5G • Spectrum
HARQ
Hybrid Automatic Repeat Request
The fast Layer 1/2 error correction mechanism combining Forward Error Correction (FEC) and automatic retransmissions.
Technical Explanation
In 5G NR, HARQ operates at the MAC/PHY layer. When the receiver detects corrupted transport blocks via CRC check, it buffers the soft bits and requests a retransmission with a NACK. 5G NR enhances HARQ with flexible timing (asynchronous DL and UL HARQ), Code Block Group (CBG) based retransmissions (retransmitting only failed CBGs instead of entire transport blocks to save bandwidth), up to 16 HARQ processes per cell, and low-latency HARQ feedback for URLLC.
Key Functions
- Fast acknowledgment/negative acknowledgment (ACK/NACK) feedback on PUCCH/DCI
- Code Block Group (CBG) level selective retransmission
- Incremental Redundancy (IR) combining with LDPC (Low-Density Parity-Check) channel coding
- Asynchronous adaptive retransmission in both Downlink and Uplink
Specifications
3GPP TS 38.212, TS 38.213, TS 38.321
Interfaces
Uu MAC and Physical Layer
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