A wireless device may receive one or more radio resource control (RRC) messages comprising configuration parameters of a first uplink carrier and a second uplink carrier of an unlicensed cell. Based on switching from the first uplink carrier to the second uplink carrier and a priority class of a listen-before-talk (LBT) procedure for a first transmission of a transport block on the second uplink carrier, the wireless device may determine a contention window to be a minimum contention window of a plurality of contention windows for the LBT procedure. The wireless device may transmit, via the second uplink carrier, the transport block based on the LBT procedure using the contention window.