1. A method of performing channel access on an unlicensed band by a base station in a wireless communication system, the method comprising:sensing a carrier of an unlicensed band for transmitting a downlink signal; and
transmitting the downlink signal when power detected by sensing the carrier is less than an energy detection threshold that is configured by the base station,
wherein the energy detection threshold is configured to be equal to or less than a maximum energy detection threshold determined by the base station,
wherein when a different radio access technology (RAT) sharing the carrier is able to exist, the maximum energy detection threshold is determined adaptively to a bandwidth of the carrier using a decibel value of a ratio between a reference bandwidth and the bandwidth of the carrier,
wherein the maximum energy detection threshold is configured to be equal to or greater than a first power value which is a sum of a lower bound of the maximum energy detection threshold for the reference bandwidth and the decibel value, and
wherein the first power value is obtained by a first equation ?72+10*log 10(BWMHz/20 MHz) [dBm], where 20 MHz of the first equation corresponds to the reference bandwidth, BWMHz corresponds to the bandwidth of the carrier represented in a unit of MHz, 10*log 10(BWMHz/20 MHz) corresponds to the decibel value, and ?72 corresponds to the lower bound of the maximum energy detection threshold for the reference bandwidth represented in a unit of dBm.