process of being dropped responsive to the step of notifying (602). The active call is established (604) with the second base station (116) in the second coverage area (122) before the active call with the first base station (115) is dropped responsive to the step of determining (603).

Claims

1. In a cellular radiotelephone communication system including at least one switch controller, a plurality of base stations including first and second base stations, and at least one mobile station, wherein a switch controller is coupled to the first and second base stations to provide radiotelephone communication throughout first and second coverage areas, respectively, wherein a mobile station is in an active call in the first coverage area with the first base station on a first traffic channel and substantially shadowed from transmissions from the second base station in the second coverage area, a method, performed by the mobile station, for preventing the active call from being dropped when the mobile station suddenly transitions from the first coverage area to the second coverage area, the method comprising the steps of:

determining that the active call is in danger of being dropped responsive to the mobile station suddenly transitioning from the first coverage area to the second coverage area;

notifying the first base station that the active call is in danger of being dropped responsive to the step of determining that the active call is in danger of being dropped;

determining that the active call with the first base station is in the process of being dropped responsive to the step of notifying the first base station that the active call is in danger of being dropped; and

establishing the active call with the second base station in the second coverage area before the active call with the first base station is dropped responsive to the step of determining that the active call with the first base station is in the process of being dropped.

2. A method for preventing the active call from being dropped according to claim 1 wherein the step of determining the active call is in danger of being dropped further comprises the step of:

monitoring a total energy received by the mobile station;

measuring a signal energy for the active call received by the mobile station;

calculating a ratio of the signal energy to the total energy; and

determining that the ratio is unfavorable to indicate that active call is in danger of being dropped.

3. A method for preventing the active call from being dropped according to claim 2 wherein the step of monitoring the total energy received by the mobile station further comprises the steps of:

measuring the total energy received by the mobile station at selected points in time to produce samples of the total energy including at least one previous sample and a current sample of the total energy;

determining that the current sample of the total energy is greater than the at least one previous sample of the total energy by a predetermined amount;

integrating the current sample of total energy with the at least one previous sample of the total energy to produce an integrated total energy; and

storing the integrated total energy to produce a stored total energy.

4. A method for preventing the active call from being dropped according to claim 1 wherein the step of notifying the first base station that the active call is in danger of being dropped further comprises the step of:

sending a pilot signal strength message to the first base station indicative of signal strengths of a plurality of pilot signals transmitted by the plurality of base stations, respectively.

5. A method for preventing the active call from being dropped according to claim 1, wherein the active call includes a plurality of sequential frames, wherein the step of determining that the active call with the first base station is in the process of being dropped further comprises the step of:

determining that a predetermined number of the plurality of sequential frames are unacceptable; and

determining, within a predetermined amount of time, that no two sequential frames of the plurality of sequential frames are acceptable.

6. A method for preventing the active call from being dropped according to claim 1 wherein the step of establishing the active call with the second base station in the second coverage area further comprises the steps of:

identifying the second base station;

sending a reconnect request message to the second base station responsive to the step of identifying the second base station;

receiving a reconnect grant message from the second base station within a predetermined amount of time responsive to the step of sending a reconnect request message to the second base station; and

resuming the active call in the second coverage area with the second base station on a second traffic channel responsive to the step of receiving a reconnect grant message from the second base station.

7. A method for preventing the active call from being dropped according to claim 6 wherein the step of identifying the second base station further comprises the steps of:

searching for a plurality of pilot signals transmitted by the plurality of base stations, respectively; and

selecting a pilot signal having the strongest signal strength from among the plurality of pilot signals, wherein the pilot signal having the strongest signal strength corresponds to the pilot signal transmitted by the second base station.

8. A method for preventing the active call from being dropped according to claim 6 wherein the step of receiving a reconnect request message further comprises the step of:

receiving the reconnect request message on a paging code channel.

