Mobile-synchronized handoff in a wireless communications system

• US 5,444,766 A
• Filed: 10/01/1993
• Issued: 08/22/1995
• Est. Priority Date: 10/01/1993
• Status: Expired due to Term

• Current Assignee: Avaya Incorporated
• Original Assignee: AT&T Corporation (AT&T, Inc.)
• Inventors: Windhausen, Richard A., Miller, Richard D., Farwell, Charles Y.
• Primary Examiner(s): Kuntz, Curtis
• Assistant Examiner(s): TROST IV, WILLIAM GEORGE
• Application Number:
  US08/131,041
• Time in Patent Office: 690 Days
• Field of Search: 455/33.1, 455/33.2, 455/54.1, 455/56.1, 379/60, 379/59, 379/58, 379/63
• US Class Current: 455/437
• CPC Class Codes: H04W 36/02 Buffering or recovering information during reselection ; Modification of the traffic flow during hand-off
• CPC Class Codes: H04W 36/36 by user or terminal equipment

Abstract

An arrangement for effecting call handoffs in a wireless communications system (FIG. 1 ) in a substantially glitchless manner. The arrangement relies on the mobile terminal (4) that is involved in a handoff to trigger the handoff by transmitting a handoff coordination signal simultaneously to the base stations (1, 2) that are involved in the handoff. The base stations receive the signal substantially simultaneously, and in response the base station that has been serving the mobile terminal'"'"'s call ceases to do so while the base station that is henceforth to serve the mobile terminal'"'"'s call commences to do so at the same instant in time.

Claims

1. In a wireless communications system, a method of coordinating a handoff of a call involving a mobile terminal from a first base station to a second base station, comprising the steps of:

transmitting a handoff coordination signal from the mobile terminal simultaneously to both the first base station which is presently transmitting the call to the mobile terminal and the second base station which is presently not transmitting the call to the mobile terminal;

receiving the handoff coordination signal substantially simultaneously at both the first and the second base stations;

in response to the receipt, the first base station ceasing to transmit the call to the mobile terminal at an instant in time; and

in response to the receipt, the second base station commencing to transmit the call to the mobile terminal at effectively same said instant in time;

whereby timing of the handoff is coordinated by the mobile terminal and is performed substantially simultaneously by both the first and the second base stations.

2. The method of claim 1 wherein:

the step of transmitting a handoff coordination signal comprises the step of

transmitting a sequence of signals each having a different value including a predetermined value;

the step of ceasing to transmit comprises the step of upon receipt of the sequence of signals including the signal having the predetermined value, the first base station ceasing to transmit the call to the mobile terminal; and

the step of commencing to transmit comprises the step of upon receipt of the sequence of signals including the signal having the predetermined value, the second base station commencing to transmit the call to the mobile terminal.

3. The method of claim 2 wherein:

the step of transmitting a sequence of signals comprises the step of transmitting a sequence of frames each including a count field having a different count.

4. The method, of claim 3 wherein:

the step of transmitting a sequence of call traffic-carrying frames comprises the step of transmitting the sequence of said frames each including a successively changing count wherein the predetermined value is a predetermined count.

5. A wireless communications system comprising:

a mobile terminal;

a first and a second base station for serving a call involving the mobile terminal;

means in the mobile terminal for transmitting a handoff coordination signal simultaneously to both the first base station which is presently transmitting the call to the mobile terminal and the second base station which is presently not transmitting the call to the mobile terminal to trigger a handoff of the call involving the mobile terminal from the first base station to the second base station;

means in the first base station responsive to receipt of the handoff coordination signal for causing the first base station to cease transmitting the call to the mobile terminal at an instant in time; and

means in the second base station responsive to receipt of the handoff coordination signal for causing the second base station to commence transmitting the call to the mobile terminal at effectively same said instant in time;

whereby timing of the handoff is coordinated by the mobile terminal and is performed substantially simultaneously by both the first and the second base stations.

6. The system of claim 5 wherein:

the transmitting means in the mobile terminal comprise means for transmitting a sequence of signals each having a different value, including a predetermined value that triggers the handoff;

the causing means in the first base station comprise means responsive to receipt of the sequence of signals including the signal having the predetermined value, for causing the first base station to cease transmitting the call to the mobile terminal; and

the causing means in the second base station comprise means responsive to receipt of the sequence of signals including the signal having the predetermined value, for causing the second base station to commence transmitting the call to the mobile terminal.

7. The system of claim 6 wherein:

the sequence of signals comprises a sequence of frames each including a count field having a different count.

8. The system of claim 7 wherein:

the frames of the sequence of frames each includes a successively changing count, and the predetermined value is a predetermined count.

9. A mobile terminal for a wireless communications system wherein base stations involved in a handoff of a call involving the mobile terminal from one of the base stations to another of the base stations respond to receipt of a handoff coordination signal by performing the handoff, comprising:

means for generating the handoff coordination signal; and

means responsive to the generation for transmitting the generated handoff coordination signal simultaneously to both the one base station which is presently transmitting the call to the mobile terminal and the other base station which is presently, not transmitting the call to the mobile terminal to cause the one base station to cease transmitting the call to the mobile terminal at an instant in time and to cause the other base station to commence transmitting the call to the mobile terminal at effectively same said instant in time;

so that timing of the handoff is coordinated by the mobile terminal and is performed substantially simultaneously by both the one and the other base stations.

10. The mobile terminal of claim 9 wherein:

the generating means comprise means for generating a sequence of signals each having a different value, including a predetermined value that triggers the handoff; and

the transmitting means transmit the generated sequence of signals.

11. The mobile terminal of claim 10 wherein:

the sequence of signals comprises a sequence of frames each including a count field having a different count.

12. The mobile terminal of claim 11 wherein:

the frames of the sequence of frames each includes a successively changing count, and the predetermined value is a predetermined count.

13. An individual base station for a wireless communications system wherein base stations involved in a handoff of a call involving a mobile terminal from one of the base stations to another of the base stations respond to receipt of a handoff coordination signal by performing the handoff, comprising:

means for receiving the handoff coordination signal from a mobile terminal involved in a call that is being handed off between the individual base station and another base station wherein a first one of the individual base station and the other base station is presently transmitting the call to the mobile terminal and a second one of the individual base station and the other base station is presently not transmitting the call to the mobile terminal, the handoff signal having been transmitted by the mobile terminal simultaneously to both the individual base station and the other base station to trigger performance of the handoff wherein the first one of the base stations ceases to transmit the call to the mobile terminal at an instant in time and the second one of the base stations commences to transmit the call to the mobile terminal at effectively same said instant in time; and

means responsive to the receipt for performing the handoff of the call between the individual base station and the other base station at said instant in time;

so that timing of the handoff is coordinated by the mobile terminal and is performed substantially simultaneously by both the first and the second base stations.

14. The base station of claim 13 wherein:

the receiving means receive from the mobile terminal a sequence of signals each having a different value, including a predetermined value to trigger the handoff; and

the means for performing the handoff comprise means responsive to receipt of the sequence of signals including the signal having the predetermined value, for performing the handoff of the call between the individual base station and the other base station upon receipt of the signal having the predetermined value.

15. The base station of claim 14 wherein:

the sequence of signals comprises a sequence of frames each including a count field having a different count.

16. The base station of claim 15 wherein:

the frames of the sequence of frames each includes a successively changing count, and the predetermined value is a predetermined count.