Method and Apparatus for Power Headroom Reporting

1. A method for performing Power Headroom Reporting (PHR) in a user equipment (UE) of a wireless communication system, the wireless communication system supporting Carrier Aggregation (CA) such that the UE is able to perform transmission through multiple carriers, the method comprising: configuring a plurality of uplink carriers; and generating a plurality of Power Headroom (PH) values, each corresponding to one of the plurality of uplink carriers; wherein the number of the plurality of PH values is equal to the number of the plurality of uplink carriers; and wherein all of the plurality of PH values are carried by a PHR Medium Access Control (MAC) Control Element and the PHR MAC Control Element has an identification field in the PHR MAC Control Element for identifying which uplink carrier each of the carried PH values is corresponding to.

2. The method of claim 1 , wherein the UE has a plurality of timers, each corresponding to one of the plurality of uplink carriers, for controlling PHR triggering in the plurality of uplink carriers.

3. The method of claim 2 , wherein the plurality of timers are each a periodic timer periodicPHR-Timer, for periodically performing the PHR triggering in the plurality of uplink carriers.

4. The method of claim 2 , wherein the plurality of timers are each a timer prohibitPHR-Timer, for prohibiting the PHR triggering in the plurality of uplink carriers during operation.

5. The method of claim 2 , wherein the number of the plurality of timers is equal to the number of the plurality of uplink carriers.

6. A communication device for performing Power Headroom Reporting (PHR) in a user equipment (UE) of a wireless communication system, the wireless communication system supporting Carrier Aggregation (CA) such that the UE is able to perform transmission through multiple carriers, the communication device comprising: a processor for executing a program code; and a memory coupled to the processor for storing the program code; wherein the program code comprises: configuring a plurality of uplink carriers; and generating a plurality of Power Headroom (PH) values, each corresponding to one of the plurality of uplink carriers; wherein the number of the plurality of PH values is equal to the number of the plurality of uplink carriers; and wherein all of the plurality of PH values are carried by a PHR Medium Access Control (MAC) Control Element and the PHR MAC Control Element has an identification field in the PHR MAC Control Element for identifying which uplink carrier each of the carried PH values is corresponding to.

7. The communication device of claim 6 , wherein the UE has a plurality of timers, each corresponding to one of the plurality of uplink carriers, for controlling PHR triggering in the plurality of uplink carriers.

8. The communication device of claim 7 , wherein the plurality of timers are each a periodic timer periodicPHR-Timer, for periodically performing the PHR triggering in the plurality of uplink carriers.

9. The communication device of claim 7 , wherein the plurality of timers are each a timer prohibitPHR-Timer, for prohibiting the PHR triggering in the plurality of uplink carriers during operation.

10. The communication device of claim 7 , wherein the number of the plurality of timers is equal to the number of the plurality of uplink carriers.

11. The method of claim 1 , wherein the plurality of PH values is carried by a single PHR MAC Control Element.

12. The communication device of claim 6 , wherein the plurality of PH values is carried by a single PHR MAC Control Element.

13. A method for performing Power Headroom Reporting (PHR) for each of a plurality of uplink carriers in a user equipment (UE) of a wireless communication system, the wireless communication system supporting Carrier Aggregation (CA) such that the UE is able to perform transmission through multiple carriers, the method comprising: configuring the plurality of uplink carriers; generating a plurality of Power Headroom (PH) values, each corresponding to one of the plurality of uplink carriers, wherein the number of the plurality of PH values is equal to the number of the plurality of uplink carriers; and triggering the PHR corresponding to a second uplink carrier of the plurality of uplink carriers when the PHR corresponding to a first uplink carrier of the plurality of uplink carriers is triggered, wherein the PHR corresponding to the first uplink carrier is triggered due to a path loss change of the first uplink carrier.

14. The method of claim 13 , wherein the plurality of PH values is carried by a single PHR Medium Access Control (MAC) Control Element.

15. The method of claim 13 , wherein each of the plurality of PH values is transmitted by a corresponding uplink carrier of the plurality of uplink carriers.

16. The method of claim 13 , wherein a timer prohibitPHR-Timer of the second uplink carrier is not running.

17. The method of claim 13 , wherein the UE has a plurality of timers, each corresponding to one of the plurality of uplink carriers, for controlling PHR triggering in the plurality of uplink carriers.

18. The method of claim 17 , wherein the plurality of timers are each a periodic timer periodicPHR-Timer, for periodically performing the PHR triggering in the plurality of uplink carriers.

19. The method of claim 17 , wherein the plurality of timers are each a timer prohibitPHR-Timer, for prohibiting the PHR triggering in the plurality of uplink carriers during operation.

20. The method of claim 17 , wherein the number of the plurality of timers is equal to the number of the plurality of uplink carriers.

21. A communication device for performing Power Headroom Reporting (PHR) for each of a plurality of uplink carriers in a user equipment (UE) of a wireless communication system, the wireless communication system supporting Carrier Aggregation (CA) such that the UE is able to perform transmission through multiple carriers, the communication device comprising: a processor for executing a program code; and a memory coupled to the processor for storing the program code; wherein the program code comprises: configuring a plurality of uplink carriers; generating a plurality of Power Headroom (PH) values, each corresponding to one of the plurality of uplink carriers, wherein the number of the plurality of PH values is equal to the number of the plurality of uplink carriers; and triggering the PHR corresponding to a second uplink carrier of the plurality of uplink carriers when the PHR corresponding to a first uplink carrier of the plurality of uplink carriers is triggered, wherein the PHR corresponding to the first uplink carrier is triggered due to a path loss change of the first uplink carrier.

22. The communication device of claim 21 , wherein the plurality of PH values is carried by a single PHR Medium Access Control (MAC) Control Element.

23. The communication device of claim 21 , wherein the plurality of PH values is transmitted by a corresponding uplink carrier of the plurality of uplink carriers.

24. The communication device of claim 21 , wherein a timer prohibitPHR-Timer of the second uplink carrier is not running.

25. The communication device of claim 21 , wherein the UE has a plurality of timers, each corresponding to one of the plurality of uplink carriers, for controlling PHR triggering in the plurality of uplink carriers.

26. The communication device of claim 25 , wherein the plurality of timers are each a periodic timer periodicPHR-Timer, for periodically performing the PHR triggering in the plurality of uplink carriers.

27. The communication device of claim 25 , wherein the plurality of timers are each a timer prohibitPHR-Timer, for prohibiting the PHR triggering in the plurality of uplink carriers during operation.

28. The communication device of claim 25 , wherein the number of the plurality of timers is equal to the number of the plurality of uplink carriers.