HARQ is a MAC layer functionality which is used for FEC (Forward Error Correction). In this case, receiver stores incorrectly received coded data and the sender sends same transport block (TB) as re-transmitted data with different set of information. Receiver combines both the data blocks i.e. previously received incorrect data block and the received recent one. This concept is known as HARQ with soft combining.
LTE supports two variants of HARQ soft combining one is known as "Incremental Redundancy" and the other one is known as "Chase combining".
That's is the reason RLC layer does not re-transmit PDU just after getting first NACK for HARQ. Since HARQ job is to use either of two available soft combining techniques and try to achieve FEC at the receiver side. In case maximum of no of HARQ re-transmission reaches then RLC ARQ feature comes into picture do take further action.
