⚡ electrical engineering

Ampli-Flection for 6G: Active-RIS-Aided Aerial Backhaul with Full 3D Coverage

This paper proposes a novel 6G aerial backhaul architecture utilizing an active reconfigurable intelligent surface (RIS) mounted on a high-altitude platform to provide full 3D coverage and energy-efficient, amplified signal transmission for both UAV-BSs and ground users, overcoming blockage and multiplicative fading through joint optimization of platform placement, array partitioning, and phase configuration.

Original authors: Hong-Bae Jeon, Chan-Byoung Chae

Published 2026-01-27

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Hong-Bae Jeon, Chan-Byoung Chae

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 shout a message to a friend standing on a tall building in a crowded city. But there's a problem: a massive skyscraper is blocking your direct line of sight. You can't just shout louder because your voice would get too weak by the time it reaches them, and shouting too loud might hurt your throat (or in this case, drain your battery).

This is the exact problem the researchers in this paper are solving for the future of 6G wireless networks. Here is a simple breakdown of their solution, using everyday analogies.

The Problem: The "Double Whammy" of Distance

In current technology, if you want to send a signal around a building, you might use a "mirror" (called a Passive RIS).

The Passive Mirror: Imagine a regular mirror. If you shine a flashlight at it, it bounces the light to your friend. But the light gets dimmer on the way to the mirror, and it gets even dimmer on the way from the mirror to your friend. This is called "multiplicative fading." It's like whispering to a mirror, and the mirror whispering to your friend; the message gets very faint.
The Aerial Problem: The researchers wanted to put this mirror in the sky (on a drone) to cover both people on the ground and other drones in the air (3D coverage). But putting a passive mirror in the sky makes the "double whammy" even worse because the distances are so long.

The Solution: The "Super-Mirror" (Active RIS)

The authors propose a new invention: an Active RIS.

The Analogy: Instead of a regular mirror that just reflects light, imagine a mirror with a built-in megaphone.
How it works: This "Super-Mirror" is mounted on a high-altitude drone. When it catches your weak signal, it doesn't just bounce it; it amplifies (turns up the volume of) the signal before sending it on its way.
The Benefit: This fixes the "double whammy." Even though the signal travels a long way, the mirror boosts it so it arrives strong and clear at the destination (whether that's a ground user or another drone).

The Challenge: The "Battery vs. Volume" Trade-off

There is a catch. Using a megaphone takes battery power.

If you turn the volume up too high, the mirror uses too much energy.
If you turn it up too low, the signal is still too weak.
The researchers had to find the perfect "Goldilocks" volume that makes the signal strong enough without draining the battery.

The "Smart Placement" Strategy

The paper isn't just about the mirror; it's also about where to put it.

The Puzzle: The researchers had to figure out the perfect spot in the sky for the drone, how to split the mirror into different sections, and exactly how to angle the "megaphone" to hit multiple targets at once.
The Math: They used complex math to solve a puzzle. They found that the best place to put the drone is usually very close to the signal source (the ground station) but high enough to see everyone.
The Result: By placing the drone in this specific spot and tuning the "volume" perfectly, they can save a massive amount of energy compared to older methods.

The Results: A Big Win for Efficiency

The researchers ran computer simulations to test their idea against other methods (like using a standard drone relay or a passive mirror).

The Comparison: They compared their "Super-Mirror" setup to a standard "Relay Drone" (which acts like a walkie-talkie repeater).
The Outcome: Their method used significantly less power. In some scenarios, the energy savings were huge (over 30 dB, which is a massive difference in engineering terms).
The Conclusion: They proved that by using an active mirror in the sky, we can create a 6G network that covers the ground and the air efficiently, without burning through batteries.

Summary in One Sentence

The paper proposes putting a "smart, volume-boosting mirror" on a drone to bounce 6G signals around city buildings, proving that this method saves a tremendous amount of energy compared to current technologies.

Technical Summary: Ampli-Flection for 6G: Active-RIS-Aided Aerial Backhaul with Full 3D Coverage

Problem Statement
The rapid expansion of data-driven applications in beyond fifth-generation (B5G) and sixth-generation (6G) networks has created an unprecedented demand for ultra-reliable, high-capacity communications. Unmanned Aerial Vehicle Base Stations (UAV-BSs) are a promising solution for extending coverage, particularly in dense urban areas or emergency scenarios. However, a critical bottleneck remains: backhaul connectivity. In dense urban environments, obstacles often block direct line-of-sight (LoS) links from terrestrial sources, forcing signals through non-LoS (NLoS) paths that severely degrade energy efficiency.

