Ibrahim Altunbas

Free-space optical (FSO) communication is emerging as a key backhaul technology for next-generation vertical heterogeneous networks (VHetNets), whose architecture spans satellites, high-altitude platform stations (HAPSs), autonomous aerial vehicles (AAVs), and terrestrial nodes. Along these vertical and slant paths, optical beams traverse successive atmospheric layers that may contain clouds, fog, rain, and aerosols, conditions that conventional single-coefficient Beer–Lambert (BL) models typically handle only in isolation. Instead of such simplified formulas, we present a unified attenuation model that incorporates aerosols, fog, rain, cloud layers, and drizzle, accounts for the zenith angle, and provides a holistic estimate of the cumulative power loss across atmospheric layers. Numerical results show several-decibel attenuation variations across representative weather scenarios, while the difference between the proposed model predictions and the layer-resolved Moderate Resolution Atmospheric Transmission (MODTRAN) simulations remains within 1 dB, thereby validating the accuracy of the proposed model and its practical relevance for VHetNet link budget studies.

In this paper we investigate the physical layer security in aerial communication with reconfigurable intelligent surface (RIS)-aided joint transmit-receive pattern selection (JTRPS), where the base station and the unmanned aerial vehicle (UAV) are equipped with reconfigurable antennas (RAs), in the presence of an aerial eavesdropper. In this setup, RIS is used to adjust its reflecting elements to enhance the end-to-end signal-to-noise ratio (SNR) of legitimate communication. To quantify the overall secrecy performance of the proposed communication model, we derived closed-form transmission secrecy outage probability (TSOP) and capacity-based secrecy outage probability (SOP) expressions to comprehensively evaluate aerial security. Our analyses show that the presence of RIS improves the outage probability, and JTRPS boosts the impact of RIS by adding spatial diversity with mirror activation patterns (MAPs) of RA. The efficiency of the proposed scenario and the correctness of the resulting secrecy performance expressions are confirmed with simulations.

Satellite networks have become central to the evolution of modern communications systems due to their potential for extensive global coverage. However, high latency in satellite systems poses critical challenges for delay-sensitive applications, underscoring the need for performance metrics that characterize ultra-reliable low-latency communications. To address these challenges, data-oriented approach, which is already widely studied in terrestrial networks, provides a fresh perspective by evaluating the transmission performance where it prioritizes both reliability and latency. This work introduces the data-oriented approach to uplink hybrid satellite-terrestrial networks (HSTNs), focusing on non-orthogonal multiple access (NOMA)-assisted semi-grant-free (SGF) transmission where terrestrial relays support the transmission between the satellite and users. Specifically, a novel grant-free user (GFU) admission protocol based on distributed contention control, the corresponding power allocation scheme for GFUs, and the relay selection procedure presented following the data-oriented approach. Then, the maximum number of GFUs that can be admitted under given data-oriented design requirements is determined. The analytical results are verified by extensive Monte-Carlo simulations, while a wide body of numerical results are also offered to understand and demonstrate the impacts of different system parameters. The proposed analytical framework offers useful insights toward the design of practical HSTNs, particularly in the context of delay-sensitive applications, by revealing the relationship between the amount of information data, the power consumption, and the satellite distance. The results demonstrate that the proposed scheme improves the DOR performance compared to conventional grant-free access and frequency division multiple access methods by dynamically selecting GFUs and allocating transmit power based on channel conditions. It is also shown that more GFUs can be admitted, especially with larger bandwidths and/or relaxed threshold settings.

Pointing error has a significant impact on the performance of millimeter wave (mmWave) and terahertz (THz) communications due to directional transmission. Currently existing pointing error models are lacking in capturing the mmWave/THz radiation characteristics and/or the impact of antenna design and array design. Therefore, in this letter, a simple analytical pointing error model for highly directional mmWave/THz transmission is proposed. By utilizing the Gaussian beam approximation for both the array element radiation pattern and the array factor, the presented model incorporates antenna design parameters (maximum gain and 3 dB beamwidth) and array design parameters (number of array elements, element spacing, and 3 dB beamwidth). This approximation is validated via electromagnetic simulations in CST Microwave Studio. Afterwards, the statistics of the pointing error are derived according to the Gaussian beam approximation, and it is demonstrated that the pointing error follows a special case of the negative log-Gamma distribution with the shape parameter of 2 and the scale parameter depending on antenna/array design and jitter variance. It is shown that the proposed model aligns perfectly with the simulation results. Moreover, the outage performance of an aerial communication scenario is analyzed to examine the impact of pointing errors. The results have revealed that the antenna and array design parameters are as influential as the jitter variance, and they cannot be neglected.

