BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and more particularly, to a random access method in a wireless communication system, and an apparatus using the method.

Discussion of the Related Art

With the explosive increase in mobile data traffic in recent years, a service provider has utilized a wireless local area network (WLAN) to distribute the data traffic. Since the WLAN uses an unlicensed band, the service provider can address a demand for a significant amount of data without the cost of an additional frequency. However, there is a problem in that an interference phenomenon becomes serious due to a competitive WLAN installation between the providers, quality of service (QoS) cannot be guaranteed when there are many users, and mobility cannot be supported. As one of methods for compensating this, a long term evolution (LTE) service in the unlicensed band is emerged.

LTE in unlicensed spectrum (LTE-U) or licensed-assisted access using LTE (LAA) is a technique in which an LTE licensed band is used as an anchor to combine a licensed band and an unlicensed band by the use of carrier aggregation (CA). A user equipment (UE) first accesses a network in the licensed band. A base station (BS) may offload traffic of the licensed band to the unlicensed band by combining the licensed band and the unlicensed band according to a situation.

The LTE-U may extend an advantage of LTE to the unlicensed band to provide improved mobility, security, and communication quality, and may increase a throughput since the LTE has higher frequency efficiency than the legacy radio access technique.

Unlike the licensed band in which exclusive utilization is guaranteed, the unlicensed band is shared with various radio access techniques such as the WLAN. Therefore, each communication node acquires a channel to be used in the unlicensed band in a contention-based manner, and this is called a carrier sense multiple access with collision avoidance (CSMA/CA). Each communication node must perform channel sensing before transmitting a signal to confirm whether a channel is idle, and this is called clear channel assessment (CCA).

A random access is a procedure for the UE to acquire uplink synchronization with a serving cell. Since the conventional random access procedure is designed under assuming that operating in a licensed band, it may be inefficient in the unlicensed band considering an interference between communication nodes.

SUMMARY OF THE INVENTION

The present invention provides a random access method operating in a unlicensed band and an apparatus using the method.

In an aspect, a random access method for an unlicensed band is provided. The method includes transmitting, by a user equipment (UE), a random access preamble in a first cell of the unlicensed band, and receiving, by the UE, a random access response in a second cell. The second cell is a cell of the unlicensed band, which is included in a cell group to which the first cell belongs, or a cell of the unlicensed band, which is included in a timing advance group (TAG) within the cell group to which the first cell belongs.

The second cell may be a cell of the unlicensed band in which downlink (DL) transmission for the UE is scheduled most quickly after the random access preamble is transmitted in the cell group.

The method may further include transmitting, by the UE, a scheduled message in the third cell according to an uplink (UL) grant in the random access response.

The third cell may be a cell of a licensed band in the cell group to which the first cell belongs.

The random access response may comprise information regarding the third cell.

The method may further include adjusting, by the UE, a UL transmission timing for a target cell on the basis of a time alignment command (TAC) comprised in the random access response. The random access response may comprise information regarding the target cell.

In another aspect, an apparatus for performing a random access by using an unlicensed band is provided. The apparatus includes a radio frequency (RF) unit, and a processor for controlling the RF unit and configured to control the RF unit to transmit a random access preamble in a first cell of the unlicensed band and control the RF unit to receive a random access response in a second cell. The second cell is a cell of the unlicensed band, which is included in a cell group to which the first cell belongs, or a cell of the unlicensed band, which is included in a timing advance group (TAG) within the cell group to which the first cell belongs.

A random access may be performed by considering interference with various communication nodes in an unlicensed band.

