Method and apparatus providing handoff of a mobile station between base stations using parallel communication links established with different time slots

• US 5,483,668 A
• Filed: 06/22/1993
• Issued: 01/09/1996
• Est. Priority Date: 06/24/1992
• Status: Expired due to Fees

• Current Assignee: Nokia Mobile Phones UK Limited (Nokia Corporation)
• Original Assignee: Nokia Mobile Phones UK Limited (Nokia Corporation)
• Inventors: Jokinen, Harri, Malkamaki, Esa
• Primary Examiner(s): Pham, Chi H.
• Application Number:
  US08/080,934
• Time in Patent Office: 931 Days
• Field of Search: 455/33.1, 455/33.2, 455/33.4, 455/34.1, 455/54.1, 455/54.2, 455/56.1, 455/67.1, 379/59, 379/60, 370/95.1, 370/95.3
• US Class Current: 455/442
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection

Abstract

The soft handover of a channel or a base station according to the invention functions especially well in a TDMA system with a high TDMA milo. For the time of preparation of handover parallel physical links are formed between the mobile station (MS) and two or more base stations (BTS1, BTS2), whereby these base stations transmit the same signal to the mobile station (MS) in different time slots, and possibly in a different frequency.

Claims

1. A method for enhancing handover in a digital cellular radio telephone system based on time division multiple access (TDMA), the system having at least one mobile switching center (MSC), at least first and second base stations (BTS1, BTS2) each of which transmits and receives information during a frame comprised of sequentially occurring timeslots, and at least one mobile station (MS) which also transmits and receives information during a frame comprised of time slots, characterized by the steps of:

prior to performing a handover of the at least one mobile station (MS) from the first base station to the second base station, establishing parallel signal links between the mobile station (MS) and the at least first and second base stations (BTS1, BTS2) by using first time slots between the mobile station (MS) and the first base station (BTS1) and second, different time slots between the mobile station (MS) and the second base station (BTS2);

communicating the same user information over the established parallel signal links using the first and second time slots;

combining received user information in at least one of the mobile switching center (MSC) and the mobile station (MS); and

severing the signal link with the first base station (BTS1) while continuing to communicate the user information with the second base station (BTS2) in the second time slots;

wherein

the step of communicating includes the steps of;

monitoring, at the mobile station (MS), a signal received at least over the signal link established with the second base station (BTS2); and

communicating an indication of the monitored signal to the first base station (BTS1) over the signal link established with the first base station (BTS1).

2. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

bidirectionally communicating information between the mobile station and the first base station using at least one first time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel;

establishing bidirectional communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

terminating bidirectional communication between the mobile station and the first base station while maintaining the established bidirectional communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the step of bidirectionally communicating information includes the steps of;

monitoring, at the mobile station, a signal received at least from the second downlink communication channel; and

communicating an indication of the monitored signal to the first base station through the at least one second time slot in the first uplink communication channel; and

wherein the step of communicating information further includes the steps of;

communicating information from the first base station to the mobile station through the at least one first time slot in the first downlink communication channel while also communicating the same information from the second base station to the mobile station through the at least one third time slot in the second downlink communication channel; and

combining, in the mobile station, the information received through the at least one first time slot with the information received through the at least one third time slot.

3. A method as set forth in claim 2 and further including the steps of:

transferring the communicated indication of the monitored signal to a cellular communication control node from the first base station;

determining at the control node, based at least in part on the transferred indication of the monitored signal, a time to initiate a handover of the mobile station from the first base station to the second base station; and

sending a first command from the control node to the mobile station via the first base station and the first downlink communication channel.

4. A method as set forth in claim 3 wherein the mobile station executes the step of establishing bidirectional communication between the mobile station and the second base station in response to receiving the command.

5. A method as set forth in claim 2 and further including the steps of:

determining at the first base station, based at least in part on the indication of the monitored signal, a time to initiate a handover of the mobile station from the first base station to the second base station; and

sending a first command from the first base station to the mobile station via the first downlink communication channel;

wherein

the mobile station executes the step of establishing bidirectional communication between the mobile station and the second base station in response to receiving the command.

