Researcher:
Yıldırım, İbrahim

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İbrahim

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Yıldırım

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Yıldırım, İbrahim

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2021 - 20227

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    Over-the-air equalization with reconfigurable intelligent surfaces
    (Inst Engineering Technology-Iet, 2022) N/A; N/A; N/A; Department of Electrical and Electronics Engineering; Arslan, Emre; Yıldırım, İbrahim; Kılınç, Fatih; Başar, Ertuğrul; PhD Student; PhD Student; Researcher; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 282628; N/A; 149116
    Reconfigurable intelligent surface (RIS)-empowered communications is on the rise and is a promising technology envisioned to aid in 6G and beyond wireless communication networks. RISs can manipulate impinging waves through their electromagnetic elements enabling some sort of control over the wireless channel. The potential of RIS technology is explored to perform a sort of virtual equalization over-the-air for frequency-selective channels, whereas equalization is generally conducted at either the transmitter or receiver in conventional communication systems. Specifically, using an RIS, the frequency-selective channel from the transmitter to the RIS is transformed to a frequency-flat channel through elimination of inter-symbol interference (ISI) components at the receiver. ISI is eliminated by adjusting the phases of impinging signals particularly to maximize the incoming signal of the strongest tap. First, a general end-to-end system model is provided and a continuous to discrete-time signal model is presented. Subsequently, a probabilistic analysis for elimination of ISI terms is conducted and reinforced with computer simulations. Furthermore, a theoretical error probability analysis is performed along with computer simulations. It is analysed and demonstrated that conventional RIS phase alignment methods can successfully eliminate ISI, and the RIS-aided communication channel can be converted from frequency-selective to frequency-flat.
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    RIS-aided Angular-based hybrid beamforming design in mmWave massive MIMO systems
    (Institute of Electrical and Electronics Engineers Inc., 2022) Koç, Asil; Le-Ngoc, Tho; Department of Electrical and Electronics Engineering; N/A; Başar, Ertuğrul; Yıldırım, İbrahim; Faculty Member; PhD Student; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 149116; 282628
    This paper proposes a reconfigurable intelligent surface (RIS)-aided and angular-based hybrid beamforming (AB-HBF) technique for the millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems. The proposed RIS-AB-HBF architecture consists of three stages: (i) RF beamformer, (ii) baseband (BB) precoder/combiner, and (iii) RIS phase shift design. First, in order to reduce the number of RF chains and the channel estimation overhead, RF beamformers are designed based on the 3D geometry-based mmWave channel model using slow time-varying angular parameters of the channel. Second, a BB precoder/combiner is designed by exploiting the reduced-size effective channel seen from the BB stages. Then, the phase shifts of the RIS are adjusted to maximize the achievable rate of the system via the nature-inspired particle swarm optimization (PSO) algorithm. Illustrative simulation results demonstrate that the use of RISs in the AB-HBF systems has the potential to provide more promising advantages in terms of reliability and flexibility in system design.
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    Channel modelling in RIS-empowered wireless communications
    (Wiley Blackwell, 2022) Department of Electrical and Electronics Engineering; N/A; Başar, Ertuğrul; Yıldırım, İbrahim; Faculty Member; PhD Student; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 149116; 282628
    Despite various intelligent reconfigurable surface (IRS)-assisted use cases in the existing literature, there is no strong consensus to identify the killer applications that effectively exploit the potential of RISs to control the transmission medium with intelligent reflections for enhanced end-to-end system performance. The first step in clearing this ambiguity is to form a unified and physical RIS-assisted channel model that can be adapted to different use cases and operating frequencies, taking into account the physical characteristics of RISs. As intelligent reflection is the art of manipulating the channel characteristics brilliantly and dynamically, modeling the RIS-assisted transmission link is essential to provide detailed insights into the practical use cases. This chapter mainly aims to present a vision of channel modeling strategies for the RIS-empowered communications systems considering the state-of-the-art channel and propagation modeling efforts in the literature. Another objective of the chapter is to draw attention to open-source and standard-compliant physical channel modeling efforts to provide comprehensive insights regarding the practical use cases of RISs in future wireless networks. Furthermore, the extensive numerical results are provided via the SimRIS Channel Simulator MATLAB package for the detailed evaluation of how RISs can be effectively used in future wireless networks to enrich and improve the existing communication systems.
