Up to 16 HARQ Processes
Each UE supports up to 16 parallel HARQ processes per cell direction. Multiple in-flight transport blocks at any moment, identified by HARQ Process ID (3 or 4 bits in DCI). This pipelines transmission — UE doesn't have to wait for ACK before next TB starts.
NDI · New Data Indicator
NDI bit in DCI tells UE: "is this a new data TB (NDI flipped) or a retransmission (NDI unchanged)?" NDI flip means UE can flush the soft buffer for that HARQ Process. No flip means soft-combine with previous attempt.
RV · Redundancy Version
4 RVs (0, 1, 2, 3) define different starting positions in the rate-matched bit sequence. RV=0 starts with systematic bits (best for first transmission). RV=2 starts with parity (best for retransmission). RV=3 is rare — used for IR.
Soft Combining: Chase vs IR
Chase combining: same coded bits transmitted again, soft-combined at decoder. Incremental Redundancy (IR): different parity bits transmitted, increasing the effective code rate. NR uses IR when RVs differ; Chase when RVs match.
Feedback Timing · k1 and k2
k1: PDSCH→PUCCH ACK delay (signaled in DCI). k2: PDCCH→PUSCH start delay. Both are in slots, BWP-numerology-relative. The UE schedules ACK transmission k1 slots after PDSCH; gNB knows exactly when to listen.
HARQ Codebook · Semi-static vs Dynamic
Semi-static codebook: ACK/NACK bit positions are pre-computed by RRC — fixed structure. Dynamic codebook: positions change based on received DCIs (counter DAI / total DAI fields in DCI signal which slots got PDSCH). Dynamic is more efficient when scheduling is sparse.
Type 3 HARQ Codebook (Rel-16)
A new codebook type added in Rel-16 enables one-shot ACK/NACK reporting for all configured HARQ processes — useful for URLLC and unlicensed spectrum (LBT).
Frequently Asked Questions
How many HARQ processes can 5G NR support?
Up to 16 parallel HARQ processes per UE per cell direction. Each is identified by a 3- or 4-bit HARQ Process ID in DCI.
What is the difference between Chase combining and Incremental Redundancy?
Chase combining: same coded bits transmitted on retransmission, soft-combined at decoder. IR: different parity bits transmitted, lowering the effective code rate. NR uses IR when RVs change between transmissions.
What is the k1 timing parameter?
k1 is the slot offset between PDSCH reception and the corresponding PUCCH ACK/NACK transmission. The DCI scheduling the PDSCH includes k1, telling the UE exactly when to send feedback.
What is a HARQ codebook?
A HARQ codebook is the structure the UE uses to multiplex ACK/NACK bits into PUCCH. Semi-static codebooks have a fixed structure pre-computed by RRC. Dynamic codebooks adapt based on DCI activity.