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Publication Metadata only Antenna array structures for enhanced cluster index modulation(IEEE, 2023) Koc, Asil; Le-Ngoc, Tho; Department of Electrical and Electronics Engineering; Raeisi, Mahmoud; Yıldırım, İbrahim; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; Communications Research and Innovation Laboratory (CoreLab)This paper investigates the effect of various antenna array structures, i.e., uniform linear array (ULA), uniform rectangular array (URA), uniform circular array (UCA), and concentric circular array (CCA), on cluster index modulation (CIM) enabled massive multiple-input multiple-output (mMIMO) millimeter-wave (mmWave) communications systems. As the CIM technique indexes spatial clusters to convey additional information bits, the different radiation characteristics caused by different array structures can significantly affect system performance. By analyzing the effects of array characteristics such as radiation pattern, array directivity, half-power beam width (HPBW), and radiation side lobes on bit error rate (BER) performance, we reveal that URA achieves better error performance than its counterparts in a CIM-enabled mmWave system. We demonstrate that narrower beams alone cannot guarantee better BER performance in a CIM-based system. Instead, other radiation characteristics, especially radiation side lobes, can significantly influence system performance by entailing extra interference in the non-intended directions. Illustrative results show that URA owes its superiority to its lower side lobes. We also propose an algorithm to implement fixed phase shifters (FPS) as a hardware-efficient (HE) analog network structure (beamformer/combiner) to reduce cost and energy consumption in mmWave systems and investigate the effect of a non-ideal analog network on the BER performance for different array structures. It is demonstrated that HE systems with a few FPSs can achieve similar BER performance compared to the optimum (OP) analog network structure.Publication Metadata only Cluster index modulation for reconfigurable intelligent surface-assisted mmwave massive MIMO(IEEE-Inst Electrical Electronics Engineers Inc, 2024) Koç, Asil; Le-Ngoc, Tho; Department of Electrical and Electronics Engineering; Raeisi, Mahmoud; Yıldırım, İbrahim; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; ; Graduate School of Sciences and Engineering; College of Engineering; Communications Research and Innovation Laboratory (CoreLab)In this paper, we propose a transmission mechanism for a reconfigurable intelligent surface (RIS)-assisted millimeter wave (mmWave) system based on cluster index modulation (CIM), named best-gain optimized cluster selection CIM (BGCS-CIM). The proposed BGCS-CIM scheme considers effective cluster power gain and spatial diversity gain obtained by the additional paths within the indexed cluster to construct an efficient codebook. We also integrate the proposed scheme into a practical system model to create a virtual path between transmitter and receiver where the direct link has been blocked. Thanks to the designed whitening filter, a closed-form expression for the upper bound on the average bit error rate (ABER) is derived and used to validate the simulation results. It has been shown that the proposed BGCS-CIM scheme outperforms the existing benchmarks thanks to its higher effective cluster gain, spatial diversity of indexed clusters, and lower inter-cluster interference.Publication Metadata only Coordinate interleaved OFDM with repeated in-phase/quadrature index modulation(IEEE-Inst Electrical Electronics Engineers Inc, 2024) ; Department of Electrical and Electronics Engineering; Tuğtekin, Ömer Furkan; Doğukan, Ali Tuğberk; Arslan, Emre; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; ; Graduate School of Sciences and Engineering; College of Engineering; Communications Research and Innovation Laboratory (CoreLab)Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which transmits information bits through ordinary constellation symbols and indices of active subcarriers, is a promising multicarrier transmission scheme and has attracted the attention of researchers due to numerous benefits such as flexibility and simplicity. Nonetheless, OFDM-IM cannot satisfy the needs of future wireless communication services such as superior reliability, high data rates, and low complexity. In this article, we propose a novel OFDM-IM scheme named coordinate interleaved OFDM with repeated in-phase/quadrature IM (CI-OFDM-RIQIM), which provides superior error performance and enhanced spectral efficiency due to its diversity order of two and clever subcarrier activation pattern (SAP) detection mechanism, respectively. In addition, CI-OFDM-RIQIM is further extended to coordinate interleaved OFDM with in-phase/quadrature IM (CI-OFDM-IQIM) by doubling information bits transmitted by IM. Furthermore, log-likelihood ratio (LLR) based low-complexity detectors are designed for both proposed schemes. Theoretical analyses are performed and an upper bound on the bit error probability is derived. Comprehensive computer simulations under perfect and imperfect channel state information (CSI), are conducted to compare the proposed and reference schemes. It is shown that CI-OFDM-RIQIM and CI-OFDM-IQIM show superior results and can be considered promising candidates for next-generation wireless communication systems.Publication Metadata only FractalRG: advanced fractal region growing