Base station transmit power control in a CDMA cellular telephone system

• US 6,708,041 B1
• Filed: 11/10/1998
• Issued: 03/16/2004
• Est. Priority Date: 12/15/1997
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Butovitsch, Peter, Andersson, Peter B.
• Primary Examiner(s): Tran, Congvan
• Application Number:
  US09/189,629
• Time in Patent Office: 1,953 Days
• Field of Search: 455/482, 455/423, 455/435, 455/438, 455/436, 455/439, 455/442, 455/517, 455/522, 455/561, 455/566, 455/132, 455/133, 455/134, 455/13.4, 455/127.1, 370/317, 370/318, 370/320
• US Class Current: 455/522
• CPC Class Codes: H04W 52/40 during macro-diversity or soft handoff
• CPC Class Codes: H04W 52/50 at the moment of starting communication in a multiple access environment

Abstract

A power control method in accordance with the present invention synchronizes the transmit power levels at the base stations currently serving a mobile station and an initial transmit power level of a new target base station being added in a diversity handover situation. At the beginning of the handover, a radio network controller orders the serving base stations to measure their respective transmit powers to the mobile station. Each serving base station then measures its transmit power to the mobile station and reports the power level measurements to the radio network controller. An initial transmit power setting is determined for the target base station, and new transmit power settings are determined for the serving base stations synchronized for example to a particular time (t₀). At or about synchronizing time t₀, the target base station transmits to the mobile station at the initial power setting, and at or about the same time, the serving base stations preferably also adjust their transmit powers toward the new values. Alternatively, the serving base stations transmit at their respective new values at or about time t₀. In a preferred embodiment of the present invention, the serving base station power adjustment is performed gradually.

Claims

1. A power control method for a mobile radio communications system including a controller connected to plural base stations, each base station communicating with mobile stations located within or near a geographical area associated with the base station, comprising:

for a call involving a mobile station, establishing an initial communication between the mobile station and a first base station with the first base station transmitting call traffic to the mobile station at a transmit power level over a first traffic channel;

for the same call, establishing a diversity handover communication between the mobile station and a second base station over a second traffic channel in addition to the initial communication;

determining an initial transmit power level for the second base station for the diversity handover communication transmissions from the second base station to the mobile station over the second traffic channel; and

controlling the transmit power level of one of the first and second base stations to achieve a desired transmit power offset between the first and second power levels.

2. The method in claim 1, wherein the desired offset is zero.

3. The method in claim 1, wherein the desired offset is non-zero.

4. The method in claim 1, wherein signals received by the first and second base stations from the mobile station are diversity-combined in the controller and the signals received by the mobile station from the first and second base stations are diversity-combined in a RAKE receiver in the mobile station.

5. The method in claim 1, the controlling step further comprising:

gradually changing the transmit power level of one of the first and second base stations to reduce the difference between the first and second transmit power levels.

6. The method in claim 1, wherein the controlling step includes:

adjusting the transmit power levels of the first and second base stations to the same power level.

7. The method in claim 1, wherein the determining step further includes:

establishing a predetermined time associated with a desired transmit power level for the first base station over the first traffic channel and the initial transmit power level of the second base station over the second traffic channel.

8. The method in claim 7, the controlling step further comprising:

determining a difference between an actual transmit power level of the first base station at time t₀ and the desired transmit power level of the first base station at time t₀.

9. The method in claim 8, wherein if the difference exceeds a threshold, the controlling step further comprising:

comparing the actual and desired transmit power levels of the first base station;

if the actual transmit power level exceeds the desired transmit power level of the first base station, decreasing the power level; and

if the actual transmit power level is less than or equal to the desired transmit power level of the first base station, increasing the power level.

10. The method in claim 9, wherein the transmit power level of the first base station is changed incrementally with a first delay period between each incremental increase and a second delay period between each incremental decrease.

11. The method in claim 1, further comprising after the second base station is transmitting:

determining whether an undesired transmit power level offset exists between the first and second base station, and

controlling the transmit power level of one of the first and second base stations to reduce the undesired transmit power level offset.

12. Apparatus for use in a mobile communications system, comprising:

a first base station for establishing a communication with a mobile station with the first base station transmitting to the mobile station at a transmit power level over a first traffic channel;

a second base station for establishing a handover communication with the mobile station over a second traffic channel; and

a base station controller, coupled to the first and second base stations, for setting an initial power level for the second base station to transmit to the mobile station for the handover communication over the second traffic channel and adjusting the transmit power level of the first base station to the mobile station over the first traffic channel in conjunction with setting the initial power level of the second base station for the handover communication over the second traffic channel.