6. A method as set forth in claim 2 wherein the first downlink communication channel and the first uplink communication channel use different frequencies.

7. A method as set forth in claim 2 wherein the first downlink communication channel and the first uplink communication channel use the same frequencies.

8. A method as set forth in claim 2 wherein the first time slot and the second time slot occur within a single frame and are offset from one another by a predetermined period of time, and wherein the third time slot and the fourth time slot occur within the same frame.

9. A method as set forth in claim 2 and further comprising a step of combining the information received through the at least one second time slot with the information received through the at least one fourth time slot.

10. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

bidirectionally communicating information between the mobile station and the first base station using at least one first time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel;

establishing bidirectional communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

terminating bidirectional communication between the mobile station and the first base station while maintaining the established bidirectional communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the step of bidirectionally communicating information includes the steps of;

monitoring, at the mobile station, a signal received at least from the second downlink communication channel; and

communicating an indication of the monitored signal to the first base station through the at least one second time slot in the first uplink communication channel, the method further including the steps of;

transferring the communicated indication of the monitored signal to a cellular communication control node from the first base station;

determining at the control node, based at least in part on the transferred indication of the monitored signal, a time to initiate a handover of the mobile station from the first base station to the second base station; and

sending a first command from the control node to the mobile station via the first base station and the first downlink communication channel;

wherein the mobile station executes the step of establishing bidirectional communication between the mobile station and the second base station in response to receiving the command; and

wherein the step of communicating information includes the steps of;

continuing to monitor, at the mobile station, a signal received from at least the second downlink communication channel;

communicating an indication of the monitored signal to the first base station through the at least one second time slot in the first uplink communication channel and also to the second base station through the at least one fourth time slot in the second uplink communication channel;

transferring the communicated indication of the monitored signal to the cellular communication control node from at least one of the first base station and the second base station;

determining at the control node, based at least in part on the transferred indication of the monitored signal, a time to handover the mobile station from the first base station to the second base station;

sending a second command from the control node to the mobile station via at least one of the first base station and the second base station; and

executing the step of terminating in response to the mobile station receiving the second command.

11. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

bidirectionally communicating information between the mobile station and the first base station using at least one first time slot in a first downlink communication channel add at least one second time slot in a first uplink communication channel;

establishing bidirectional communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

terminating bidirectional communication between the mobile station and the first base station while maintaining the established bidirectional communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the step of bidirectionally communicating information includes the steps of;

monitoring, at the mobile station, a signal received at least from the second downlink communication channel; and

communicating an indication of the monitored signal to the first base station through the at least one second time slot in the first uplink communication channel, the method further including the steps of;

determining at the first base station, based at least in part on the indication of the monitored signal, a time to initiate a handover of the mobile station from the first base station to the second base station; and

sending a first command from the first base station to the mobile station via the first downlink communication channel;

wherein

the mobile station executes the step of establishing bidirectional communication between the mobile station and the second base station in response to receiving the command;

wherein the step of communicating information includes the steps of;

continuing to monitor, at the mobile station, a signal received from at least the second downlink communication channel;

communicating an indication of the monitored signal to the first base station through the at least one second time slot in the first uplink communication channel and also to the second base station through the at least one fourth time slot in the second uplink communication channel;

determining at one of the first base station and the second base station, based at least in part on the indication of the monitored signal, a time to handover the mobile station from the first base station to the second base station;

sending a second command to the mobile station from at least one of the first base station and the second base station; and

executing the step of terminating in response to the mobile station receiving the second command.

12. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

bidirectionally communicating information between the mobile station and the first base station using at least one first time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel;

establishing bidirectional communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

terminating bidirectional communication between the mobile station and the first base station while maintaining the established bidirectional communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the step of bidirectionally communicating information includes the steps of;

monitoring, at the mobile station, a signal received at least from the second downlink communication channel;

determining at the mobile station, based at least in part on the monitored signal, a time to initiate a handover of the mobile station from the first base station to the second base station; and

executing, at the determined time to initiate handover, the step of establishing bidirectional communication between the mobile station and the second base station; and

wherein the step of communicating information further includes the steps of;

communicating information from the first base station to the mobile station through the at least one first time slot in the first downlink communication channel while also communicating the same information from the second base station to the mobile station through the at least one third time slot in the second downlink communication channel; and

combining, in the mobile station, the information received through the at least one first time slot with the information received through the at least one third time slot.

