A communications system is described in which user devices are allocated sub-carriers on which to transmit uplink data to a base station. ACK/NACK messages for the data transmitted on the uplink are then transmitted by the base station on sub-carriers that depend on the sub-carriers used to carry the uplink data. A direct mapping function is preferably used to determine the sub-carriers to be used for the ACK/NACK messages from the uplink sub-carriers. In another embodiment, the ACK/NACK messages are transmitted to the user devices on sub-carriers that are previously identified to the user devices, preferably by transmitting one or more index values to the user device in a control channel thereof.