ARQ
1. Works at RLC layer
2. If the received data has an error (as detected by ARQ) then it is discarded, and a new re-transmission is requested from the sender
HARQ
1. Works at PHY layer but controlled by MAC layer
2. If the received data has an error then the Receiver buffers the data and requests a re-transmission from the sender. When the receiver receives the re-transmitted data, it then combines it with buffered data prior to channel decoding and error detection. This helps the performance of the re-transmissions.
The ARQ functionality provides error correction by retransmissions in AM at Layer 2,where as the HARQ functionality ensures delivery between peer entities at Layer 1.
The reasons for having a two-level retransmission structure,is to achieve both fast and reliable transmission. The hybrid-ARQ mechanism provides very fast retransmissions which is suitable for high speeds used in LTE, whereas the ARQ is responsible for reliability. HARQ feedback is fast and frequent to correct transmission errors as soon as possible, so that the end-to-end RTT is low. HARQ feedback is more susceptible to error as compared to ARQ feedback. For high-speed transmissions using protocols like TCP, low packet error rate is desirable and ARQ adds robustness to the transmission. In addition, services like voice which place more importance on the RTT might skip ARQ and use the UM mode. Thus, the combination of HARQ and ARQ can be used intelligently depending on the services and their error and delay requirements.
In LTE-FDD, eight HARQ processes are available in both downlink and uplink. The typical delay between the original transmission and the first retransmission is 8 ms. The HARQ process to which a transport block belongs is identified by a unique three bit HARQ process IDentifier (HARQ ID).Hence,MAC will do HARQ retransmission once it receives NACK from from the intended receiver for the corresponding HARQ processor id. It is operational on both the UL and the DL. For the ARQ operation there is a RLC SN which will be used when sending the ACK/NACK message.