9. In a code division multiple access (CDMA) cellular radiotelephone communication system including at least one switch controller, a plurality of base stations including first and second base stations, and at least one mobile station, wherein a switch controller is coupled to the first and second base stations to provide radiotelephone communication throughout first and second coverage areas, respectively, wherein a mobile station is in an active call in the first coverage area with the first base station on a first traffic channel and substantially shadowed from transmissions from the second base station in the second coverage area, a method, performed by the mobile station, for preventing the active call from being dropped when the mobile station suddenly transitions from the first coverage area to the second coverage area, the method comprising the steps of:

monitoring a total energy received by the mobile station;

measuring a signal energy for the active call received by the mobile station;

calculating a ratio of the signal energy to the total energy;

determining that the ratio is unfavorable to indicate that active call is in danger of being dropped;

sending a pilot signal strength message to the first base station indicative of signal strengths of a plurality of pilot signals transmitted by the plurality of base stations, respectively, responsive to the step of determining that the ratio is unfavorable;

determining that the active call with the first base station is in the process of being dropped responsive to the step of sending a pilot signal strength message to the first base station; and

establishing the active call with the second base station in the second coverage area before the active call with the first base station is dropped responsive to the step of determining that the active call with the first base station is in the process of being dropped.

10. A method for preventing the active call from being dropped according to claim 9 wherein the step of monitoring the total energy received by the mobile station further comprises the steps of:

measuring the total energy received by the mobile station at selected points in time to produce samples of the total energy including at least one previous sample and a current sample of the total energy;

determining that the current sample of the total energy is greater than the at least one previous sample of the total energy by a predetermined amount;

integrating the current sample of total energy with the at least one previous sample of the total energy to produce an integrated total energy; and

storing the integrated total energy to produce a stored total energy.

11. A method for preventing the active call from being dropped according to claim 9 wherein the active call includes a plurality of sequential frames, wherein the step of determining that the active call with the first base station is in the process of being dropped further comprises the step of:

determining that a predetermined number of the plurality of sequential frames are unacceptable; and

determining, within a predetermined amount of time, that no two sequential frames of the plurality of sequential frames are acceptable.

12. A method for preventing the active call from being dropped according to claim 9 wherein the step of establishing the active call with the second base station in the second coverage area further comprises the steps of:

identifying the second base station;

sending a reconnect request message to the second base station responsive to the step of identifying the second base station;

receiving a reconnect grant message from the second base station within a predetermined amount of time responsive to the step of sending a reconnect request message to the second base station; and

resuming the active call in the second coverage area with the second base station on a second traffic channel responsive to the step of receiving a reconnect grant message from the second base station.

13. A method for preventing the active call from being dropped according to claim 12 wherein the step of identifying the second base station further comprises the steps of:

searching for a plurality of pilot signals transmitted by the plurality of base stations, respectively; and

selecting a pilot signal having the strongest signal strength from among the plurality of pilot signals, wherein the pilot signal having the strongest signal strength corresponds to the pilot signal transmitted by the second base station.

14. A method for preventing the active call from being dropped according to claim 12 wherein the step of receiving a reconnect request message further comprises the step of:

receiving the reconnect request message on a paging code channel.

15. In a code division multiple access (CDMA) cellular radiotelephone communication system including at least one switch controller, a plurality of base stations including first and second base stations, and at least one mobile station, wherein a switch controller is coupled to the first and second base stations to provide radiotelephone communication throughout first and second coverage areas, respectively, wherein a mobile station is in an active call in the first coverage area with the first base station on a first traffic channel and substantially shadowed from transmissions from the second base station in the second coverage area, a method, performed by the mobile station, for preventing the active call from being dropped when the mobile station suddenly transitions from the first coverage area to the second coverage area, the method comprising the steps of:

determining that the active call is in danger of being dropped responsive to the mobile station suddenly transitioning from the first coverage area to the second coverage area;

notifying the first base station that the active call is in danger of being dropped responsive to the step of determining that the active call is in danger of being dropped;

determining that the active call with the first base station is in the process of being dropped responsive to the step of notifying the first base station that the active call is in danger of being dropped;

searching for a plurality of pilot signals transmitted by the plurality of base stations, respectively, responsive to the step of determining that the active call with the first base station is in the process of being dropped;

selecting a pilot signal having the strongest signal strength from among the plurality of pilot signals, wherein the pilot signal having the strongest signal strength corresponds to the pilot signal transmitted by the second base station;

sending a reconnect request message to the second base station responsive to the step of identifying the second base station responsive to the step of determining the pilot signal having the strongest signal strength from among the plurality of pilot signals;

receiving a reconnect grant message from the second base station within a predetermined amount of time responsive to the step of sending a reconnect request message to the second base station; and

resuming the active call in the second coverage area with the second base station on a second traffic channel responsive to the step of receiving a reconnect grant message from the second base station.

