A wireless device receives configuration parameters of a first uplink carrier and a second uplink carrier of an unlicensed cell. A switch is made from the first uplink carrier to the second uplink carrier. A first contention window size determining based on the switch. Based on the first contention window size, a listen before talk procedure is performed for transmission of a transport block in one or more subbands of the second uplink carrier. A transport block is transmitted based on the listen before talk procedure indicating a clear channel.