A base station transmits radio resource control message(s) to a wireless device configuring second carrier(s). Each of the second carrier(s) is assigned to a carrier group in a plurality of carrier groups comprising a first carrier group and at least one second carrier group. The control message(s) comprise parameters. If parameters comprise a carrier group index for the second carrier, the wireless device assigns the second carrier to one of the at least one second carrier group identified by the carrier group index. Otherwise, the wireless device assigns the second carrier to the first carrier group.