Summary
The paper studies two ways to boost wireless signals from high-altitude stations: using special surfaces called RIS or relay devices, both under a system called NOMA. It finds that RIS technology works better in real-world conditions where hardware isn’t perfect and channels are uncertain, because it doesn’t add extra noise like relays do. The authors show that how these RIS elements are arranged and where users are located can greatly affect network speed and efficiency. While RIS is more sensitive to mistakes in knowing the channel, having more RIS elements helps make up for this, keeping it better than traditional relay systems. This work helps guide future designs of high-altitude 6G communication systems that need to work well even with practical challenges.
High-altitude platform station (HAPS)Reconfigurable intelligent surfaces (RIS)Relay stations (RS)Non-orthogonal multiple access (NOMA)Hardware impairments (HWI)Channel state information (CSI)Sum-rateEnergy efficiency6G networksImpairment-aware communication
Authors
Bilal Karaman, Faicel Khennoufa, Ilhan Basturk, Metin Ozturk, Ferdi Kara, Sezai Taskin, Halim Yanikomeroglu
Abstract
This paper investigates the performance of high-altitude platform station (HAPS)-assisted communication systems employing either reconfigurable intelligent surfaces (RIS) or relay stations (RS) under non-orthogonal multiple access (NOMA) scheme. Practical system impairments, including hardware impairments (HWI) and imperfect channel state information (CSI), are explicitly considered. The results show that HAPS-RIS outperforms HAPS-RS in terms of both sum-rate and energy efficiency under non-ideal conditions due to its passive nature, which avoids noise amplification. Furthermore, it is demonstrated that RIS element allocation and user spatial distribution significantly impact NOMA performance, where increased user separation and proper allocation enhance channel disparity and improve system efficiency. Despite its higher sensitivity to imperfect CSI, HAPS-RIS can effectively compensate for performance degradation through large-scale RIS element deployment, maintaining a performance advantage over half-duplex RS-based systems. These insights provide useful design guidelines for impairment-aware HAPS-assisted 6G communication systems.