HARQ

Hybrid Automatic Repeat reQuest (HARQ) is a protocol for reliable data transmission over noisy or fading channels. It combines forward error correction (FEC) and automatic repeat request (ARQ) to correct errors with limited retransmissions. In HARQ, a data block is encoded with an error-correcting code and transmitted. The receiver attempts decoding; if successful, it sends an ACK; if decoding fails, it sends a NACK prompting a retransmission. HARQ typically uses soft information from previous transmissions to improve decoding efficiency rather than discarding earlier attempts.

Two main HARQ modes are used in practice. Chase combining (CC) retransmits the same coded block; the receiver coherently combines multiple receptions to increase the likelihood of decoding. Incremental redundancy (IR) transmits additional parity information on retransmissions, so the receiver accumulates more redundancy across transmissions. IR generally achieves higher efficiency, especially in varying channel conditions.

HARQ is a core mechanism in modern wireless networks such as LTE and 5G NR. It is implemented with several parallel HARQ processes and a feedback channel that carries ACK/NACK signals to schedule retransmissions. While HARQ improves reliability and throughput and reduces latency in fading channels, it adds control signaling and receiver complexity.