Apparatuses, methods, and systems are disclosed for contention window size adjustment. One apparatus includes a transmitter that transmits data on a carrier to a set of devices in a first transmission burst having a duration of at least one subframe. In some embodiments, the set of devices includes one or more devices. In various embodiments, the apparatus includes a receiver that receives feedback information from each device. In certain embodiments, the apparatus includes a processor that determines, based on the feedback information, whether interference above a predetermined level exists on the carrier during the first transmission burst at each device, adjusts a contention window size based on the determination of whether interference above the predetermined level exists on the carrier during the first transmission burst at each device, and determines a value N between a predetermined minimum contention window size and the adjusted contention window size.