A method for performing downlink transmission in a specific cell which is an unlicensed cell by a base station by aggregating the specific cell and a licensed cell in a cellular wireless communication system, the method comprising: transmitting downlink channels in at least one downlink subframe for a downlink burst; receiving a plurality of hybrid automatic repeat request acknowledgement, HARQ-ACK, responses for the downlink channels of the specific cell, wherein the HARQ-ACK responses may include positive acknowledgement, ACK, negative acknowledgement, NACK, or discontinuous transmission, DTX; determining a contention window size based on the plurality of HARQ-ACK responses; generating a random number N, N≥0, within the contention window size; and performing the downlink transmission in the specific cell after standing by for N slots while the specific cell is idle, wherein when a ratio of NACK responses in the plurality of HARQ-ACK responses is equal to or more than a reference value, the contention window size becomes larger than a previous value, and wherein when the ratio of the NACK responses in the plurality of HARQ-ACK responses is less than the reference value, the contention window size is reset to a minimum value, characterized in that when the plurality of HARQ-ACK responses include DTX: the DTX is counted as NACK in the ratio of NACK responses when the downlink channels are scheduled by a control channel transmitted on the specific cell, and the DTX is excluded for determining the contention window size by not being counted in the ratio of NACK responses when the downlink channels are scheduled by a control channel transmitted on the licensed cell. transmitting downlink channels in at least one downlink subframe for a downlink burst; receiving a plurality of hybrid automatic repeat request acknowledgement, HARQ-ACK, responses for the downlink channels of the specific cell, wherein the HARQ-ACK responses may include positive acknowledgement, ACK, negative acknowledgement, NACK, or discontinuous transmission, DTX; determining a contention window size based on the plurality of HARQ-ACK responses; generating a random number N, N≥0, within the contention window size; and performing the downlink transmission in the specific cell after standing by for N slots while the specific cell is idle, wherein when a ratio of NACK responses in the plurality of HARQ-ACK responses is equal to or more than a reference value, the contention window size becomes larger than a previous value, and wherein when the ratio of the NACK responses in the plurality of HARQ-ACK responses is less than the reference value, the contention window size is reset to a minimum value, characterized in that when the plurality of HARQ-ACK responses include DTX: the DTX is counted as NACK in the ratio of NACK responses when the downlink channels are scheduled by a control channel transmitted on the specific cell, and the DTX is excluded for determining the contention window size by not being counted in the ratio of NACK responses when the downlink channels are scheduled by a control channel transmitted on the licensed cell. the DTX is counted as NACK in the ratio of NACK responses when the downlink channels are scheduled by a control channel transmitted on the specific cell, and the DTX is excluded for determining the contention window size by not being counted in the ratio of NACK responses when the downlink channels are scheduled by a control channel transmitted on the licensed cell.