Simulation results demonstrate the effectiveness of the proposed framework and show that the BD-RIS-assisted system achieves up to a 45.2% sum-rate improvement compared with conventional single-connected RIS (SC-RIS) architectures while maintaining robustness against CSI uncertainty and satisfying all system constraints.
Abstract
Non-terrestrial networks (NTNs) have emerged as a promising technology for providing ubiquitous connectivity in remote, underserved, and disaster-stricken regions. In particular, high-altitude platforms (HAPs) can offer wide-area coverage; however, their performance is often limited by severe path loss, multi-user interference, and spectrum-sharing constraints. To address these challenges, this paper investigates a multi-antenna HAP-based underlay aerial-to-ground communication network employing rate-splitting multiple access (RSMA) and assisted by a terrestrial beyond-diagonal reconfigurable intelligent surface (BD-RIS). The objective is to maximize the system sum rate while satisfying user rate requirements, HAP transmit-power constraints, and interference-temperature constraints imposed to protect the primary network. The design further accounts for imperfect channel state information (CSI) through a worst-case robust optimization framework. The resulting problem is highly non-convex due to the coupled optimization of RSMA precoding and BD-RIS beamforming. To address this challenge, the transmit precoding design is reformulated as a convex semidefinite program and solved using successive convex approximation and the MOSEK solver, while the BD-RIS scattering matrix is optimized over the unitary manifold using Riemannian manifold optimization. Simulation results demonstrate the effectiveness of the proposed framework and show that the BD-RIS-assisted system achieves up to a 45.2% sum-rate improvement compared with conventional single-connected RIS (SC-RIS) architectures while maintaining robustness against CSI uncertainty and satisfying all system constraints.
Simulation results demonstrate that the proposed GNN-RSMA interference management algorithm outperforms conventional multiple access schemes while achieving fairness and worst-user performance comparable to successive convex approximation (SCA)-based optimization at only a fraction of its computational cost.
We consider a downlink multicell multiple-input multiple-output (MIMO) system in an urban region, with a focus on improving the capacity of cell-edge user equipments (UEs). These UEs typically experience lower rates than near UEs because of shadowing, path loss, and inter-cell interference (ICI). To address this issue, we integrate a high-altitude platform station (HAPS) with the terrestrial network as a relay for edge-UE transmissions. We assume that the HAPS operates in full-duplex (FD) mode and exploits its large physical size to enhance passive self-interference (SI) suppression by separating its transmit and receive antennas. In the proposed scheme, each terrestrial base station (BS) forwards edge-UE data to the FD-HAPS, which then relays the data to the intended edge UEs. To design beams at both BSs and HAPS, we formulate a sum-rate maximization problem for under total transmit-power and minimum quality-of-service (QoS) constraints. To solve the resulting non-convex problem, we develop a centralized algorithm based on successive convex approximation (SCA) and alternating optimization (AO) for fast convergence. Simulation results show that relaying information via FD-HAPS significantly improves the capacity of cell-edge UEs compared with a terrestrial-only network.
High-altitude platform station (HAPS)-assisted freespace optics (FSO) communication has emerged as a promising solution to enhance the coverage, flexibility, and resilience of existing network infrastructures through aerial relaying. By employing HAPS as relay nodes, the strict line-of-sight (LoS) constraints of conventional FSO links can be effectively alleviated. In this work, we investigate a multi-HAPS-based FSO communication system with decode-and-forward (DF) relaying, where opportunistic relay selection (ORS) is utilized to select the most suitable HAPS for data forwarding. The optical wireless channel is modeled using the doubly inverted Gamma-Gamma (IGGG) distribution, which jointly accounts for atmospheric turbulence, attenuation, and pointing errors. Closed-form analytical expressions are derived for outage probability (OP), while the average symbol error rate (ASER) and ergodic capacity (EC) are evaluated using efficient Gauss-Laguerre (GL) quadrature approximations. In addition, asymptotic analysis in the high signal-to-noise ratio (SNR) regime is performed to characterize the diversity gain of the proposed system. The analytical results are validated through Monte-Carlo simulations. Numerical results demonstrate the impact of relay selection strategies, turbulence conditions, and system parameters on system performance, providing useful insights into the reliability and diversity benefits of multi-HAPSassisted FSO systems under realistic atmospheric environments.
