Method for transmitting downlink signal of user equipment having dual connectivity in heterogeneous cell environment

1. A method for transmitting a downlink (DL) signal of a user equipment (UE) with dual connectivity by a small cell in a heterogeneous cell deployment in which a macro cell and the small cell coexist, the method comprising:

checking, by the small cell, that the UE performs handover to a second macro cell from a first macro cell;

determining whether the small cell supports dual connectivity of the UE together with the second macro cell, upon the checking that the UE performs the handover;

when an X2 interface with the second macro cell is capable of being formed and dual connectivity of the UE is not capable of being supported, transmitting, by the small cell, a first data forwarding request message for requesting the second macro cell to transmit DL data of the small cell to the UE;

receiving from the second macro cell, by the small cell, a first data forwarding response message for configuring a radio bearer with the second macro cell;

transmitting to the second macro cell, by the small cell, a serial number (SN) status transfer message including information about a count value of a first packet to be transmitted to the UE by the second macro cell; and

transmitting, by the small cell, the DL data to the second macro cell through the radio bearer configured based on the first data forwarding response message.

2. The method according to claim 1 , further comprising receiving a target PCell information message indicating that the handover is performed, from the first macro cell,

wherein the handover is checked based on the target PCell information message.

3. The method according to claim 1 , further comprising:

when the second macro cell is not capable of transmitting the DL data to the UE, receiving, from the second macro cell, a data forwarding failure message comprising information about a reason why transmission of the DL data is not possible instead of receiving the first data forwarding response message from the second macro cell.

4. The method according to claim 1 , further comprising, when the X2 interface with the second macro cell is not capable of being formed, transmitting a second data forwarding request message to the second macro cell through a mobility management entity (MME) that supports the small cell.

5. The method according to claim 4 , further comprising:

receiving a second data forwarding response message through the MME from the second macro cell in response to the second data forwarding request message; and

transmitting the DL data to the second macro cell through a radio bearer via a serving gateway (S-GW) that supports the small cell.

6. A small cell base station (BS) for transmitting a downlink (DL) signal of a user equipment (UE) with dual connectivity in a heterogeneous cell deployment in which a macro cell and the small cell coexist, the small cell BS comprising:

a transmitter;

a receiver; and

a processor connected to the transmitter and the receiver,

wherein the processor is configured to:

control transmission of a DL signal of the UE,

check that the UE performs handover to a second macro cell from a first macro cell,

determine whether the small cell supports dual connectivity of the UE together with the second macro cell, upon the checking that the UE performs the handover;

when an X2 interface with the second macro cell is capable of being formed and dual connectivity of the UE is not capable of being supported, control the transmitter to transmit a first data forwarding request message for requesting the second macro cell to transmit DL data of the small cell BS to the UE,

control the receiver to receive a first data forwarding response message for configuring a radio bearer with the second macro cell from the second macro cell,

control the transmitter to transmit a serial number (SN) status transfer message including information about a count value of a first packet to be transmitted to the UE by the second macro cell, to the second macro cell, and

control the transmitter to transmit the DL data to the second macro cell through the radio bearer configured based on the first data forwarding response message.

7. The small cell BS according to claim 6 , wherein:

the processor controls the receiver to receive a target primary cell (PCell) information message indicating that the handover is performed, from the first macro cell, wherein the handover is checked based on the target PCell information message.

8. The small cell BS according to claim 6 , wherein, when the second macro cell is not capable of transmitting the DL data to the UE, the processor controls the receiver to receive, from the second macro cell, a data forwarding failure message comprising information about a reason why transmission of the DL is not possible instead of receiving the first data forwarding response message from the second macro cell.

9. The small cell BS according to claim 6 , wherein, when the X2 interface with the second macro cell is not capable of being formed, the processor controls the transmitter to transmit a second data forwarding request message to the second macro cell through a mobility management entity (MME) that supports the small cell BS.

10. The small cell BS according to claim 9 , wherein the processor controls the receiver to receive a second data forwarding response message through the MME from the second macro cell in response to the second data forwarding request message and controls the transmitter to transmit the DL data to the second macro cell through a radio bearer via a serving gateway (S-GW) that supports the small cell BS.