A method performed by a user equipment, UE, operating in a wireless communication system, the method comprising: receiving ratio information and a threshold value (S1503, S1513) related to a split bearer; determining an uplink data volume for initial transmission that comprises a packet data convergence protocol, PDCP, data volume, wherein the PDCP data volume indicates an amount of data that is available for initial transmission at a PDCP entity of the UE; and wherein the PDCP entity is connected with a first Medium Access Control, MAC, entity (MAC-M) and a second MAC entity (MAC-S) for the split bearer; and wherein, if the uplink data volume for initial transmission is less than the received threshold value, the method further comprises: indicating, to the first MAC entity (MAC-M) configured to communicate with a first base station, BS, in the wireless communication system by the PDCP entity, the determined PDCP data volume as a PDCP data volume for the first MAC entity (MAC-M) for transmitting a buffer status report, BSR, to the first BS by the first MAC entity (MAC-M); and indicating to the second MAC (MAC-S) entity configured to communicate with a second BS in the wireless communication system by the PDCP entity, "0" as a PDCP data volume for the second MAC entity (MAC-S) for not transmitting the BSR to the second BS by the second MAC entity (MAC-S), and wherein, if the uplink data volume for initial transmission is equal to or greater than the received threshold value, the method further comprises: calculating the PDCP data volume for the first MAC entity (MAC-M) and the PDCP data volume for the second MAC entity (MAC-S) based on the ratio information; indicating, to the first MAC entity (MAC-M) by the PDCP entity, the calculated PDCP data volume for the first MAC (MAC-M) entity for transmitting the BSR to the first BS by the first MAC entity (MAC-M); and indicating, to the second MAC entity (MAC-S) by the PDCP entity, the calculated PDCP data volume for the second MAC entity (MAC-S) for transmitting the BSR to the second BS by the second MAC entity (MAC-S). receiving ratio information and a threshold value (S1503, S1513) related to a split bearer; determining an uplink data volume for initial transmission that comprises a packet data convergence protocol, PDCP, data volume, wherein the PDCP data volume indicates an amount of data that is available for initial transmission at a PDCP entity of the UE; and wherein the PDCP entity is connected with a first Medium Access Control, MAC, entity (MAC-M) and a second MAC entity (MAC-S) for the split bearer; and wherein, if the uplink data volume for initial transmission is less than the received threshold value, the method further comprises: indicating, to the first MAC entity (MAC-M) configured to communicate with a first base station, BS, in the wireless communication system by the PDCP entity, the determined PDCP data volume as a PDCP data volume for the first MAC entity (MAC-M) for transmitting a buffer status report, BSR, to the first BS by the first MAC entity (MAC-M); and indicating to the second MAC (MAC-S) entity configured to communicate with a second BS in the wireless communication system by the PDCP entity, "0" as a PDCP data volume for the second MAC entity (MAC-S) for not transmitting the BSR to the second BS by the second MAC entity (MAC-S), and indicating, to the first MAC entity (MAC-M) configured to communicate with a first base station, BS, in the wireless communication system by the PDCP entity, the determined PDCP data volume as a PDCP data volume for the first MAC entity (MAC-M) for transmitting a buffer status report, BSR, to the first BS by the first MAC entity (MAC-M); and indicating to the second MAC (MAC-S) entity configured to communicate with a second BS in the wireless communication system by the PDCP entity, "0" as a PDCP data volume for the second MAC entity (MAC-S) for not transmitting the BSR to the second BS by the second MAC entity (MAC-S), and wherein, if the uplink data volume for initial transmission is equal to or greater than the received threshold value, the method further comprises: calculating the PDCP data volume for the first MAC entity (MAC-M) and the PDCP data volume for the second MAC entity (MAC-S) based on the ratio information; indicating, to the first MAC entity (MAC-M) by the PDCP entity, the calculated PDCP data volume for the first MAC (MAC-M) entity for transmitting the BSR to the first BS by the first MAC entity (MAC-M); and indicating, to the second MAC entity (MAC-S) by the PDCP entity, the calculated PDCP data volume for the second MAC entity (MAC-S) for transmitting the BSR to the second BS by the second MAC entity (MAC-S). calculating the PDCP data volume for the first MAC entity (MAC-M) and the PDCP data volume for the second MAC entity (MAC-S) based on the ratio information; indicating, to the first MAC entity (MAC-M) by the PDCP entity, the calculated PDCP data volume for the first MAC (MAC-M) entity for transmitting the BSR to the first BS by the first MAC entity (MAC-M); and indicating, to the second MAC entity (MAC-S) by the PDCP entity, the calculated PDCP data volume for the second MAC entity (MAC-S) for transmitting the BSR to the second BS by the second MAC entity (MAC-S).