13. The apparatus in claim 12, wherein the base station controller includes a diversity handover unit for diversity combining signals corresponding to a communication from the mobile station received from the first and second base stations.

14. The apparatus in claim 12, wherein the transmit power level of the first base station may be adjusted towards the set initial power level of the second base station.

15. The apparatus in claim 12, wherein the transmit power level of the first base station is adjusted to a desired offset from the set initial power level of the second base station.

16. The apparatus in claim 12, wherein the first and second base stations each include a transmit power controller for varying the transmit power level of the base station to the mobile station.

17. The apparatus in claim 16, wherein one of the first and second base station transmit power controllers is configured to vary its base station transmit power in response to a transmit power level control signal from the base station controller.

18. The apparatus in claim 12, wherein the mobile station includes:

a RAKE receiver for diversity combining the signals received by the mobile station from the first and second base stations to generate a diversity combined received signal, and

a mobile station controller for measuring a parameter of the diversity combined received signal and transmitting to the first and second base stations a transmit power level control command to increase or decrease the transmit power based on the measured parameter.

19. The apparatus in claim 12, wherein the transmit power level of the first base station is adjustable to reduce a difference between the initial second transmit power level and the first transmit power level.

20. The apparatus in claim 12, wherein the base station controller is configured to gradually change the transmit power level of one of the first and second base stations.

21. The apparatus in claim 12, wherein the base station controller is configured to establish a predetermined synchronization time for the handover communication associated with desired transmit power levels for the first base station and the second base station over the first and second traffic channels, respectively.

22. The apparatus in claim 21, wherein the first base station is configured to determine a difference between an actual transmit power level of the first base station at synchronization time and a desired transmit power level of the first base station at synchronization time and adjust the first base station transmit power level over the first traffic channel to decrease the difference.

23. A method comprising coordinating a transmit power level of a serving base station transmitting to a mobile station over a first traffic channel with an initial transmit power level of a target base station transmitting to the mobile station in a diversity handover operation over a second traffic channel, wherein a new transmit power level is determined for the serving base station transmitting to the mobile station over the first traffic channel based on the initial power level of the target base station over the second traffic channel, and the initial and new transmit power levels are provided to the serving and target base stations with a predetermined synchronization time associated with the handover operation.

24. The method in claim 23 applied to a code division multiple access (CDMA) mobile communications system.

25. The method in claim 23 including coordinating respective transmit power levels of plural serving base stations supporting a soft handover communication with the mobile station with the initial transmit power level of the target base station.

26. The method in claim 23, further comprising adjusting the transmit power level at the serving base station to reduce a difference between an actual transmit power level at the predetermined synchronization time and the new transmit power level.

27. The method in claim 26, wherein the reduction is performed gradually.

28. The method in claim 23, further comprising adjusting the transmit power level at one or both of the serving and target base stations to reduce a difference between an actual transmit power level at the predetermined time and the new or initial transmit power level.

29. A power control method for a mobile radio communications system including a controller connected to plural base stations, each base station communicating with mobile stations located within or near a geographical area associated with the base station and having plural sectors each having one or more antennas, comprising:

for a call involving a mobile station, establishing an initial communication between the mobile station and a first sector with the first sector transmitting call traffic to the mobile station at a transmit power level over a first traffic channel;

for the same call, establishing a diversity handover communication between the mobile station and a second sector over a second traffic channel in addition to the initial communication;

determining an initial transmit power level for the second sector for the diversity handover communication transmissions from the second sector to the mobile station over the second traffic channel; and

controlling the transmit power level of one of the first and second sectors to achieve a desired transmit power offset between the first and second power levels.

30. The method in claim 29, wherein the desired offset is approximately zero.

31. The method in claim 29, the controlling step further comprising:

gradually changing the transmit power level of one of the first and second sectors to reduce the difference between the first and second transmit power levels.

32. The method in claim 29, wherein the controlling step includes:

adjusting the transmit power levels of the first and second sectors to the same power level.

33. The method in claim 29, wherein the determining step further includes:

establishing a predetermined time (t₀) associated with a desired transmit power levels for the first sector over the first traffic channel and the initial transmit power level of the second sector over the second traffic channel.

34. The method in claim 33, the controlling step further comprising:

determining a difference between an actual transmit power level of the first sector at time t₀ and the desired transmit power level of the first sector at time t₀.

35. The method in claim 34, wherein if the difference exceeds a threshold, the controlling step further comprising:

comparing the and desired transmit power levels of the first sector;

if the actual transmit power level exceeds the desired transmit power level of the first sector, decreasing the power level of the first sector; and

if the actual transmit power level is less than or equal to the desired transmit power level of the first sector, increasing the power level of the first sector.