New control signaling is associated to a protocol data unit conveying user data in a mobile communication system and to the control channel signal. A mobile station and a base station and their respective operation are described in view of the newly defined control channel signals. To reduce the control channel overhead, a common field is defined for the transport format and redundancy version in the control channel information format. According to one approach, the common field is used to jointly encode transport format and redundancy version therein. According to another aspect, one shared field is provided on the control channel signal that indicates either a transport format or a redundancy version depending on whether the control channel signal relates to an initial transmission or a retransmission. In another embodiment, further enhancements to an HARQ protocol address certain error cases.