1. Introduction

Random Access Procedure (RACH) is a fundamental mechanism in 5G NR that enables a User Equipment (UE) to:

1. Establish initial network access
2. Perform uplink synchronization
3. Request scheduling resources

In terrestrial networks, RACH is already optimized for low latency and predictable propagation. However, in Non Terrestrial Networks (NTN), particularly LEO based systems, RACH design faces significant challenges due to:

1. Long propagation delays
2. Large cell sizes (spot beams)
3. High Doppler variations

This blog explores how RACH is adapted for NTN in 3GPP Release 17 and beyond.

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2. Standard RACH Procedure in 5G NR

The contention based RACH procedure consists of four main steps:

1. Msg1: Preamble Transmission
   • UE transmits a PRACH preamble
2. Msg2: Random Access Response (RAR)
   • gNB provides timing advance and uplink grant
3. Msg3: RRC Request / UL Message
   • UE sends identity or connection request
4. Msg4: Contention Resolution
   • gNB resolves contention and confirms access

Key Characteristics in Terrestrial Networks

Parameter	Typical Value
Round Trip Time	< 1 ms
Timing Advance Range	Small
Cell Radius	Few km
Preamble Collision	Moderate

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3. NTN-Specific Challenges for RACH

3.1 Large Propagation Delay

1. LEO RTT can range between 20 ms to 50 ms
2. GEO RTT can exceed 500 ms
3. Impacts:
   • Delayed RAR reception
   • Increased contention window

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3.2 Large Cell Coverage (Spot Beams)

1. NTN beams can cover hundreds of kilometers
2. Leads to:
   • Wide timing uncertainty
   • Large timing advance requirements

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3.3 Doppler Shift and Frequency Offset

1. Rapid satellite movement introduces Doppler
2. Affects:
   • PRACH detection accuracy
   • Preamble correlation

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3.4 UE Location Uncertainty

1. UE may not know precise timing offset
2. Particularly critical for:
   • Non GNSS capable devices
   • Initial access scenarios

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4. 3GPP Release 17 Enhancements for NTN RACH

To address NTN challenges, Release 17 introduces several adaptations:

4.1 Extended Timing Advance

1. Support for larger TA values
2. Enables compensation for long distances

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4.2 PRACH Configuration Adaptation

1. Longer preamble formats
2. Increased cyclic prefix
3. Improved robustness against delay spread

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4.3 RACH Window Extension

1. Extended RAR window duration
2. UE waits longer for response

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4.4 GNSS Assisted Timing Estimation

1. UE uses GNSS for:
   • Initial timing alignment
   • Doppler pre-compensation

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4.5 Beam Aware RACH

1. RACH resources mapped per beam
2. Reduces contention probability

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5. NTN RACH Procedure Flow (Simplified)

Step by Step Flow

1. UE selects PRACH resource based on beam
2. UE applies:
   • GNSS based timing estimation (if available)
   • Doppler pre compensation
3. UE transmits preamble (Msg1)
4. Satellite forwards signal to gateway/gNB
5. gNB processes and sends RAR (Msg2)
6. UE receives RAR after extended delay
7. UE transmits Msg3 using granted resources
8. Contention resolution (Msg4) completes access

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6. Comparison: Terrestrial vs NTN RACH

Feature	Terrestrial NR	NTN NR (Rel-17)
Propagation Delay	Very Low	Very High
Timing Advance Range	Limited	Extended
PRACH Format	Standard	Extended CP
RAR Window	Short	Extended
GNSS Dependency	Optional	Highly Beneficial
Beam Awareness	Limited	Critical

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7. Practical Deployment Insights

7.1 Impact on Access Delay

1. NTN RACH introduces significant access delay
2. Impacts:
   • IoT device responsiveness
   • Call setup time

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7.2 Collision Probability Management

1. Larger beams → more UEs per RACH occasion
2. Mitigation:
   • Beam based resource partitioning
   • Adaptive preamble allocation

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7.3 Power Control Considerations

1. UEs must transmit at higher power levels
2. Challenges for:
   • Battery powered devices
   • NB IoT NTN scenarios

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7.4 Integration with NTN Mobility

1. Frequent beam changes affect RACH triggers
2. Requires:
   • Efficient re access strategies
   • Fast synchronization recovery

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8. Future Enhancements (Release 18 and Beyond)

1. AI based RACH optimization
2. Predictive access based on UE mobility
3. Reduced signaling overhead for IoT NTN
4. Enhanced beam coordination

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9. Conclusion

RACH in NTN is not just a scaled version of terrestrial access, it is a fundamentally re engineered procedure to cope with:

1. Extreme propagation delays
2. Large coverage footprints
3. Dynamic satellite movement

3GPP Release 17 lays the foundation, but further enhancements will be critical for enabling:

• Massive IoT over satellites
• Direct to device communication
• Seamless global coverage

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