HARQ Optimization for NR NTN

Ref: 38.821

Options for HARQ Optimization

HARQ Process Count and Feedback:

  • Option 1: Keep 16 HARQ processes, disable UL HARQ feedback, and rely on RLC ARQ.
  • Option 2: Increase HARQ processes beyond 16, enable UL HARQ feedback, and maintain a 4-bit HARQ process ID field.

Solutions for Managing >16 HARQ Process IDs:

  1. Slot-number-based approach.
  2. Virtual process ID with retransmission timing restrictions.
  3. Reuse HARQ process ID within RTT time window.
  4. Reinterpret existing DCI fields with higher-layer assistance.

HARQ Enhancements for Soft Buffer Management

  1. Pre-emptive HARQ to reduce stop-and-wait time.
  2. Configurable HARQ buffer usage per UE and HARQ process.
  3. HARQ buffer status reports from the UE.

Recommendations and Next Steps

  1. Use adaptive approaches to configure HARQ processes and feedback mechanisms based on system conditions (e.g., RTT, channel type, UE density).

  2. Further investigate trade-offs among HARQ process count, RLC ARQ dependency, and buffer management.

  3. Incorporate traffic patterns, mobility, and scheduling in system-level simulations.

  4. Ensure backward compatibility for UEs with limited HARQ process support.

  5. Engage in specification development to refine HARQ feedback, buffer size, and process count configurations.

“Though Many Ue & gNB stack vendor started supporting “disable UL HARQ feedback” still many works need to do to manage the Latency and improve Overall Throughput.”

HARQ Optimization for NR NTN

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