using Gaussian mixture models for left atrium segmentation(Academic Press Inc Elsevier Science, 2024) Firouznia, Marjan; Koupaei, Javad Alikhani; Faez, Karim; Jabdaragh, Aziza Saber; Department of Computer Engineering; Demir, Çiğdem Gündüz; Department of Computer Engineering; Koç Üniversitesi İş Bankası Yapay Zeka Uygulama ve Araştırma Merkezi (KUIS AI)/ Koç University İş Bank Artificial Intelligence Center (KUIS AI); College of EngineeringThis paper presents an advanced region growing method for precise left atrium (LA) segmentation and estimation of atrial wall thickness in CT/MRI scans. The method leverages a Gaussian mixture model (GMM) and fractal dimension (FD) analysis in a three -step procedure to enhance segmentation accuracy. The first step employs GMM for seed initialization based on the probability distribution of image intensities. The second step utilizes fractal -based texture analysis to capture image self -similarity and texture complexity. An enhanced approach for generating 3D fractal maps is proposed, providing valuable texture information for region growing. In the last step, fractal -guided 3D region growing is applied for segmentation. This process expands seed points iteratively by adding neighboring voxels meeting specific similarity criteria. GMM estimations and fractal maps are used to restrict the region growing process, reducing the search space for global segmentation and enhancing computational efficiency. Experiments on a dataset of 10 CT scans with 3,947 images resulted in a Dice score of 0.85, demonstrating superiority over traditional techniques. In a dataset of 30 MRI scans with 3,600 images, the proposed method achieved a competitive Dice score of 0.89 +/- 0.02, comparable to Deep Learning -based models. These results highlight the effectiveness of our approach in accurately delineating the LA region across diverse imaging modalities.Publication Metadata only Frequency-domain detection for molecular communication with cross-reactive receptors(IEEE-Inst Electrical Electronics Engineers Inc, 2024) ; Department of Electrical and Electronics Engineering; Civaş, Meltem; Akan, Özgür Barış; Department of Electrical and Electronics Engineering; ; Graduate School of Sciences and Engineering; College of Engineering;Molecular Communications (MC) is a bio-inspired communication paradigm using molecules as information carriers, necessitating novel transceivers and modulation/detection techniques. In realizing practical MC receivers (MC-Rxs), biosensor field-effect transistor (bioFET)-based architectures are promising, having surface receptors that undergo reversible reactions with ligands. These interactions are converted into electrical signals via field effect, enabling the decoding of transmitted information. A significant challenge in these receivers is the limited specificity of receptors to target ligands, which leads to molecular cross-talk from similar interfering ligands co-existing in the MC channel. Decoding transmitted symbols under such interference is challenging in the time domain, especially when MC-Rx lacks prior knowledge of interferer statistics or operates near saturation. To address this, we introduce a frequency-domain detection (FDD) technique for bioFET-based MC-Rxs, which exploits the distinct binding reaction rates of different ligand types, reflected in the power spectrum of binding noise. Compared to conventional time-domain detection (TDD) technique, this method offers improved detection performance under stochastic molecular interference. We analyze the bit error probability (BEP) of FDD, confirming its superior performance in various interference scenarios. Moreover, the theoretical performance limits of FDD are validated through a particle-based spatial stochastic simulator, simulating binding reactions on MC-Rx within microfluidic channels. © 1972-2012 IEEE.Publication Metadata only Implications of node selection in decentralized federated learning(IEEE, 2023) Department of Computer Engineering; Lodhi, Ahnaf Hannan; Akgün, Barış; Özkasap, Öznur; Department of Computer Engineering; Graduate School of Sciences and Engineering; College of EngineeringDecentralized Federated Learning (DFL) offers a fully distributed alternative to Federated Learning (FL). However, the lack of global information in a highly heterogeneous environment negatively impacts its performance. Node selection in FL has been suggested to improve both communication efficiency and convergence rate. In order to assess its impact on DFL performance, this work evaluates node selection using performance metrics. It also proposes and evaluates a time-varying parameterized node selection method for DFL employing validation accuracy and its per-round change. The mentioned criteria are evaluated using both hard and stochastic/soft selection on sparse networks. The results indicate that the bias associated with node selection adversely impacts performance as training progresses. Furthermore, under extreme conditions, soft selection is observed to result in higher diversity for better generalization, while a completely random selection is shown to be preferable with very limited participation.Publication Metadata only Joint delay-Doppler index modulation for orthogonal time frequency space modulation(IEEE-Inst Electrical Electronics Engineers Inc, 2024) ; Department of Electrical and Electronics Engineering; Tek, Yusuf İslam; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; ; Graduate School of Sciences and