5G NR HARQ timing & slot scheduler visualizer
Interactive 5G NR slot schedule validator for K0, K1, K2 timing offsets, TDD symbol directions, and stop-and-wait HARQ process ACK/NACK lifecycles per TS 38.213 and TS 38.214.
2 scheduled grants across 20 slots
DL Grant #1
DL • DCI slot #100Understanding 5G NR HARQ timing & slot scheduling
In 5G New Radio (NR), the Base Station (gNB) schedules Downlink (PDSCH) and Uplink (PUSCH) transmissions using Downlink Control Information (DCI) carried over PDCCH. To ensure reliable communication and strict latency requirements, physical-layer timing offsets K0, K1, and K2 dictate the exact slot and symbol boundaries where channels and HARQ-ACK feedback occur.
When a UE detects a DL grant DCI in control slot n:
- K0: Slot offset from DCI to PDSCH reception (TS 38.214 §5.1.2.1).
- K1: Slot offset from PDSCH to HARQ-ACK feedback on PUCCH/PUSCH (TS 38.213 §9.2.3).
- SLIV: Start symbol
Sand lengthLwithin the target slot.
When a UE receives an UL grant DCI in control slot n:
- K2: Slot offset from UL grant DCI to PUSCH transmission (TS 38.214 §6.1.2.1).
- TDA table: Selected row from RRC higher-layer time-domain resource allocation list.
- HARQ PID & NDI: Process ID and New Data Indicator for stop-and-wait ARQ protocols.
Stop-and-wait HARQ process state machine
5G NR supports up to 16 (or 32 in Rel-17 extended HARQ) independent parallel stop-and-wait HARQ processes per serving cell. Each process maintains state across transmission iterations:
Frequently asked questions (FAQ)
What causes a HARQ-ACK TDD direction mismatch error?
In TDD deployments, slots are dynamically or semi-statically configured for Downlink (DL), Uplink (UL), or Flexible (FLEX) directions. If a selected K1 offset places the PUCCH HARQ-ACK transmission in a slot or symbol range that is configured as DL-only, the UE cannot transmit uplink control signals, resulting in a timing error per TS 38.213 §11.1.
How does the tool handle imported DCI 1_0 and 0_0 payloads?
When grants are imported from the WirelessBrew DCI Decoder, the tool parses fields such as HARQ process ID, NDI, RV, time-domain allocation index, and PDSCH-to-HARQ timing indicator, mapping them directly into the scheduling timeline for feasibility verification.
What is the typical Redundancy Version (RV) sequence for retransmissions?
Per 3GPP TS 38.214 §5.1.2.1, standard incremental redundancy HARQ progresses through RV values 0 → 2 → 3 → 1. Retransmissions scheduled with unchanged RV or non-standard progression trigger informational warnings in the scheduler inspector.
What is the difference between K0, K1, and K2 timing offsets?
In 3GPP Rel-17 specifications: K0 defines the slot delay between the PDCCH DL grant and the scheduled PDSCH reception; K1 defines the slot delay between PDSCH reception and HARQ-ACK feedback on PUCCH; and K2 defines the slot delay between the PDCCH UL grant and PUSCH transmission.
How are Start and Length Indicator Values (SLIV) evaluated in time-domain scheduling?
SLIV encodes both start symbol S (0–13) and symbol length L (1–14) into a single integer per TS 38.214 Table 5.1.2.1-1. The tool validates that S + L ≤ 14 and that all symbols within the allocation fall on valid TDD symbol directions.
Can multiple grants share the same HARQ process ID simultaneously?
No. Stop-and-wait HARQ protocols require a process to remain occupied from initial transmission until feedback (ACK/NACK) is received or timeout occurs. Scheduling a new transmission (toggled NDI) on an active process before ACK produces a protocol reuse warning.