A method for transmitting acknowledgement/negative acknowledgement (ACK/NACK) signals by an apparatus in a wireless communication system. The apparatus spreads ACK/NACKs using spreading codes of a spreading factor, the spreading factor being one of a first spreading factor equal to 2 and a second spreading factor equal to 4. The apparatus transmits one or more physical hybrid automatic repeat request (HARQ) indicator channel (PHICH) groups allocated in units of four resource elements, each PHICH group carrying up to four ACK/NACK signals for the first spreading factor, and up to eight ACK/NACK signals for the second spreading factor. A total number of allocated PHICH groups for the first spreading factor is determined as twice a total number of allocated PHICH groups for the second spreading factor. Each ACK/NACK signal is identified using an index pair of a PHICH group index and a spreading code index.