1. Introduction

Waveform design and physical layer behavior are at the core of any wireless communication system. In 5G NR, the waveform (CP-OFDM and DFT-s-OFDM) is optimized for terrestrial environments with low latency, limited Doppler, and relatively stable channels.

In Non Terrestrial Networks (NTN), the physical layer must operate under fundamentally different conditions, including:

1. High Doppler shifts
2. Large delay spreads
3. Long propagation delays
4. Rapidly varying channel conditions

This requires careful adaptation of waveform parameters and PHY layer procedures as defined in 3GPP Release 17.

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2. 5G NR Waveforms Overview

5G NR supports two primary waveforms:

1. CP-OFDM (Cyclic Prefix Orthogonal Frequency Division Multiplexing)
   • Used in downlink and uplink
   • Flexible numerology
2. DFT-s-OFDM (Discrete Fourier Transform spread OFDM)
   • Used in uplink
   • Lower Peak to Average Power Ratio (PAPR)

Key PHY features:

1. Scalable subcarrier spacing (SCS)
2. Slot-based transmission structure
3. Advanced channel coding (LDPC, Polar codes)

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3. NTN Specific Physical Layer Challenges

3.1 High Doppler Shift

1. Caused by high speed satellite movement
2. Results in:
   • Frequency offset
   • Inter carrier interference (ICI)

Impact:

1. Degrades OFDM orthogonality
2. Reduces decoding performance

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3.2 Large Delay Spread

1. Due to large coverage areas and varying distances
2. Leads to:
   • Inter symbol interference (ISI)
   • Timing misalignment

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3.3 Long Propagation Delay

1. LEO: 20–50 ms
2. GEO: >500 ms

Impact:

1. Affects HARQ timing
2. Impacts synchronization procedures

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3.4 Channel Variability

1. Beam movement causes time varying channel conditions
2. Link quality changes during transmission

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4. Waveform Adaptations for NTN (3GPP Release 17)

4.1 Subcarrier Spacing (SCS) Optimization

1. Larger SCS helps mitigate Doppler effects
2. Trade off:
   • Higher SCS → reduced coverage
   • Lower SCS → more Doppler sensitivity

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4.2 Extended Cyclic Prefix (CP)

1. Supports larger delay spread
2. Reduces ISI impact

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4.3 Doppler Pre Compensation

1. Applied at UE or satellite
2. Uses GNSS based estimation

Benefits:

1. Maintains OFDM orthogonality
2. Improves demodulation performance

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4.4 Timing Advance Enhancements

1. Supports wider timing range
2. Aligns uplink transmissions despite long delays

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4.5 Flexible Numerology

1. Adapts SCS and slot duration based on NTN scenario
2. Enables optimization for:
   • Coverage
   • Capacity
   • Mobility

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5. Physical Layer Signal Flow in NTN

1. UE estimates Doppler and timing offset
2. Applies pre compensation
3. Transmits OFDM signal
4. Satellite relays signal to gateway
5. Receiver performs:
   • Frequency correction
   • Channel estimation
   • Decoding

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6. Comparison: Terrestrial vs NTN PHY Layer

Feature	Terrestrial NR	NTN NR (Rel-17)
Doppler Impact	Low	High
Delay Spread	Limited	Large
SCS Optimization	Standard	Adaptive
CP Length	Normal	Extended
Synchronization	Easier	Complex
Channel Stability	Moderate	Dynamic

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7. Practical Deployment Insights

7.1 Trade off Between Coverage and Robustness

1. Lower SCS improves coverage
2. Higher SCS improves Doppler resilience

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7.2 Importance of Accurate Doppler Estimation

1. Critical for maintaining signal quality
2. GNSS plays a key role

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7.3 Impact on UE Complexity

1. Additional processing for:
   • Frequency correction
   • Timing alignment

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7.4 Satellite Payload Considerations

1. Transparent vs regenerative payload impacts PHY processing
2. Some compensation can be offloaded to satellite

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8. Future Enhancements (Release 18 and Beyond)

1. Advanced Doppler resilient waveforms
2. AI based channel estimation
3. Adaptive numerology switching
4. Improved interference mitigation techniques

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9. Conclusion

Waveform and physical layer design in NTN must address challenges that are not present in terrestrial networks.

Key adaptations include:

1. Flexible numerology
2. Doppler pre compensation
3. Extended cyclic prefix
4. Enhanced timing mechanisms

These changes ensure that 5G NR waveforms remain robust even in highly dynamic satellite environments.

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