Methods and apparatuses are provided for transmitting a set of acknowledgement (ACK) bits, by a user equipment (UE). A first subset of ACK bits is transmitted. A second subset of ACK bits is transmitted. The first subset of ACK bits is a response to a reception of one or more scheduling assignments (SAs) for a first cell group, and the second subset of ACK bits is a response to a reception of one or more SAs for a second cell group. The first subset of ACK bits is different from the second subset of ACK bits. A union of the first subset of ACK bits and the second subset of ACK bits is the set of ACK bits that the UE transmits in a subframe.