Ack Nack Repetition
As you may guess from the terminoloty itself, ACK NACK repetition is a method in which a sender (UE in current specification) transmit ACK/NACK multiple times in a row. In an aspect, we can say it is similar concept to TTI Bundling. The difference between Ack Nack Repetition and TTI Bundling is that TTI bundling is for PUSCH transmission (user data transmission) and Ack Nack Repetition is for PUCCH transmission.
Overall Ack Nack Repetition flow can be illustrated as shown below.
The advantage and disadvatage of Ack/Nack repetition is similar to that of TTI bundling. In short, the advantage is that it increase the probability of Ack/Nack information at destination. The disadvantage is that it would cause some additional time delay (resulting in low throughput) since there should be a certain time period during which UL scheduling (DCI 0) should not be transmitted.
How to enable this feature (Ack/Nack Repetition) ? Just enable a couple of IEs in RRC message (RRCConnectionSetup or RRCConnectionReconfiguration) as shown below.