13. A method as set forth in claim 12 wherein the step of communicating information includes the steps of:

continuing to monitor, at the mobile station, a signal received from at least the second downlink communication channel;

determining at the mobile station, based at least in part on the monitored signal, a time to handover the mobile station from the first base station to the second base station; and

executing, at the determined time to handover, the step of terminating.

14. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

bidirectionally communicating information between the mobile station and the first base station using at least one time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel;

establishing communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also simultaneously communicating the same information from the mobile station to the second base station through the at least one second time slot in the first uplink communication channel; and

terminating bidirectional communication between the mobile station and the first base station while maintaining the established communication with the second base station using the at least one third time slot in the second downlink communication channel, thereby handing over the mobile station from the first base station to the second base station.

15. A cellular communications system, comprising:

a first base station serving a first communications cell;

a second base station serving a second communications cell; and

a mobile station including transceiver means for bidirectionally communicating information between the mobile station and the first base station using at least one time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel, said transceiver means being controlled to also establish bidirectional communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel, said transceiver means being further controlled to communicate information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel, wherein said transceiver means is further controlled to terminate bidirectional communication between the mobile station and the first base station while maintaining the established bidirectional communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the mobile station further comprises

means for monitoring a signal received at least from the second downlink communication channel; and

wherein said transceiver means is further controlled to communicate an indication of the monitored signal to the first base station through the at least one second time slot in the first uplink communication channel;

wherein the system further comprises;

means for communicating voice information from the first base station to the mobile station through the at least one first time slot in the first downlink communication channel while also communicating the same voice information from the second base station to the mobile station through the at least one third time slot in the second downlink communication channel; and

wherein the mobile station is further comprised of means for forming an audio signal from the voice information received through the at least one first time slot and also from the voice information received through the at least one third time slot.

16. A cellular communications system, comprising:

a first base station serving a first communications cell;

a second base station serving a second communications cell; and

a mobile station including transceiver means for bidirectionally communicating information between the mobile station and the first base station using at least one time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel, said transceiver means being controlled to also establish bidirectional communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel, said transceiver means being further controlled to communicate information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel, wherein said transceiver means is further controlled to terminate bidirectional communication between the mobile station and the first base station while maintaining the established bidirectional communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the mobile station is further comprised of;

means for monitoring a signal received at least from the second downlink communication channel; and

wherein said transceiver means is controlled to communicate an indication of the monitored signal to at least one of the first base station through the at least one second time slot in the first uplink communication channel and the second base station through the at least one fourth time slot in the second uplink communication channel;

wherein the system further comprises;

means for communicating voice information from the first base station to the mobile station through the at least one first time slot in the first downlink communication channel while also communicating the same voice information from the second base station to the mobile station through the at least one third time slot in the second downlink communication channel; and

wherein the mobile station is further comprised of means for forming an audio signal from the voice information received through the at least one first time slot and also from the voice information received through the at least one third time slot.

17. A system as set forth in claim 16 and further comprising:

a cellular communication control node that is bidirectionally coupled to said first base station and to said second base station;

means for transferring the communicated indication of the monitored signal to said cellular communication control node from at least one of the first base station and the second base station;

means for determining, at the control node, based at least in part on the transferred indication of the monitored signal, a time to handover the mobile station from the first base station to the second base station; and

means for sending a handover command from the control node to the mobile station.

