In general, mobile communication systems have been developed to provide communication services to users in motion. Thanks to rapid technological advancement, mobile communication systems are capable of providing not only voice communication services but also high-speed data communication services.
Recently, 3GPP has been working to standardize specifications for the Long Term Evolution (LTE) system, which is regarded as a next generation mobile communication system. The LTE system is expected to be commercially available in 2010, and aims to realize high-speed packet based communication supporting a data rate of 100 Mbps exceeding existing data rates. With completion of LTE system standardization, to achieve higher data rates, 3GPP started to develop the LTE-Advanced (LTE-A) system by introducing various new communication schemes to the LTE system.
Carrier aggregation (CA) is a representative one of the communication schemes to be newly introduced. Unlike an existing user equipment that uses one downlink carrier and one uplink carrier for data transmission and reception, a user equipment supporting carrier aggregation may use multiple downlink carriers and multiple uplink carriers. To support carrier aggregation involving multiple downlink carriers and multiple uplink carriers, some functions of the user equipment (UE) and the base station (Node B) must be changed. The HARQ (hybrid automatic repeat request) scheme is one of such functions. In HARQ, when data is received from a base station, a user equipment may perform error correction operation on the received data and detect errors using a simple scheme like cyclic redundancy check (CRC) to determine necessity of retransmission. If retransmission is necessary, the user equipment sends a retransmission request to the base station through the uplink channel. However, in the event that carrier aggregation involving multiple carriers is employed, it is necessary to determine, in response to traffic having received through one carrier, another carrier through which HARQ feedback is to be sent.