A wireless device receives configuration parameters of a first uplink carrier and a second uplink carrier of an unlicensed cell. A first contention window is determined for a first listen before talk (LBT) procedure for a first channel of the first uplink carrier. Based on switching from the first uplink carrier to the second uplink carrier, a second contention window is determined to be a minimum contention window, of a plurality of contention windows, for a second LBT procedure of a second channel of the second uplink carrier. A transport block is transmitted, via the second uplink carrier, based on the second LBT procedure using the second contention window.