Aishanee Chatterjee, Prashant Sharma, S. R.· International Conference on...· 0 citations
Satellite-terrestrial integrated networks with simultaneous wireless information and power transfer (SWIPT) provide wide-area connectivity and sustainable service support, but they also face serious security challenges due to the broadcast nature of satellite links and the possibility that an energy receiver may act as potential eavesdropper. To address this issue, this paper proposes a secure precoding design for a high-altitude platform (HAP)-assisted rate-splitting multiple access (RSMA) architecture under a quasi-static transmission model. Specifically, a cooperative direct and relay transmission (CDRT) framework is developed, in which the HAP assists the satellite transmission to improve the physical layer security for multi-user SWIPT services. By assuming the energy receiver near the target user as potential eavesdropper, we formulate a sum secrecy rate maximization problem subject to energy harvesting and transmit power constraints. To transform the original nonconvex optimization problem into a tractable convex problem, we employ techniques such as first-order Taylor expansion approximation, rank-one constraint relaxation, successive convex approximation, and semidefinite relaxation. Numerical results demonstrate that the proposed CDRT-RSMA scheme significantly outperforms conventional non-orthogonal and time-division multiple access schemes in terms of security performance.
Mengyan Huang, Xingwang Li, Chengjun Jiang et al.· IEEE Journal on Selected Are...· 0 citations
Due to their resilience and global coverage, satellite networks are poised to become a key component for non-terrestrial networks in the future. However, given the scarcity of spectrum resources, the dense deployment of low Earth orbit (LEO) satellites introduces significant interference challenges. Meanwhile, the limited computing power and backhaul capacity of satellites have become bottlenecks hindering the development of advanced interference mitigation techniques. This paper studies beamforming in GEO-LEO heterogeneous multi-satellite systems. For the GEO system, we develop a multicast beamforming approach based on a nonlinear eigenvalue problem (NEPv) for beam direction design and Lagrange dual decomposition (LDD) for power allocation. For the LEO system, we propose a general distributed beamforming framework and two distributed beamforming methods. Specifically, we first leverage equivalent multi-dimensional fractional programming (FP) to decompose the objective function. The resulting subproblems are then optimized in a distributed manner across multiple satellites via the parallel block coordinate descent (PBCD) method. For the distributed optimization subproblems, we derive semi-closed-form solutions using Lagrangian dual ascent (LDA) and alternating direction method of multipliers (ADMM) for scenarios without and with GEO-LEO interference avoidance, respectively. Simulation results show that the proposed NEPv-LDD method strictly satisfies the QoS constraints of users and achieves near-optimal performance with low complexity. For the LEO beamforming, the developed distributed FP (DiFP) framework exhibits strong scalability in large-scale constellations. Built upon the DiFP framework, the proposed DiFP-NoSIA incurs almost no performance loss, while DiFP-ADMM shows only an 8.58% performance degradation compared to the centralized benchmark.
Xin Chen, Zhiyong Luo· IEEE Transactions on Wireles...· 0 citations
In this paper, we investigate the downlink performance of multi-cell RSMA-enabled ISAC networks in which base stations (BSs), communication users, and sensing targets are spatially distributed according to independent Poisson point processes (PPPs). Each BS simultaneously serves multiple users using RSMA while exploiting the common stream as a dual-functional communication and sensing waveform. The users are equipped with FAS that selects the best antenna port to maximize the received signal quality. Closed-form analytical expressions are derived for the ergodic sum-rates by combining stochastic geometry, order statistics, and Laplace-transform-based interference analysis. Furthermore, a tractable approximation for the average radar SINR is developed by characterizing the statistical properties of the common precoder. Leveraging the derived analytical expressions, a low-complexity analytical resource allocation framework is proposed to jointly optimize the RSMA power allocation, the communication-sensing beam tradeoff, and the number of scheduled users while sat- isfying the sensing quality-of-service constraint. Compared with conventional iterative optimization approaches, the proposed analytical design significantly reduces computational complexity while achieving nearly identical communication performance. Simulation results verify the accuracy of the developed analytical expressions and demonstrate substantial improvements in both RSMA sum-rate and sensing performance over conventional transmission schemes.
Abdelhamid Salem, Hana Shamata, Salma M. Elkawafi et al.· 0 citations