1. Introduction

Hybrid Automatic Repeat reQuest (HARQ) is a critical mechanism in 5G NR that ensures reliable data transmission by combining forward error correction with retransmissions. In terrestrial networks, HARQ operates efficiently due to low latency and tight feedback loops. However, in Non Terrestrial Networks (NTN), HARQ behavior is fundamentally impacted by long propagation delays, making traditional mechanisms less effective.

This article explores how HARQ operates in NTN, its limitations, and the adaptations introduced in 3GPP Release 17.

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2. HARQ Fundamentals in 5G NR

HARQ operates using a stop and wait mechanism with multiple parallel processes.

1. UE transmits data (Transport Block)
2. Receiver decodes and sends ACK/NACK
3. If NACK is received, retransmission occurs

Key features:

1. Soft combining improves decoding performance
2. Multiple HARQ processes maintain throughput
3. Synchronous operation in uplink and asynchronous in downlink

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3. HARQ Timing in Terrestrial Networks

Parameter	Typical Value
RTT	1–8 ms
HARQ Feedback Delay	Very Low
Number of Processes	8–16
Efficiency	High

Low latency ensures:

1. Fast retransmissions
2. Efficient spectrum utilization
3. Minimal buffering requirements

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4. NTN-Specific Challenges for HARQ

4.1 Long Round Trip Time (RTT)

1. LEO RTT: ~20–50 ms
2. GEO RTT: >500 ms

Impact:

1. Delayed ACK/NACK feedback
2. Increased retransmission cycle time

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4.2 HARQ Process Explosion

1. To maintain throughput, more parallel HARQ processes are required
2. Leads to:
   • Increased UE memory requirements
   • Higher processing complexity

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4.3 Buffering Constraints

1. UE and gNB must store multiple pending HARQ processes
2. Especially critical for:
   • IoT devices
   • Low cost NTN terminals

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4.4 Inefficient Spectral Utilization

1. Delayed feedback leads to idle transmission gaps
2. Reduces system efficiency

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4.5 Link Variability

1. Rapid channel variations due to satellite movement
2. Retransmissions may not experience the same channel conditions

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5. 3GPP Release 17 Approach to HARQ in NTN

Unlike other mechanisms, HARQ in NTN is partially de emphasized due to its inefficiency under high latency.

5.1 HARQ Reduction Strategy

1. Fewer HARQ processes configured
2. In some cases, HARQ retransmissions are minimized

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5.2 Reliance on Forward Error Correction (FEC)

1. Stronger coding schemes (e.g., LDPC)
2. Reduces dependency on retransmissions

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5.3 HARQ less or HARQ lite Operation

1. Particularly for:
   • NB IoT NTN
   • Delay-tolerant services
2. Benefits:
   • Lower complexity
   • Reduced buffering

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5.4 Configurable Feedback Timing

1. Flexible ACK/NACK timing
2. Adapted to NTN delay profiles

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6. Simplified NTN HARQ Operation Flow

1. UE transmits data to satellite
2. Satellite relays to gateway/gNB
3. gNB processes data and generates ACK/NACK
4. Feedback sent back via satellite
5. UE receives feedback after significant delay
6. Retransmission triggered if required

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7. Comparison: Terrestrial vs NTN HARQ

Feature	Terrestrial NR	NTN NR (Rel-17)
RTT	Very Low	Very High
HARQ Efficiency	High	Reduced
HARQ Processes	Moderate	Increased
Buffer Requirements	Low	High
Retransmission Speed	Fast	Slow
FEC Dependency	Moderate	High

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8. Practical Deployment Insights

8.1 Throughput vs Reliability Trade off

1. Operators must balance:
   • Higher coding (reliability)
   • Reduced retransmissions (efficiency)

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8.2 UE Design Impact

1. Increased memory for HARQ buffers
2. Higher power consumption

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8.3 Service Differentiation

1. HARQ suitable for:
   • Low latency services (limited NTN use)
2. HARQ-less approaches suitable for:
   • IoT
   • Messaging services

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8.4 Scheduler Dependency

1. HARQ inefficiency shifts burden to scheduler
2. Requires:
   • Intelligent resource allocation
   • Predictive transmission strategies

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9. Future Enhancements (Release 18 and Beyond)

1. AI-driven retransmission strategies
2. Adaptive HARQ enabling/disabling
3. Cross layer optimization with transport protocols
4. Enhanced coding techniques reducing retransmission dependency

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10. Conclusion

HARQ, a cornerstone of terrestrial 5G reliability, faces fundamental limitations in NTN due to long delays and dynamic channel conditions.

Instead of scaling HARQ directly, NTN design shifts toward:

1. Stronger forward error correction
2. Reduced reliance on retransmissions
3. Flexible and adaptive feedback mechanisms

Understanding these changes is essential for designing efficient satellite based 5G systems.

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