A base station transmits control information on a subframe of a first carrier starting from the first OFDM symbol of the subframe. The control information provides transmission format and scheduling information for data packets transmitted on a first data channel of the first carrier. The base station transmits a control message on the first data channel to a wireless device. The control message configures radio resources of a second data channel to start from the first OFDM symbol of subframes of a second carrier. The base station transmits data packets on the second data channel.