A telecommunications system communicating data to/from one or more terminals, including one or more transmitters to transmit first and second carriers over respective first and second frequency bandwidths. During a first time period the transmitters can transmit data receivable by first and second classes of terminal on the first and second carriers. During a second different time period, the transmitters can transmit data to the second class of terminal on the first and second carriers in combination. A data transmission format on the first carrier in the first time period is incompatible with a user data transmission format on the second carrier during the first time period. The transmitters can transmit control data on the first carrier in the first time period which is receivable by the second class of terminal to enable the second class of terminal to maintain synchronization with the first carrier during the first time period.