Method for triggering power headroom reporting in a dual connectivity system and a device therefor

1. A method for a User Equipment (UE) operating in a wireless communication system, the method comprising:

configuring the UE with a first Medium Access Control (MAC) entity corresponding to a first e-NodeB (eNB) and a second MAC entity corresponding to a second eNB for a radio bearer, wherein a power headroom reporting (PHR) of different MAC entities in the UE operate independently;

triggering PHRs and generating PHR MAC control elements (CEs) in the first MAC entity and the second MAC entity respectively, if a first type event occurs in the second MAC entity;

triggering a PHR and generating a PHR MAC CE in the second MAC entity only, if a second type event occurs in the second MAC entity; and

transmitting the PHR MAC CE generated by the first MAC entity to the first eNB, or transmitting the PHR MAC CE generated by the second MAC entity to the second eNB,

wherein the first type event includes: there is an uplink (UL) transmission on any cell of the second MAC entity, and a required power backoff for the any cell of the second MAC entity has changed more than a threshold, and

wherein the second type event comprises at least one of the following events: a periodicPHR-Timer of the second MAC entity expires, or upon a power headroom reporting functionality for the second MAC entity is configured or reconfigured by upper layers.

2. The method of claim 1 , wherein the first type event further includes: the required power backoff for the any cell of the second MAC entity has changed more than the threshold since a last transmission of a PHR.

3. A User Equipment (UE) operating in a wireless communication system, the UE comprising:

a Radio Frequency (RF) module; and

a processor operably coupled with the RF module and configured to:

configure the UE with a first Medium Access Control (MAC) entity corresponding to a first e-NodeB (eNB) and a second MAC entity corresponding to a second eNB for a radio bearer, wherein a power headroom reporting (PHR) of different MAC entities in the UE operate independently,

trigger PHRs and generate PHR MAC control elements (CEs) in the first MAC entity and the second MAC entity respectively, if a first type event occurs in the second MAC entity,

trigger a PHR and generate a PHR MAC CE in the second MAC entity only, if a second type event occurs in the second MAC entity, and

transmit the PHR MAC CE generated by the first MAC entity to the first eNB, or transmit the PHR MAC CE generated by the second MAC entity to the second eNB,

wherein the first type event: there is an uplink (UL) transmission on any cell of the second MAC entity, and a required power backoff for the any cell of the second MAC entity has changed more than a threshold, and

wherein the second type event comprises at least one of the following events: a periodicPHR-Timer of the second MAC entity expires, or upon a power headroom reporting functionality for the second MAC entity is configured or reconfigured by upper layers.

4. The UE of claim 3 , wherein the first type event further includes: the required power backoff for the any cell of the second MAC entity has changed more than the threshold since a last transmission of a PHR.

5. The method of claim 1 , wherein if the first type event occurs in the second MAC entity, the PHR is triggered in the first MAC entity when a prohibitPHR-timer of the first MAC entity expires, and the PHR is triggered in the second MAC entity, when a prohibitPHR-timer of the second MAC entity expires.

6. The UE of claim 3 , wherein if the first type event occurs in the second MAC entity, the PHR is triggered in the first MAC entity when a prohibitPHR-timer of the first MAC entity expires, and the PHR is triggered in the second MAC entity, when a prohibitPHR-timer of the second MAC entity expires.