Radio communication mobile station device and response signal spread sequence control method

A terminal apparatus comprising:

• a deciding section, embodied on a processor, configured to decide a resource index of an uplink control channel to which a response signal to downlink data is mapped;
• and
• a control section configured to determine a sequence, which is used for the response signal, from the decided resource index of the uplink control channel,
• wherein:
• a downlink control channel is allocated to one or several consecutive control channel element(s) (CCE(s)) in a search space, which is comprised of CCEs corresponding to a plurality of downlink control channel candidates to be attempted to be decoded;
• said deciding section decides the resource index of the uplink control channel based on the CCE(s), to which the downlink control channel is allocated;
• and
• the search space for each aggregation size corresponding to a number of the CCE(s), to which the downlink control channel is allocated, is comprised of CCEs that are defined by starting on a CCE with a CCE number, which depends on both a total number of CCEs in a subframe and the aggregation size, and at least part of the search space for the each aggregation size is comprised of CCEs with consecutive CCE numbers.