1. A method for performing a synchronous hybrid automatic repeat reQuest (HARQ), comprising:allocating a length of a transmission time interval (TTI) with variable size per HARQ process;
transmitting resource allocation information including a duration field corresponding to a length of a TTI allocated to each of the HARQ processes;
determining whether an HARQ error occurs at predetermined timing after the resource allocation information is transmitted; and
when the HARQ error occurs, handling the HARQ error,
wherein a new HARQ round trip time (RTT) for retransmission in a terminal is determined according to the length of the TTI corresponding to the duration field and then a transmission timing for next retransmission is determined according to the new HARQ RTT.