⚡ electrical engineering

Elevation-Aware Supplementary Uplink for Direct Satellite-to-Device Communications

This paper proposes an elevation-aware supplementary uplink (SUL) framework for direct satellite-to-device communications that dynamically switches between primary and lower-frequency carriers based on elevation-dependent link margins to enhance uplink reliability and coverage while maintaining user equipment power efficiency.

Original authors: Rajan Shrestha, Hayder Al-Hraishawi

Published 2026-02-24

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Rajan Shrestha, Hayder Al-Hraishawi

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to talk to a friend who is flying a very fast plane high above the Earth. You are holding a standard walkie-talkie (your smartphone), and your friend is in a Low Earth Orbit (LEO) satellite.

This paper tackles a specific problem: It's really hard to shout up to the sky when your friend is low on the horizon.

Here is the breakdown of the problem and the clever solution proposed by the authors, explained through simple analogies.

The Problem: The "Whisper vs. The Shout"

When your phone tries to send a message to a satellite, it faces three big hurdles:

Distance: The satellite is hundreds of miles away.
The "Fog" of the Atmosphere: The air gets thicker and messier near the horizon, blocking signals.
The Weak Shout: Your phone has a tiny battery and a weak antenna. It can't shout very loud.

The Current Setup (The "Ka-band"):
Currently, these systems try to use a "high-frequency" channel (like a very sharp, high-pitched whistle). This is great for sending lots of data quickly when the satellite is directly overhead (at the top of the sky).

The Issue: As the satellite moves toward the horizon (low elevation), the signal has to travel through more atmosphere and a longer distance. The "whistle" gets lost in the wind and rain. Your phone can't shout loud enough to be heard, so the connection drops.

The Solution: The "Supplementary Uplink" (SUL)

The authors suggest adding a second channel, called SUL. Think of this as switching from a high-pitched whistle to a deep, low-frequency bass drum.

High Frequency (Ka-band): Like a whistle. It cuts through the air well when things are clear and close, but it gets blocked easily by fog or distance.
Low Frequency (SUL/L-band): Like a bass drum. It doesn't carry as much "data speed" (it's slower), but it is much tougher. It can punch through fog, rain, and long distances much better.

The Magic Trick: "Elevation-Aware Switching"

The paper isn't just about having two channels; it's about knowing when to switch.

Imagine you are the pilot of the satellite. You know exactly where you are going to be in the next few minutes because your path is predictable (like a train on a track).

When the satellite is high overhead: The system uses the Whistle (Ka-band). It's fast and efficient.
When the satellite starts dipping toward the horizon: The system predicts, "Uh oh, the whistle is going to get blocked by the fog soon."
The Switch: Before the connection breaks, the system automatically tells your phone: "Stop whistling! Start drumming!" It switches your phone to the Bass Drum (SUL) channel.
The Return: When the satellite climbs back up and the air clears, it switches you back to the Whistle.

The "Hysteresis" (The "Don't Flip-Flop" Rule)

You might ask: "What if the satellite is right on the edge? Won't the phone keep switching back and forth every second?"

The authors added a safety buffer called Hysteresis.

Without Hysteresis: It's like a light switch that is too sensitive. If the light is 49% bright, it turns off. If it's 51% bright, it turns on. If it fluctuates between 49% and 51%, the light flickers annoyingly.
With Hysteresis: The rule is, "Don't switch until the light is really dim (say, 40%), and don't switch back until it's really bright (say, 60%)."
Result: Your phone only switches once per satellite pass. It stays on the "Bass Drum" until the connection is rock solid again, preventing annoying flickering.

The Results: What Did They Find?

The computer simulations showed that this "Smart Switching" system is a game-changer:

Extended Range: It allows your phone to talk to the satellite even when it's very low on the horizon (near the edge of the map), where the old system would have failed.
No Power Drain: Because the phone only uses one channel at a time (either the whistle OR the drum, never both), it doesn't drain the battery faster.
Stability: With the "Hysteresis" rule, the connection stays smooth without constant switching.

The Bottom Line

This paper proposes a smart way to keep your phone connected to satellites everywhere on Earth, even in remote areas or when the satellite is just peeking over the horizon. By intelligently switching between a "fast but fragile" signal and a "slower but tough" signal based on where the satellite is, we can ensure that your emergency text or data call gets through, no matter where you are.

1. Problem Statement

Direct Satellite-to-Device (DS2D) communication allows standard mobile devices to connect directly to Low Earth Orbit (LEO) satellites, offering global coverage without terrestrial infrastructure. However, the uplink (device-to-satellite) faces critical challenges:

Severe Path Loss: Propagation distances (slant ranges of 600–2000 km) result in free-space path loss of 160–190 dB.
Power Constraints: Standard User Equipment (UE) like smartphones are limited to ~23 dBm transmit power and use omnidirectional antennas, unlike high-gain satellite terminals.
Low Elevation & Beam Edges: Reliability degrades significantly at low elevation angles and near beam edges due to increased path loss, atmospheric attenuation, and shadowing.
Frequency Trade-offs: While higher frequencies (e.g., Ka-band) offer bandwidth, they suffer from higher path loss and rain attenuation. Lower frequencies (e.g., L-band) have better propagation but are often underutilized for uplink in DS2D.

