1. Introduction: Why Uplink Power Control is Critical in NTN

In terrestrial networks, uplink power control (UL PC) is primarily used to:

• Reduce interference
• Maintain target SINR
• Optimize UE battery usage

However, in Non Terrestrial Networks (NTN), uplink power control becomes mission critical due to:

• Extremely long propagation distances
• High path loss variability
• Rapid satellite movement
• Limited UE transmit power capability

Unlike terrestrial systems, where uplink conditions are relatively stable, NTN uplink performance is highly dynamic and sensitive to even small misconfigurations.

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2. NTN Uplink Characteristics vs Terrestrial Networks

Aspect	Terrestrial Networks	NTN (LEO Based)
Distance to Node	Few km	Hundreds to thousands km
Path Loss	Moderate	Extremely high
UE Power Margin	Sufficient	Limited
Interference	Dense network	Sparse but beam based
Variation	Slow	Rapid (due to movement)

Practical Insight:

In NTN, uplink is often the bottleneck, not downlink, especially for handheld devices.

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3. Fundamentals of Uplink Power Control in NTN

NTN follows 3GPP based uplink power control with adaptations.

General Uplink Power Equation (Conceptual):

• UE transmit power depends on:
  • Path loss estimation
  • Target received power at satellite
  • Fractional compensation factor
  • UE power limits

Key Components:

• Open loop power control
• Closed loop adjustments
• Maximum UE transmit power (Pmax constraint)

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4. Key Challenges in NTN Uplink Power Control

4.1 Extreme Path Loss

• Signal travels very long distances
• UE often operates near maximum transmit power

Impact:

• Limited room for power adjustment
• Increased risk of link failure

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4.2 Rapid Path Loss Variation

• Satellite movement changes distance and elevation angle
• Path loss changes dynamically

Impact:

• Static power control parameters become ineffective
• Requires adaptive tuning

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4.3 UE Power Limitation

• Handheld devices have strict power caps

Impact:

• Cell edge (beam edge) users may not reach required power
• Leads to:
  • High BLER
  • Access failures

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4.4 Uplink Interference in Beam Overlap Regions

• Multiple UEs transmit in overlapping beams

Impact:

• Increased interference
• Reduced uplink SINR

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4.5 Delay in Closed Loop Control

• High latency affects feedback loop

Impact:

• Slower adaptation to changing conditions
• Reduced effectiveness of closed loop control

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5. Uplink Power Control Parameters for Optimization

5.1 P0 (Target Received Power)

• Defines baseline uplink power

Optimization:

• Higher P0 → better coverage, more interference
• Lower P0 → reduced interference, risk of link failure

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5.2 Alpha (Path Loss Compensation Factor)

• Determines how much path loss is compensated

Values:

• Alpha = 1 → full compensation
• Alpha < 1 → partial compensation

Optimization Insight:

• Full compensation may overload UE
• Fractional compensation balances performance and power

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5.3 Delta Adjustments (Closed Loop Control)

• Fine tuning based on feedback

Challenge in NTN:

• Delayed feedback reduces responsiveness

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5.4 Maximum UE Power (Pmax)

• Hard limit on UE transmission

Optimization Focus:

• Identify users frequently hitting Pmax
• Optimize parameters to reduce saturation

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6. Beam Level Uplink Power Optimization

In NTN, uplink must be optimized per beam.

Key Considerations:

• Beam center vs edge users
• Load distribution across beams
• Interference from overlapping beams

Strategies:

• Adjust P0 per beam
• Apply beam specific alpha values
• Monitor beam wise uplink KPIs

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7. Beam Edge Uplink Optimization

Beam edges are the weakest uplink regions.

Challenges:

• High path loss
• Low SINR
• UE power saturation

Optimization Techniques:

• Increase P0 for edge users
• Improve beam overlap
• Optimize handover thresholds to avoid prolonged edge stay

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8. Uplink KPI Monitoring for Optimization

Key KPIs to track:

• UL BLER
• UL Throughput
• PUSCH SINR
• UE Power Headroom
• Pmax hit ratio

Practical Insight:

• High Pmax usage indicates uplink coverage limitation
• Low SINR + high power = coverage issue
• Low SINR + low power = parameter issue

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9. Trade Offs in Uplink Power Optimization

Optimization Goal	Risk
Increase coverage	Higher interference
Reduce interference	Coverage degradation
Improve throughput	UE battery drain
Aggressive compensation	Power saturation

Key Principle:

Balance is critical, over optimization in one direction creates new problems.

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10. Practical Optimization Workflow

Step 1: KPI Analysis

• Identify uplink bottlenecks
• Detect high BLER or low throughput zones

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Step 2: Power Headroom Analysis

• Check how often UE hits maximum power

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Step 3: Parameter Tuning

• Adjust P0 and alpha
• Optimize beam level settings

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Step 4: Interference Monitoring

• Analyze impact on neighboring beams

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Step 5: Validation

• Track KPI improvement over satellite passes

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11. Common Uplink Issues and Root Causes

Issue	Root Cause
High UL BLER	Insufficient transmit power
Low UL throughput	High path loss / poor SINR
Frequent access failure	UE cannot reach required power
Interference spikes	Over-aggressive power settings
Uneven performance	Poor beam level tuning

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12. Future Direction: Intelligent Uplink Control

NTN uplink optimization is evolving toward:

• AI based power control adaptation
• Predictive path loss compensation
• UE assisted reporting for better tuning
• Real time beam aware optimization

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13. Conclusion: Uplink Becomes the Limiting Factor

In NTN, uplink performance is often the weakest link.

Effective uplink power control requires:

• Understanding dynamic path loss behavior
• Managing UE power constraints
• Balancing coverage and interference
• Adapting to beam level variations

For RF engineers, mastering uplink optimization in NTN is essential to ensure reliable and efficient network performance.

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