after, method 1). More specifically, when DCI intended for a terminal served by a base station is allocated in a PDCCH region, each PDCCH occupies a resource consisting of one or a plurality of contiguous CCEs. In addition, as the number of CCEs occupied by a PDCCH (i.e., the number of aggregated CCEs: CCE aggregation level), one of aggregation levels 1, 2, 4 and 8 is selected according to the number of information bits of the assignment control information or a propagation path condition of the terminal, for example.
The other method is to previously report a PUCCH resource to each terminal from a base station (i.e., explicit signaling) (hereinafter, method 2). To put it differently, each terminal transmits response signals using the PUCCH resource previously indicated by the base station in method 2.
In method 2, PUCCH resources common to a plurality of terminals (e.g., four PUCCH resources) may be previously indicated to the terminals from a base station. For example, terminals may employ a method to select one PUCCH resource to be actually used, on the basis of a transmit power control (TPC) command of two bits included in DCI in SCell. In this case, the TPC command is called an ACK/NACK resource indicator (ARI). Such a TPC command allows a certain terminal to use an explicitly signaled PUCCH resource in a certain frame while allowing another terminal to use the same explicitly signaled PUCCH resource in another subframe in the case of explicit signaling.
Regarding PUCCH Format 3 and channel selection, which are methods of indicating results of error detection when carrier aggregation is applied, a method of determining PUCCH resources will be described with reference to FIGS. 5A and 5B and FIGS. 6A and 6B.
In PUCCH Format 3, as shown in FIG. 5B, a terminal indicates results of error detection corresponding to a plurality of downlink data items for each downlink component carrier received in a maximum of five downlink component carriers to a base station using PUCCH Format 3 resources or PUCCH Format 1b resou