A method for performing downlink transmission in an unlicensed cell by a base station by aggregating the unlicensed cell and a licensed cell in a cellular wireless communication system, the method comprising: transmitting downlink channels on the unlicensed cell; receiving a plurality of hybrid automatic repeat request acknowledgement, HARQ-ACK, responses for the downlink channels, wherein the HARQ-ACK responses include at least one of positive acknowledgement, ACK, negative acknowledgement, NACK, or discontinuous transmission, DTX; determining a contention window size based on the plurality of HARQ-ACK responses; in response to determining that the unlicensed cell is idle for a predetermined time, verifying that the unlicensed cell is idle for a random time; and performing the downlink transmission on the unlicensed cell in response to the unlicensed cell being idle for the random time in the contention window, 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 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 set 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 unlicensed 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 on the unlicensed cell; receiving a plurality of hybrid automatic repeat request acknowledgement, HARQ-ACK, responses for the downlink channels, wherein the HARQ-ACK responses include at least one of positive acknowledgement, ACK, negative acknowledgement, NACK, or discontinuous transmission, DTX; determining a contention window size based on the plurality of HARQ-ACK responses; in response to determining that the unlicensed cell is idle for a predetermined time, verifying that the unlicensed cell is idle for a random time; and performing the downlink transmission on the unlicensed cell in response to the unlicensed cell being idle for the random time in the contention window, 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 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 set 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 unlicensed 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 unlicensed 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.