5G Uplink HARQ Timing Explained | PUCCH/PUSCH ACK/NACK Mapping, K1 Delay & DCI Traces (Post + Video)

Mohamed ELAdawi

29 Aug 2025 — 2 min read

Introduction

In 4G FDD, the uplink HARQ feedback timing was fixed. In 5G, this changes because of TDD operation and low-latency requirements. 5G introduces two methods for uplink HARQ feedback:

Flexible timing
Multiplexing timing
This allows the network to optimize feedback delay depending on system configuration and UE capability.

Flexible Uplink HARQ Timing

In flexible mode, the feedback is not fixed. The UE receives a parameter called K1, which defines after how many slots the ACK/NACK should be sent.
- Example: If K1=4, the feedback is sent 4 slots after receiving the data.
- Other offsets like 7 or 8 can also be used, depending on network setup.

This approach gives 5G the flexibility needed in dynamic TDD systems.

Multiplexing Uplink HARQ Timing

In multiplexing mode, multiple HARQ feedbacks can be sent at the same uplink timing.
- The gNB manages this by assigning PUCCH resources to each UE to avoid collisions.
- PUCCH formats 2/3/4 are required when more than two ACK/NACKs are sent at the same time (since PUCCH format 0 supports only 2 HARQ ACKs).

UE Capability Processing – Challenges in UR Processing Time

Baseline & Variable Timing: All devices support baseline processing times, with some supporting faster processing.
- Minimum baseline for 5G devices is ~0.36–0.57 ms, much lower than LTE Rel-8 which required around 2.3 ms.
Reference Signal Dependency: Additional reference signals delay processing start, increasing overall time.
Faster than LTE: Despite processing delays, NR is significantly faster than LTE, prioritizing low-latency design.

Traces & Real Example (Refer to the video below for better understanding)

From the traces, we can see how the UE gets the exact K1 timing:

The DCI message provides parameters like HARQ process ID and PDSCH-to-HARQ feedback timing.
The UE then maps these bits to the RRC-configured dl-DataToUL-ACK values, which finally tell it when to send ACK/NACK.
Example: Slot 4 with K1=4 means the feedback will be sent 4 slots later.

*Refer to the video below for better understanding

DCI Fields Explanation

✅ In summary:

5G NR uplink HARQ timing is more flexible and efficient compared to 4G, supporting dynamic TDD and low latency applications.
By checking logs and DCI, we can estimate minimum and maximum HARQ timing, which is critical for network design and troubleshooting.