Method for performing a change of mode in devices directly communicating with each other in a wireless connection system, and apparatus for same

A method of performing a mode change, which is performed by a first device performing a device-to-device communication in a wireless access system, comprising the steps of:

• determining whether a mode change condition for performing a handover to a cellular network is satisfied while performing the device-to-device communication;
• transmitting a RACH request message to a base station when the mode change condition is satisfied;
• transmitting a mode change indication message to the base station via a resource obtained by transmitting the RACH request message;
• receiving a mode change command message from the base station to initiate the mode change;
• transmitting a first data packet index for a data packet, which is lastly transmitted to a second device via the device-to-device communication, to the base station,
• when a device-to-device communication termination request message is received from the base station, terminating the device-to-device communication with the second device;
• and
• transmitting data to the second device via the cellular network,
• wherein the device-to-device communication termination request message includes a third data packet index for a data packet firstly transmitted via the cellular network by comparing the first data packet index and a second data packet index, and
• wherein the second data packet index indicates a data packet which is lastly received by the second device via the device-to-device communication.

A device for performing a mode change while performing a device-to-device communication in a wireless access system, comprising:

• a radio frequency (RF) unit configured to comprise a transmitter and a receiver;
• and
• a processor configured to support the mode change,
• the processor configured to:
• determine whether a mode change condition for performing a handover to a cellular network is satisfied while performing the device-to-device communication,
• transmit a mode change indication message to a base station via the RF unit when the mode change condition is satisfied,
• receive a mode change command message from the base station via the RF unit to initiate the mode change,
• transmit a first data packet index for a data packet, which is lastly transmitted by a second device via the device-to-device communication, to the base station via the RF unit, when a device-to-device communication termination request message is received from the base station,
• terminate the device-to-device communication with the second device, and
• transmit data to the second device via the cellular network using the RF module,
• wherein the device-to-device communication termination request message includes a third data packet index for a data packet firstly transmitted via the cellular network by comparing the first data packet index and a second data packet index, and
• wherein the second data packet index indicates a data packet which is lastly received by the second device via the device-to-device communication.