The core problem is maintaining reliable uplink connectivity for power-limited UEs across the entire satellite pass, particularly at low elevation angles where high-frequency carriers fail to meet Signal-to-Noise Ratio (SNR) requirements.

2. Methodology

The authors propose an Elevation-Aware Supplementary Uplink (SUL) framework that integrates 3GPP Release 15 SUL concepts into DS2D systems.

Dual-Carrier Strategy: The system utilizes two carriers:
- Primary Uplink (PUL): High frequency (e.g., Ka-band, 30 GHz) for spectral efficiency.
- Supplementary Uplink (SUL): Low frequency (e.g., L-band, 1.6 GHz) for coverage and robustness.
Geometry-Aware Switching: Unlike carrier aggregation (which transmits simultaneously), this framework schedules the UE to transmit on either PUL or SUL at any given time. This avoids power splitting and simplifies RF front-end design.
Deterministic Prediction: Leveraging the predictable geometry of LEO orbits, the system estimates the UE's elevation angle ( $\theta$ $θ$ ) in real-time. This allows for the deterministic calculation of:
- Slant range ( $d$ ).
- Free-space path loss ( $PL_{FS}$ ).
- Atmospheric attenuation ( $L_{atm}$ ).
- Predicted Link Margin ( $M$ ).
Hysteresis-Based Activation Algorithm:
- The algorithm calculates the predicted link margin for both carriers.
- It switches from PUL to SUL if the PUL margin drops below a safety threshold ( $\Delta_s$ ) while the SUL margin is positive.
- It switches back from SUL to PUL only when the PUL margin exceeds the SUL margin by a hysteresis margin ( $\Delta_h$ ).
- This hysteresis prevents "ping-pong" effects (frequent switching) caused by minor fluctuations in link margin.

3. Key Contributions

Elevation-Aware SUL Framework: A novel architecture that adapts uplink carrier selection based on the satellite's elevation angle and predicted link margin, specifically designed for power-constrained DS2D UEs.
Hysteresis-Enabled Switching Algorithm: The introduction of a specific algorithm (Algorithm 1) that uses safety and hysteresis margins to ensure stable carrier transitions, minimizing signaling overhead and switching frequency.
Performance Validation: Comprehensive simulation results demonstrating that the SUL framework significantly extends coverage and improves availability compared to PUL-only systems, all while adhering to strict UE power limits.

4. Simulation Results

The study simulated a LEO satellite pass (600 km altitude) with a UE transmit power of 23 dBm, comparing a PUL-only system (Ka-band, 30 GHz) against the proposed SUL-enabled system (switching to L-band, 1.6 GHz).

Coverage Extension:
- For moderate SNR targets (0–5 dB), the PUL-only system requires elevation angles above 30°–40° to maintain connectivity.
- The SUL-enabled system maintains connectivity down to the minimum elevation limit of ~10°.
- This represents an elevation coverage extension of over 20°, effectively covering beam edges and low-elevation regions.
Uplink Availability:
- The Cumulative Distribution Function (CDF) of uplink availability shows that SUL enables near-uniform connectivity throughout the satellite pass.
- PUL-only systems exhibit a "delayed onset" of availability, only becoming viable at high elevations.
Switching Stability:
- The study analyzed the number of carrier switches per pass relative to the hysteresis margin ( $\Delta_h$ ).
- With a hysteresis margin of >3.5 dB, the system consistently requires only one uplink transition per satellite pass (switching from PUL to SUL as the satellite rises or vice versa), effectively eliminating oscillatory switching.

5. Significance

This paper addresses a critical bottleneck in the commercialization of Direct-to-Device satellite services: uplink reliability at the edge of coverage.

Practical Viability: By utilizing existing 3GPP SUL concepts adapted for LEO geometry, the solution offers a low-complexity, software-defined enhancement to existing hardware without requiring high-power amplifiers on the device.
Global Connectivity: The ability to maintain links at low elevation angles (10°) is crucial for maritime, aeronautical, and remote terrestrial applications where the satellite is near the horizon.
Resource Efficiency: The "either-or" switching mechanism ensures that the UE does not exceed its power budget, making the technology feasible for standard smartphones.

In conclusion, the proposed Elevation-Aware SUL framework provides a robust, stable, and power-efficient solution to extend the operational envelope of DS2D communications, ensuring reliable connectivity from the moment a satellite becomes visible until it sets.