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arXiv · 2604.25331

Performance Analysis of HAPS-RIS-Assisted MIMO Systems Under Phase-Dependent Amplitude Response Using Saddle Point Approximation

Tayfun Yilmaz·Haci Ilhan·Ali Gorcin·Halim Yanikomeroglu
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Abstract

The integration of HAPS, RISs, and MIMO technologies is emerging as a promising paradigm for extending the coverage and reliability of future wireless communication networks. However, in a HAPS-mounted RIS-assisted MIMO (HAPS-RIS-MIMO) system, the received SNR statistics become difficult to characterize due to the cascaded Rician small-scale fading and log-normal large-scale shadowing effects. To address this challenge, this paper develops a tractable analytical framework for the SNR characterization of HAPS-RIS-MIMO systems under LoS-aligned precoding. Specifically, saddlepoint approximation is employed to characterize the distribution of the small-scale effective channel power, while Gauss-Hermite quadrature is used to incorporate the composite log-normal large-scale fading effect. Based on the resulting cumulative distribution function, the outage probability expression is derived and validated through Monte Carlo simulations. The numerical results provide both theoretical validation and practical design insights by analyzing the effects of transmit power, HAPS altitude, transmit antenna number, RIS size, RIS amplitude response, and RIS phase resolution. It is shown that optimizing the RIS phases to enhance the LoS power contribution provides substantial transmit-power savings compared with random RIS phase configurations. Moreover, LoS-aligned precoding achieves a performance close to eigenmode precoding when the RIS phases are properly optimized, indicating a promising low-complexity alternative for practical HAPS-RIS-MIMO deployments. Furthermore, sufficiently large RIS deployments with LoS-aware phase optimization can mitigate the degradation caused by increased HAPS altitude and limited transmit-power budgets, while practical RIS hardware improvements in amplitude response and phase resolution provide additional transmit-power gains of approximately 3-5 dB.

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Tayfun Yilmaz, Haci Ilhan, Ali Gorcin, Halim Yanikomeroglu. 2026-04-28. Performance Analysis of HAPS-RIS-Assisted MIMO Systems Under Phase-Dependent Amplitude Response Using Saddle Point Approximation. https://arxiv.org/abs/2604.25331

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