16. A method for preventing the active call from being dropped according to claim 15 wherein the step of determining the active call is in danger of being dropped further comprises the step of:

monitoring a total energy received by the mobile station;

measuring a signal energy for the active call received by the mobile station;

calculating a ratio of the signal energy to the total energy; and

determining that the ratio is unfavorable to indicate that active call is in danger of being dropped.

17. A method for preventing the active call from being dropped according to claim 16 wherein the step of monitoring the total energy received by the mobile station further comprises the steps of:

measuring the total energy received by the mobile station at selected points in time to produce samples of the total energy including at least one previous sample and a current sample of the total energy;

determining that the current sample of the total energy is greater than the at least one previous sample of the total energy by a predetermined amount;

integrating the current sample of total energy with the at least one previous sample of the total energy to produce an integrated total energy; and

storing the integrated total energy to produce a stored total energy.

18. A method for preventing the active call from being dropped according to claim 15 wherein the step of notifying the first base station that the active call is in danger of being dropped further comprises the step of:

sending a pilot signal strength message to the first base station indicative of signal strengths of a plurality of pilot signals transmitted by the plurality of base stations, respectively.

19. A method for preventing the active call from being dropped according to claim 15 wherein the active call includes a plurality of sequential frames, wherein the step of determining that the active call with the first base station is in the process of being dropped further comprises the step of:

determining that a predetermined number of the plurality of sequential frames are unacceptable; and

determining, within a predetermined amount of time, that no two sequential frames of the plurality of sequential frames are acceptable.

20. In a code division multiple access (CDMA) cellular radiotelephone communication system including at least one switch controller, a plurality of base stations including first and second base stations, and at least one mobile station, wherein a switch controller is coupled to the first and second base stations to provide radiotelephone communication throughout first and second coverage areas, respectively, wherein a mobile station is in an active call in the first coverage area with the first base station on a first traffic channel and substantially shadowed from transmissions from the second base station in the second coverage area, a method, performed by the mobile station, for preventing the active call from being dropped when the mobile station suddenly transitions from the first coverage area to the second coverage area, the method comprising the steps of:

monitoring a total energy received by the mobile station;

measuring a signal energy for the active call received by the mobile station;

calculating a ratio of the signal energy to the total energy;

determining that the ratio is unfavorable to indicate that active call is in danger of being dropped;

sending a pilot signal strength message to the first base station indicative of signal strengths of a plurality of pilot signals transmitted by the plurality of base stations, respectively, responsive to the step of determining that the ratio is unfavorable;

determining that the active call with the first base station is in the process of being dropped responsive to the step of sending a pilot signal strength message to the first base station;

searching for a plurality of pilot signals transmitted by the plurality of base stations, respectively, responsive to the step of determining that the active call with the first base station is in the process of being dropped;

selecting a pilot signal having the strongest signal strength from among the plurality of pilot signals, wherein the pilot signal having the strongest signal strength corresponds to the pilot signal transmitted by the second base station;

sending a reconnect request message to the second base station responsive to the step of identifying the second base station responsive to the step of selecting the pilot signal having the strongest signal strength from among the plurality of pilot signals;

receiving a reconnect grant message from the second base station within a predetermined amount of time responsive to the step of sending a reconnect request message to the second base station; and

resuming the active call in the second coverage area with the second base station on a second traffic channel responsive to the step of receiving a reconnect grant message from the second base station.

21. A method for preventing the active call from being dropped according to claim 20 wherein the step of monitoring the total energy received by the mobile station further comprises the steps of:

measuring the total energy received by the mobile station at selected points in time to produce samples of total energy including at least one previous sample and a current sample of the total energy;

determining that the current sample of the total energy is greater than the at least one previous sample of the total energy by a predetermined amount;

integrating the current sample of total energy with the at least one previous sample of the total energy to produce an integrated total energy; and

storing the integrated total energy to produce a stored total energy.

22. A method for preventing the active call from being dropped according to claim 20 wherein the active call includes a plurality of sequential frames, wherein the step of determining that the active call with the first base station is in the process of being dropped further comprises the step of:

determining that a predetermined number of the plurality of sequential frames are unacceptable; and

determining, within a predetermined amount of time, that no two sequential frames of the plurality of sequential frames are acceptable.