Downlink Transmissions of UAV-RIS-Assisted Cell-Free Massive MIMO Systems: Location and Trajectory Optimization

Area of Science:

• Wireless Communication Systems
• Intelligent Reflecting Surfaces
• Aerial Networks

Background:

• Cell-free massive MIMO (CF-mMIMO) enhances spectral efficiency.
• Reconfigurable intelligent surfaces (RIS) can improve wireless environments.
• Unmanned aerial vehicles (UAVs) offer dynamic aerial platforms.

Purpose of the Study:

• Investigate UAV-borne RIS (UAV-RIS) for CF-mMIMO systems.
• Maximize user equipment (UE) achievable downlink rate.
• Determine optimal UAV location and RIS phase shifts.

Main Methods:

• Derive closed-form approximation of UE achievable downlink rate.
• Correlate UAV location with Rician K-factor.
• Employ alternating optimization for UAV-RIS parameters.

Main Results:

• Achieved significant UE sum rate improvement.
• Verified accuracy of the derived rate approximation.
• Identified optimal UAV-RIS placement based on UE distribution.

Conclusions:

• UAV-RIS enhances CF-mMIMO performance.
• Optimal UAV-RIS deployment depends on UE density.
• Designed efficient UAV-RIS trajectory for sustained performance.