Device and method for handling a bearer type change for a radio bearer

1. A communication device for handling a bearer type change, comprising:

at least one storage device; and

at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of:

receiving a first radio resource control (RRC) message on a signaling radio bearer (SRB) from a first base station (BS), wherein the first RRC message configures a data radio bearer (DRB) which is a secondary cell group (SCG) bearer type or a split bearer type, and configures a first packet data convergence protocol (PDCP) configuration for the DRB;

transmitting a first RRC response message to the first BS on the SRB, in response to the first RRC message;

establishing a first PDCP entity for the DRB according to the first PDCP configuration;

transmitting a first plurality of PDCP Service Data Units (SDUs) to a second BS according to the PDCP entity and a TX_NEXT associated to the PDCP entity;

receiving a second RRC message on the SRB from the first BS, wherein the second message configures the DRB from the SCG bearer type or the split bearer type to a master cell group (MCG) bearer type, and configures a second PDCP configuration for the DRB;

transmitting a second RRC response message on the SRB to the first BS, in response to the second RRC message;

releasing the first PDCP entity, and establishing a second PDCP entity according to the second PDCP configuration; and

transmitting a second plurality of PDCP SDUs to the first BS according to the second PDCP entity and a TX_HFN and a Next_PDCP_TX_SN associated to the second PDCP entity.

2. The communication device of claim 1 , wherein the instructions further comprise:

receiving a third plurality of PDCP SDUs from the second BS according to the first PDCP entity and a first RX_NEXT associated to the PDCP entity; and

receiving a fourth plurality of PDCP SDUs from the first BS according to the second PDCP entity and a RX_HDN and a Next_PDCP_RX_SN associated to the second PDCP entity.

3. The communication device of claim 1 , wherein the first BS is a master evolved Node-B (MeNB), and the second BS is a secondary next-generation Node-B (SgNB).

4. A network comprising a first base station (BS) of a first radio access technology (RAT) and a second BS of a second RAT, for handling a bearer type change, comprising:

at least one storage device; and

at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of:

the first BS transmitting a first radio resource control (RRC) message on a signaling radio bearer (SRB) to a communication device, wherein the first RRC message configures a data radio bearer (DRB) which is a secondary cell group (SCG) bearer type or a split bearer type, and configuring a first packet data convergence protocol (PDCP) configuration for the DRB;

the first BS receiving a first RRC response message from the communication device on the SRB, in response to the first RRC message;

the second BS establishing a first PDCP entity for the DRB according to the first PDCP configuration;

the second BS transmitting a first plurality of PDCP Service Data Units (SDUs) to the communication device according to the first PDCP entity and a first TX_NEXT associated to the first PDCP entity;

the first BS transmitting a second RRC message on the SRB to the communication device, wherein the second RRC message configures the DRB from the SCG bearer type or the split bearer type to a master cell group (MCG) bearer type, and configures a second PDCP configuration for the DRB;

the first BS receiving a second RRC response message responding the second RRC message on the SRB from the communication device;

the second BS transmitting the first TX_NEXT or a first COUNT value of a PDCP SDU in the first plurality of PDCP SDUs, to the first BS, when the DRB is changed from the SCG bearer type or the split bearer type to the MCG bearer;

the first BS establishing a second PDCP entity according to the second PDCP configuration; and

the first BS transmitting a second plurality of PDCP SDUs to the communication device according to the second PDCP entity and the first TX_NEXT or according to a second TX_NEXT derived from the first COUNT value.

5. The network of claim 4 , wherein the PDCP SDU of the first plurality of PDCP SDUs indicates that a successful transmission of a corresponding PDCP Protocol Data Unit (PDU) is not confirmed by a lower layer of a PDCP in the second BS.

6. The network of claim 4 , wherein the first BS derives the second TX_NEXT by setting the second TX_NEXT to the first COUNT value.

7. The network of claim 4 , wherein the instructions further comprise:

the second BS receiving a third plurality of PDCP SDUs from the communication device according to the first PDCP entity and a first RX_NEXT associated to the first PDCP entity;

the second BS transmitting the first RX_NEXT or a second COUNT value associated to a missing PDCP SDU, to the first BS; and

the first BS receiving a fourth plurality of PDCP SDUs from the communication device according to the second PDCP entity and the first RX_NEXT, or according to a second RX_NEXT derived from the second COUNT value.

8. The network of claim 7 , wherein the first BS derives the second RX_NEXT by setting the second RX_NEXT to the second COUNT value.

9. A network comprising a first base station (BS) of a first radio access technology (RAT) and a second BS of a second RAT, for handling a bearer type change, comprising:

at least one storage device; and

at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of:

the first BS transmitting a first radio resource control (RRC) message on a signaling radio bearer (SRB) to a communication device, wherein the first RRC message configures a data radio bearer (DRB) which is a secondary cell group (SCG) bearer type or a split bearer type, and configuring a first packet data convergence protocol (PDCP) configuration for the DRB;

the first BS receiving a first RRC response message from the communication device on the SRB, in response to the first RRC message;

the second BS establishing a first PDCP entity for the DRB according to the first PDCP configuration;

the second BS transmitting a first plurality of PDCP Service Data Units (SDUs) to the communication device according to the first PDCP entity and a TX_NEXT associated to the first PDCP entity;

the first BS transmitting a second RRC message on the SRB to the communication device, wherein the second RRC message configures the DRB from the SCG bearer type or the split bearer type to a master cell group (MCG) bearer type, and configures a second PDCP configuration for the DRB;

the first BS receiving a second RRC response message responding the second RRC message on the SRB from the communication device;

the first BS establishing a second PDCP entity according to the second PDCP configuration; and

the first BS transmitting a second plurality of PDCP SDUs to the communication device according to the second PDCP entity, a Next_PDCP_TX_SN and a TX_HFN.

10. The network of claim 9 , wherein the instructions further comprise:

the second BS receiving a third plurality of PDCP SDUs from the communication device according to the first PDCP entity and a RX_NEXT associated to the first PDCP entity;

the second BS transmitting the RX_NEXT or a second COUNT value associated to a missing PDCP SDU, to the first BS;

the first BS receiving a fourth plurality of PDCP SDUs from the communication device according to the second PDCP entity, a RX_HFN and a Next_PDCP_RX_SN.