User terminal, radio communication method and radio communication system

A radio communication method for a radio base station and a user terminal that communicate by means of time division duplexing, in a communication band that is broadbanded by carrier aggregation between a first cell and a second cell, the radio communication method comprising the steps in which the user terminal:

• receives downlink control information for the first cell and downlink control information for the second cell allocated to a downlink control channel of the first cell, by cross-carrier scheduling;
• determines retransmission control for downlink signals transmitted from each cell;
• selects resources for uplink control channels to feed back retransmission control signals;
• and
• feeds back the retransmission control signals to the radio base station using the uplink control channel of the first cell,
• wherein, when a ratio of downlink subframes is higher in the second cell than in the first cell, the resources for the uplink control channel to allocate the retransmission control signals corresponding to the downlink signals of the first cell to, and the resources for the uplink control channel to allocate the retransmission control signals corresponding to the downlink signals of the second cell to, are determined by different methods, so that these resources are distributed.

A radio communication system comprising a user terminal that communicates with a radio base station by means of time division duplexing, in a communication band that is broadbanded by carrier aggregation between a first cell and a second cell, wherein:

• the radio base station comprises:
• a transmitting section that, by applying cross-carrier scheduling, allocates downlink control information for the first cell and downlink control information for the second cell to downlink control channels of the first cell, and transmits the downlink control information;
• a receiving section that receives retransmission signals fed back from the user terminal;
• and
• a retransmission control section that controls retransmission of downlink signals based on the received retransmission control signals;
• the user terminal comprises:
• a receiving section that receives the downlink control information for the first cell and the downlink control information for the second cell transmitted from the radio base station;
• a retransmission control determining section that determines retransmission control for downlink signals transmitted from each cell;
• a transmitting section that feeds back the retransmission signals to the radio base station using the uplink control channel of the first cell;
• and
• a resource selection section that selects resources for the uplink control channels to feed back the transmission control signals;
• and
• when a ratio of downlink subframes is higher in the second cell than in the first cell, the resource selection section determines the resources for the uplink control channel to allocate the retransmission control signals corresponding to the downlink signals of the first cell to, and the resources for the uplink control channel to allocate the retransmission control signals corresponding to the downlink signals of the second cell to, by different methods, so that these resources are distributed.