HARQ stands for "Hybrid Automatic Repeat Request" . HARQ mechanism is used to make successful delivery of data from eNB MAC layer to UE MAC layer or vice a versa . Maximum 8 harq processes can be run in the system. HARQ process is run by MAC layer and controlled by PHY layer. LTE utilizes Incremental Redundancy HARQ with a 1/3 turbo encoder used for forward error correction. In LTE, a user can be scheduled as short as 1 millisecond. As one HARQ process is stop and wait mechanism. Suppose a HARQ process transmit a data at N millisecond then It will stop and it will not transmit a single bit and wait till N+4 millisecond for ACK/NACK. That's why multiple HARQ processes used to run in parallel to achieve high throughput and spectral efficiency.
In Incremental HARQ, transmitter doesn't send same data when it receives a NACK from receiver. Even receiver store the corrupt data and wait for new data from transmitter. Transmitter sendssome different coded bits so that receiver can combine new data with corrupt data and increase the possibility to correct the data itself.
It is useful since transmitter not need to send the complete data when it receives NACK from receiver.
Even there a ARQ mechanism is still there in RLC layer. It is used when max harq retransmission gets over ed