In this article, we study the timely notion of short-packet communications, with specific focus on uplink rate-splitting multiple access (RSMA) systems under the finite blocklength regime . Specifically, we consider rate-adaptive and power-adaptive uplink RSMA mechanisms, incorporating multiple user groups, and derive analytical expressions for the fundamental delay-outage rate (DOR) metric. The analytical derivations are validated through the corresponding Monte Carlo numerical simulations, reflecting high accuracy. Then, the derived DOR expressions are exploited for providing optimal DOR performance under diverse individual transmission parameters, such as different information delivery time thresholds, varying blocklengths and channel bandwidths, while also considering the non-orthogonal multiple access (NOMA) as a particular special case. Importantly, DOR optimization in terms of the message splitting ratio is also pursued, and a feasible optimization algorithm is proposed. A vast collection of numerical results is then provided, comparing between the rate-adaptive and the power-adaptive schemes, assessing the impact of message splitting ratio optimization while also comparing between RSMA and NOMA. Additionally, the overall power efficiency and impacts of imperfect channel state information (CSI) are assessed and shown. Overall, the offered analysis methods and numerical results provide valuable tools and insight for deploying, designing and optimizing data-oriented uplink RSMA mechanisms in future wireless systems such as the emerging 6G networks.

Satellite communication (SatCom) has become an important field of research to accomplish the requirements of next-generation wireless communication systems such as high data rates and capacity while providing global coverage. Besides traditional communication applications, SatCom is gathering more and more attention as it enhances the performance of sensing applications, such as environmental monitoring, atmospheric pollution monitoring, etc. In order to deliver worldwide service and improve accurate data collection in sensor networks, high-altitude platform station (HAPS) systems can be employed in SatCom. Moreover, utilizing terahertz (THz) frequencies in HAPS-assisted SatCom systems offers incredibly high bandwidths, enabling extreme data rates and higher resolution in sensing applications. In this paper, a THz SatCom system model is considered where K number of HAPS systems are deployed to assist the transmission between a low Earth orbit satellite and a ground station by utilizing variable-gain amplify-and-forward relaying. Symbol error rate and ergodic capacity analyses are performed in the presence of attenuation depending on the atmospheric conditions, fading, pointing error, and in-phase and quadrature imbalance. Theoretical findings are validated through Monte-Carlo simulations.

Satellites and high altitude platform station (HAPS) systems are expected to become the key elements of non-terrestrial networks with the recent advances on sixth-generation (6G) wireless networks. In this article, motivated by the mounting interest in HAPS systems both from academia and industry, we propose a multi-HAPS aided optical satellite communication architecture using hybrid radio frequency (RF)/free space optical (FSO) communication to provide enhanced throughput, and reliability. The proposed architecture can be used in railways, transcontinental highways or maritime communications to provide enhanced coverage, throughput and reliability. To quantify the overall performance of the proposed scenario, outage probability, ergodic capacity, throughput, energy efficiency are obtained and validated. Additionally, the impact of outdated channel state information and channel estimation errors are considered, severely affecting the system performance by causing signal-to-noise ratio loss and outage floors. Furthermore, we find optimum HAPS distance and obtain the number of HAPS systems that is required for reliable communications. The results show that serial placement of HAPS systems at optimum distances can enhance the system performance and energy efficiency.

In this letter, we consider a rate-splitting multiple access (RSMA)-based uplink satellite network and propose a dynamic power allocation and decoding order scheme depending on the channel coefficients. The exact outage probability expression of the considered system is derived for the shadowed Rician fading channel. The asymptotic outage probability expression is also obtained, and the coding gain and diversity order of the system are found. Simulation results validate the derived expressions, and it is shown that full diversity can be achieved using the proposed channel-dependent power allocation and decoding order scheme. Furthermore, the proposed scheme is compared with the conventional decoding order and power allocation schemes, and its superior performance is demonstrated.