18. A cellular Time Division Multiple Access (TDMA) communications system, comprising:

a first base station serving a first communications cell;

a second base station serving a second communications cell; and

a mobile station that includes means for bidirectionally communicating with said first and second base stations, wherein a handover of said mobile station between base stations is accomplished in three phases, wherein in a first phase said mobile station bidirectionally communicates user information and also signalling information only with said first base station using at least one time slot in a first downlink communication channel and at least one time slot in a first uplink communication channel, wherein in a third phase said mobile station bidirectionally communicates said user information and also signalling information only with said second base station using at least one time slot in a second downlink communication channel and at least one time slot in a second uplink communication channel, and wherein in a second phase said mobile station bidirectionally communicates said user information and also signalling information with said first base station using said at least one time slot in said first downlink communication channel and said at least one time slot in said first uplink communication channel while also bidirectionally communicating said user information and also signalling information with said second base station using said at least one time slot in said second downlink communication channel and said at least one time slot in said second uplink communication channel;

wherein the mobile station is further comprised of

means for monitoring a signal received at least from the second downlink communication channel; and

wherein

said means for bidirectionally communicating is controlled to communicate an indication of the monitored signal to at least one of the first base station through the at least one time slot in the first uplink communication channel and the second base station through the at least one time slot in the second uplink communication channel;

wherein the system further comprises;

means for communicating voice information from the first base station to the mobile station through the at least one first time slot in the first downlink communication channel while also communicating the same voice information from the second base station to the mobile station through the at least one third time slot in the second downlink communication channel; and

wherein the mobile station is further comprised of means for forming an audio signal from the voice information received through the at least one first time slot and also from the voice information received through theist least one third time slot.

19. A cellular Time Division Multiple Access (TDMA) communications system, comprising:

a first base station serving a first communications cell;

a second base station serving a second communications cell; and

a mobile station that includes means for bidirectionally communicating with said first and second base stations, wherein a handover of said mobile station between base stations is accomplished in three phases, wherein in a first phase said mobile station bidirectionally communicates user information and also signalling information only with said first base station using at least one time slot in a first downlink communication channel and at least one time slot in a first uplink communication channel, wherein in a third phase said mobile station bidirectionally communicates said user information and also signalling information only with said second base station using at least one time slot in a second downlink communication channel and at least one time slot in a second uplink communication channel, and wherein in a second phase said mobile station bidirectionally communicates said user information and also signalling information with said first base station using said at least one time slot in said first downlink communication channel and said at least one time slot in said first uplink communication channel while also bidirectionally communicating said user information and also signalling information with said second base station using said at least one time slot in said second downlink communication channel and said at least one time slot in said first uplink communication channel;

wherein the mobile station is further comprised of

means for monitoring a signal received at least from the second downlink communication channel; and

wherein

said means for bidirectionally communicating is controlled to communicate an indication of the monitored signal to at least one of the first base station through the at least one time slot in the first uplink communication channel and the second base station through the at least one time slot in the first uplink communication channel.

20. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

communicating information between the mobile station and the first base station using at least one first time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel;

establishing communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

terminating communication between the mobile station and the first base station while maintaining the established communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the step of communicating information further includes the steps of;

communicating voice information from the first base station to the mobile station through the at least one first time slot in the first downlink communication channel while also communicating the same voice information from the second base station to the mobile station through the at least one third time slot in the second downlink communication channel; and

in the mobile station, forming an audio signal from the voice information received through the at least one first time slot and also from the voice information received through the at least one third time slot.

21. In a cellular communications system, a method for performing a handover of a mobile station from a first base station to a second base station, comprising the steps of:

communicating information between the mobile station and the first base station using at least one first time slot in a first downlink communication channel and at least one second time slot in a first uplink communication channel;

establishing communication between the mobile station and the second base station using at least one third time slot in a second downlink communication channel and at least one fourth time slot in a second uplink communication channel;

communicating information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

terminating communication between the mobile station and the first base station while maintaining the established communication with the second base station, thereby handing over the mobile station from the first base station to the second base station;

wherein the step of communicating information further includes the steps of;

communicating voice information from the mobile station to the first base station through the at least one second time slot in the first uplink communication channel while also communicating the same voice information from the mobile station to the second base station through the at least one fourth time slot in the second uplink communication channel; and

combining the voice information received through the at least one second time slot with the voice information received through the at least one fourth time slot.