Connection modification method applicable to base station

1. A connection modification method applicable to a first base station serving a user equipment (UE) capable of dually connecting to the first base station and a second base station, the method comprising:

receiving, by the first base station, a first secondary cell group (SCG) configuration to establish a first SCG bearer between the second base station and the UE from the second base station;

transmitting, by the first base station, a first radio resource control (RRC) message comprising the first SCG configuration, to the UE;

transmitting, by the first base station, a first SCG addition request comprising the first SCG configuration for establishing a second SCG bearer between a third base station and the UE, to the third base station;

receiving, by the first base station, a first SCG addition response comprising a second SCG configuration which is generated by the third base station based on the first SCG configuration in response to transmitting the first SCG addition request, from the third base station;

transmitting, by the first base station, a SCG release request to release the first SCG bearer, to the second base station;

transmitting, by the first base station, a second RRC message comprising the second SCG configuration to establish the second SCG bearer, to the UE, wherein the second SCG configuration includes information different from the first SCG configuration;

receiving, from the second base station, a Packet Data Convergence Protocol (PDCP) service data unit (SDU) of the first SCG bearer, a transmitter hyper frame number (TX_HFN) and a PDCP sequence number (SN), wherein the PDCP SDU is not successfully transmitted and the TX_HFN and PDCP SN are associated with the PDCP SDU; and

transmitting to the third base station the PDCP SDU of the first SCG bearer, the TX_HFN and the PDCP SN in response to receiving from the second base station the PDCP SDU of the first SCG bearer, the TX_HFN and the PDCP SN.

2. The method of claim 1 , wherein receiving the first SCG configuration comprising:

transmitting a second SCG addition request to the second base station for establishing the first SCG bearer with the UE; and

receiving a second SCG addition response comprising the first SCG configuration from the second base station.

3. The method of claim 1 further comprising:

receiving, from the second base station, a receiver hyper frame number (RX_HFN) and a PDCP SN which are associated with another PDCP SDU which was not received in response to transmitting the SCG release request; and

transmitting, to the third base station, the RX_HFN and the PDCP SN which are associated with the other PDCP SDU.

4. A base station which serves a user equipment (UE) that is capable of dually connecting to the base station and a second base station, the base station comprising:

a transmitter;

a receiver; and

a processing circuit coupled to the transmitter and the receiver and configured to:

receive, via the receiver, a first secondary cell group (SCG) configuration to establish a first SCG bearer between the second base station and the UE from the second base station;

transmit, via the transmitter, a first radio resource control (RRC) message comprising the first SCG configuration, to the UE;

transmit, via the transmitter, a first SCG addition request comprising the first SCG configuration for establishing a second SCG bearer between a third base station and the UE, to the third base station;

receive, via the receiver, a first SCG addition response comprising a second SCG configuration which is generated by the third base station based on the first SCG configuration in response to transmitting the first SCG addition request, from the third base station;

transmit, via the transmitter, a SCG release request to release the first SCG bearer, to the second base station;

transmit, via the transmitter, a second RRC message comprising the second SCG configuration to establish the second SCG bearer, to the UE, wherein the second SCG configuration comprises information different from the first SCG configuration;

receive, from the second base station, a Packet Data Convergence Protocol (PDCP) service data unit (SDU) of the first SCG bearer, a transmitter hyper frame number (TX_HFN) and a PDCP sequence number (SN), wherein the PDCP SDU is not successfully transmitted and the TX_HFN and PDCP SN are associated with the PDCP SDU; and

transmit to the third base station the PDCP SDU of the first SCG bearer, the TX_HFN and the PDCP SN in response to receiving from the second base station the PDCP SDU of the first SCG bearer, the TX_HFN and the PDCP SN.

5. The base station of claim 4 , wherein the processing circuit is configured to receive, via the receiver, the first SCG configuration comprising:

transmit, via the transmitter, a second SCG addition request to the second base station for establishing the first SCG bearer with the UE; and

receive, via the receiver, a second SCG addition response comprising the first SCG configuration from the second base station.

6. The base station of claim 4 , wherein the processing circuit is further configured:

receive, from the second base station via the receiver, a receiver hyper frame number (RX_HFN) and a PDCP SN which are associated with another PDCP SDU which was not received in response to transmitting the SCG release request; and

transmit, to the third base station via the transmitter, the RX_HFN and the PDCP SN which are associated with the other PDCP SDU.