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

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.

The video below explains the same:

References:

• 3GPP TS 38.214
• 5G NR in Bullets
• The new generation wireless access technology book