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    PublicationOpen Access
    Reconfigurable intelligent surfaces for future wireless networks: a channel modeling perspective
    (Institute of Electrical and Electronics Engineers (IEEE), 2021) Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Yıldırım, İbrahim; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 149116; N/A
    While researchers have set their sights on future wireless networks of 2030, communications through reconfigurable intelligent surfaces (RISs) appears as one of the potential enabling technologies for 6G wireless networking. This article aims to shed light on the potential use-cases of RISs in future wireless systems by means of a novel channel modeling methodology as well as a new software tool for RIS-empowered millimeter-wave communication systems. It is shown by the open-source, user-friendly, and widely applicable SimRIS Channel Simulator, whose 2.0 version is proposed and goes online by this article, that RISs will work under certain use-cases and communication environments. Potential future research directions are also discussed to bridge the gap between the theory and practice of RIS-empowered systems toward their standardization for 6G wireless networks.
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    PublicationOpen Access
    Hybrid RIS-empowered reflection and decode and forward relaying for coverage extension
    (Institute of Electrical and Electronics Engineers (IEEE), 2021) Alexandropoulos, George C.; Department of Electrical and Electronics Engineering; Kılınç, Fatih; Yıldırım, İbrahim; Başar, Ertuğrul; Researcher; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 149116
    In this letter, we introduce two hybrid transmission schemes combining a passive reconfigurable intelligent surface (RIS) with decode-and-forward relaying in a synergistic manner. The proposed schemes offer a flexible as well as cost-and power-efficient solution for coverage extension in future generation wireless networks. We present closed-form expressions for the end-to-end signal-to-noise ratio of both schemes and a sequential optimization algorithm for the power allocation and the RIS phase configurations. Our computer simulations and theoretical analysis demonstrate that the RIS and relaying technologies enhance the achievable rate and error performance remarkably when working complementary to each other, rather than being considered as competing technologies.
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    PublicationOpen Access
    Indoor and outdoor physical channel modeling and efficient positioning for reconfigurable intelligent surfaces in mmWave bands
    (Institute of Electrical and Electronics Engineers (IEEE), 2021) Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Yıldırım, İbrahim; Kılınç, Fatih; Faculty Member; Researcher; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 149116; N/A; N/A
    Reconfigurable intelligent surface (RIS)-assisted communication appears as one of the potential enablers for sixth generation (6G) wireless networks by providing a new degree of freedom in the system design to telecom operators. Particularly, RIS-empowered millimeter wave (mmWave) communication systems can be a remedy to provide broadband and ubiquitous connectivity. This paper aims to fill an important gap in the open literature by providing a physical, accurate, open-source, and widely applicable RIS channel model for mmWave frequencies. Our model is not only applicable in various indoor and outdoor environments but also includes the physical characteristics of wireless propagation in the presence of RISs by considering 5G radio channel conditions. Various deployment scenarios are presented for RISs and useful insights are provided for system designers from the perspective of potential RIS use-cases and their efficient positioning. The scenarios in which the use of an RIS makes a big difference or might not have a big impact on the communication system performance, are revealed.
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    PublicationOpen Access
    Physical channel modeling for RIS-empowered wireless networks in Sub-6 GHz bands : (Invited paper)
    (Institute of Electrical and Electronics Engineers (IEEE), 2021) Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Kılınç, Fatih; Yıldırım, İbrahim; Faculty Member; Researcher; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 149116; N/A; N/A
    Reconfigurable intelligent surface (RIS)-assisted communications is one of the promising candidates for next generation wireless networks by controlling the propagation environment dynamically. In this study, a channel modeling strategy for RIS-assisted wireless networks is introduced in sub-6 GHz bands by considering both far-field and near-field behaviours in transmission. We also proposed an open-source physical channel simulator for sub-6 GHz bands where operating frequency, propagation environment, terminal locations, RIS location and size can be adjusted. It is demonstrated via extensive computer simulations that an improved achievable rate performance is obtained in the presence of RISs for both near-field and far-field conditions.