While Reconfigurable Intelligent Surfaces (RIS) have been proposed to mitigate blockages, conventional passive RIS architectures suffer from "multiplicative fading," where the path loss is the product of the source-to-RIS and RIS-to-destination links. This effect becomes particularly severe in aerial scenarios with long link distances. Furthermore, existing aerial RIS studies are largely limited to 2D coverage (serving only ground users) or passive operation, failing to address the energy-efficiency challenges of supporting both aerial and ground users in a full 3D space.

Methodology
The paper proposes a novel Aerial Active-RIS architecture. Unlike passive RIS, the proposed system deploys an RIS on a high-altitude platform where each element is equipped with an active amplifier. This design combines the geometric benefits of aerial deployment (enhanced LoS probability) with the signal amplification capabilities of active elements to overcome multiplicative fading.

The core of the methodology involves a joint optimization framework to maximize the energy efficiency of the UAV-BS backhaul network. The authors formulate a non-convex optimization problem aimed at minimizing the total power consumption (source transmit power + active-RIS reflection power + hardware consumption) while satisfying target data rate constraints. Key components of the proposed algorithm include:

System Modeling: The system models a source with a Uniform Linear Array (ULA) communicating with multiple UAV-BSs via an aerial active-RIS. The channel model explicitly accounts for 3D coordinates, elevation-dependent path loss, and dynamic noise introduced by the active amplifiers.
Signal Processing: The authors employ Maximum-Ratio Transmission (MRT) at the source and derive optimal phase shifts for the RIS to ensure constructive signal combination at the UAV-BSs.
Optimization Strategy:
- Placement: The optimal 2D location of the aerial-RIS is determined by minimizing the product of distances to the source and UAV-BSs, utilizing a global criterion method (Fermat-Torricelli problem) to find a Pareto-optimal point.
- Array Partitioning: The algorithm determines whether to use the full RIS array or partition it into sub-arrays to accommodate UAV-BSs located outside the main lobe of a single beam.
- Amplification Gain: The authors prove that equal amplification gain across all active elements is a feasible and effective strategy for optimizing energy efficiency. They derive a closed-form solution for the optimal amplification gain ( $\alpha^*$ ) that balances signal amplification against the power consumption and dynamic noise of the active circuitry.
- Power Allocation: The source transmit power for each UAV-BS is derived in closed form to meet the backhaul rate requirements.

Key Contributions

Novel Architecture: The paper introduces the first aerial-active-RIS-assisted backhaul network capable of providing full 3D coverage for both UAV-BSs and ground users, explicitly distinguishing the geometric advantages of aerial placement from the amplification gains of active elements.
Optimization Framework: A comprehensive optimization framework is developed to jointly determine the aerial platform's placement, array partitioning strategy, phase configuration, and amplification gain. The authors derive closed-form expressions for the optimal source transmit power and amplification gain.
Feasibility and Efficiency: The study demonstrates that the proposed method achieves a 100% feasibility rate under conservative power budgets. It verifies that equal amplification gain is sufficient for optimization, avoiding complex element-wise amplitude tuning.

Results
Extensive numerical simulations were conducted in a realistic urban environment with 103 randomly distributed ground users and corresponding UAV-BSs. The proposed aerial-active-RIS scheme was compared against benchmarks, including aerial Amplify-and-Forward (AF) relays, aerial passive-RIS, and aerial active-RIS with non-optimized amplification gains.

Energy Efficiency: The proposed method significantly outperformed all benchmarks. Compared to aerial passive-RIS, the proposed scheme achieved performance gains of approximately 30–36 dB in total power consumption depending on the distance and number of RIS elements.
Comparison with AF Relays: The aerial active-RIS demonstrated substantially lower power requirements than aerial AF relays. The AF relay approach suffered from higher hardware complexity and power consumption due to the need for full RF chains and self-interference suppression, whereas the active-RIS relies on low-power element-level amplification.
Robustness: The results confirmed that the system remains feasible even as the distance between the source and the UAV-BS increases, with the optimal amplification gain adapting to maintain link reliability.

Significance and Claims
The paper claims that the proposed architecture offers a resilient and energy-efficient solution for 6G backhaul connectivity. By overcoming the multiplicative fading inherent in passive cascaded channels through active amplification, the system enables reliable LoS links in complex urban 3D environments. The authors emphasize that this is the first work to facilitate an aerial-active-RIS-assisted backhaul network capable of supporting heterogeneous users (both aerial and ground) with comprehensive 3D coverage. The study concludes that the proposed method has strong potential to deliver scalable and reliable backhaul connectivity for future 6G networks, addressing a critical challenge in deploying UAV-BSs in dense urban areas.