On the Trade-Off Between Sum-Rate and Energy Efficiency through the Convergence of HAPS and Active RIS Technologies

TL;DR

This paper explores the integration of active RIS with HAPS to improve non-terrestrial network performance, balancing sum-rate and energy efficiency through optimized configurations and architectures.

Contribution

It introduces a joint optimization framework for active RIS-HAPS systems, comparing various architectures for enhanced throughput and energy efficiency.

Findings

Active RIS outperforms passive RIS in QoS.

Fully-connected architectures achieve higher throughput.

Sub-connected schemes offer better energy efficiency.

Abstract

This paper investigates the integration of active reconfigurable intelligent surfaces (RIS) relay with high-altitude platform stations (HAPS) to enhance non-terrestrial network (NTN) performance in next-generation wireless systems. While prior studies focused on passive RIS architectures, the severe path loss and double fading in long-distance HAPS links make active RIS a more suitable alternative due to its inherent signal amplification capabilities. We formulate a sum-rate maximization problem to jointly optimize power allocation and RIS element assignment for ground user equipments (UEs) supported by a HAPS-based active RIS-assisted communication system. To reduce power consumption and hardware complexity, several sub-connected active RIS architectures are also explored. Simulation results reveal that active RIS configurations significantly outperform passive RIS in terms of quality of service (QoS). Moreover, although fully-connected architectures achieve the highest throughput, sub-connected schemes demonstrate superior energy efficiency under practical power constraints. These findings highlight the potential of active RIS-enabled HAPS systems to meet the growing demands of beyond-cellular coverage and green networking.