Systems, methods, and user equipment can involve transmitting Acknowledgement/Negative Acknowledgement (ACK/NACK) bits for carrier aggregation between a first cell and a second cell in a User Equipment (UE). With the UE, for a sub-frame, a first number of ACK/NACK bits for the first cell can be compared with a second number of ACK/NACK bits for the second cell. If a first number of ACK/NACK bits for the first cell is less than a second number of ACK/NACK bits for the second cell, an ACK/NACK bit position from the first cell can be used to transmit an ACK/NACK bit for the second cell. In some implementations, one or more DTX bits can be used to set the number of ACK/NACK bits in the first cell equal to the number of ACK/NACK bits in the second cell.