hybridiARQ

Hybrid automatic repeat request (HARQ) is a protocol for reliable data transmission that combines forward error correction coding with automatic repeat request. Data are protected by an error-correcting code before transmission. If the receiver decodes successfully, an acknowledgment is sent; if errors remain, a negative acknowledgment triggers subsequent transmissions that add redundancy and improve the chances of successful decoding without resending the entire block.

HARQ uses two common approaches. In Chase combining (CC-HARQ), the same coded block is retransmitted and the receiver performs soft combining of the multiple receptions to improve decoding probability. In incremental redundancy (IR-HARQ), retransmissions convey additional parity bits, so the receiver can decode using the accumulated redundant information. Channel codes such as convolutional codes, turbo codes, LDPC, or rate-compatible schemes are employed, depending on the system. HARQ can be designed with different coding rates and modulation schemes to adapt to changing channel conditions.

Operation hinges on feedback and state retention. After each transmission, the receiver sends an ACK or NAK; the transmitter may retransmit or send additional parity bits based on the feedback. HARQ improves reliability and spectral efficiency by blending error correction with limited retransmissions, but it adds latency and complexity, and requires a reliable feedback channel.

Applications of HARQ are widespread in wireless systems, notably cellular networks such as LTE and 5G NR, as well as in some Wi-Fi standards and satellite links, where varying channel conditions benefit from adaptive retransmission strategies.