HARQ
Hybrid Automatic Repeat Request: a combination of forward error correction and ARQ retransmission that enables rapid recovery from transmission errors with incremental redundancy.
HARQ is the fast error-recovery loop that makes a noisy radio link usable. Pure forward error correction adds redundancy up front; pure ARQ retransmits on failure. HARQ does both, and the clever part is what happens to a failed packet: the receiver doesn't discard it. It keeps the soft information and combines it with the retransmission, so even the corrupted first copy contributes to decoding the second.
Two combining styles exist. Chase combining resends an identical copy and the receiver averages them. Incremental redundancy — the NR default — sends different coded bits each time (different redundancy versions), so each retransmission adds new parity rather than repeating, which is more efficient. This is also why schedulers deliberately run around 10% initial BLER: aiming for zero would waste spectrum on an over-cautious MCS, so they push the modulation high and let HARQ mop up the failures. NR runs multiple parallel HARQ processes (up to 16) so a device isn't stalled waiting for one block's feedback while others proceed.
Frequently asked questions
- What's the difference between HARQ and ARQ?
- ARQ simply retransmits a packet that failed and discards the bad copy. HARQ adds forward error correction and, critically, keeps the failed copy: it soft-combines the retransmission with what it already received, so even a corrupted first attempt helps decode the data. In NR, HARQ operates fast at the MAC layer for quick recovery, while a slower ARQ in the RLC layer catches the rare residual failures.
Related terms
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