Hybrid automatic repeat request (HARD) is a combination of high-rate forward error-correcting coding and ARQ error-control. In standard ARQ, redundant bits are added to the data to be transmitted using an error-detecting code such as a cyclic redundancy check (CRC), with receivers detecting a corrupted message requesting a new message from the sender. In Hybrid ARQ, the original data is encoded with a FEC (forward error correction or forward error coding) code, and the parity bits are either immediately transmitted along with the message, or they are transmitted only upon request by a receiver that has detected an erroneous message. The FEC code is typically used to correct an expected subset of all errors that may occur, while the ARQ provides a fallback to correct errors that cannot be corrected through the use of only the redundancy included in the initial transmission. Therefore, hybrid ARQ offers better performance in poor signal conditions, but at the expense of significantly lower throughput during good signal conditions. A signal quality crossover point may be defined, below which simple HARQ may be preferred, and above which basic ARQ may be used.

Long Term Evolution (LTE) has become the technology of choice for the majority of wireless network operators worldwide, providing mobile broadband data and high-speed Internet access to their subscriber base. LTE defines a number of downlink (DL) physical channels, categorized as transport or control channels, to carry information blocks received from the MAC and higher layers. LTE also defines three physical layer channels for the uplink (UL).