A base station transmits downlink control information on a first carrier starting from the first symbol of a first subframe and provides first transmission format and scheduling information for first data packets to be transmitted on a first data channel of the first carrier. The base station transmits control message(s) configuring radio resources of a second control channel of a second carrier. The radio resources comprise one or more sets of resource blocks in a subset of subframes. The control message indicates a starting OFDM symbol in a subframe and a frequency allocation for said second control channel.