Engineering; College of Engineering;Orthogonal time frequency space (OTFS) is a promising waveform with outstanding performance in doubly-selective channels. Spreading information symbols across the entire delay-Doppler plane enables OTFS to exploit full diversity. Nevertheless, there is a need for enhanced reliability in OTFS systems to satisfy the rigorous requirements of forthcoming communication systems. To tackle this concern, we propose a scheme called joint delay-Doppler index modulation OTFS (JDDIM-OTFS). This scheme activates delay or Doppler resource elements in a subframe, and in order to obtain higher spectral efficiency, we use different constellations for each active resource block. Furthermore, we utilize a low-complex detector with the help of a greedy approach. Moreover, a theoretical upper bound is derived for the bit error rate (BER). Simulation results validate that JDDIM-OTFS outmatches classical OTFS and other benchmark schemes in terms of BER under perfect and imperfect channel conditions.Publication Metadata only Measurement and characteristic analysis of ris-assisted wireless communication channels in sub-6 ghz outdoor scenarios(IEEE, 2023) Lan, Jifeng; Sang, Jian; Zhou, Mingyong; Gao, Boning; Meng, Shengguo; Li, Xiao; Tang, Wankai; Jin, Shi; Cheng, Qiang; Cuit, Tie Jun; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; College of EngineeringReconfigurable intelligent surface (RIS)-empowered communication has recently drawn significant attention due to its superior capability in manipulating the wireless propagation environment. However, the channel modeling and measurement of RIS-assisted wireless communication systems in real environment has not been adequately studied. In this paper, we construct a channel measurement system using vector network analyzer (VNA) is used to investigate RIS-assisted wireless communication channel in outdoor scenarios at 2.6 GHz. New path loss (PL) models including angle domain information are proposed by refining the traditional close-in (CI) and floating-intercept (FI) models. In the proposed models, both influences of the distance from transmitter (TX) to RIS and the distance from receiver (RX) to RIS on the PL, are taken into account. In addition, the value of root mean square (RMS) delay spread of RIS-assisted wireless communication is found to be much smaller than that of the traditional non line-of-sight (NLOS) case, implying that RIS provides a virtual line-of-sight (LOS) link.Publication Metadata only Measurement-based characterization of physical layer security for ris-assisted wireless systems(IEEE, 2023) Kesir, Samed; Wikelek, Ibrahim; Pusane, Ali Emre; Gorcin, Ali; Department of Electrical and Electronics Engineering; Kayraklık, Sefa; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; CoreLab; Graduate School of Sciences and Engineering; College of EngineeringThere have been recently many studies demonstrating that the performance of wireless communication systems can be significantly improved by a reconfigurable intelligent surface (RIS), which is an attractive technology due to its low power requirement and low complexity. This paper presents a measurement-based characterization of RISs for providing physical layer security, where the transmitter (Alice), the intended user (Bob), and the eavesdropper (Eve) are deployed in an indoor environment. Each user is equipped with a software-defined radio connected to a horn antenna. The phase shifts of reflecting elements are software controlled to collaboratively determine the amount of received signal power at the locations of Bob and Eve in such a way that the secrecy capacity is aimed to be maximized. An iterative method is utilized to configure a Greenerwave RIS prototype consisting of 76 passive reflecting elements. Computer simulation and measurement results demonstrate that an RIS can be an effective tool to significantly increase the secrecy capacity between Bob and Eve.Publication Metadata only Multivariate extreme value theory based rate selection for ultra-reliable communications(IEEE-Inst Electrical Electronics Engineers Inc, 2024) ; Department of Electrical and Electronics Engineering; Mehrnia, Niloofar; Ergen, Sinem Çöleri; Department of Electrical and Electronics Engineering; ; Graduate School of Sciences and Engineering; College of Engineering;Diversity schemes play a vital role in improving the performance of ultra-reliable communication (URC) systems by transmitting over two or more communication channels to combat fading and co-channel interference. Determining an appropriate transmission strategy that satisfies the ultra-reliability constraint necessitates the derivation of the statistics of the channel in the ultra-reliable region and, subsequently, integration of these statistics into the rate selection while incorporating a confidence interval to account for potential uncertainties that may arise during estimation. In this paper, we propose a novel framework for ultra-reliable real-time transmission considering both spatial diversities and ultra-reliable channel statistics based on multivariate extreme value theory (MEVT). First, the tail distribution of the joint received power sequences obtained from different receivers is modeled while incorporating the inter-relations of extreme events occurring rarely based on the Poisson point process approach in MEVT. The optimum transmission strategies are then developed by determining the optimum transmission rate based on the estimated joint tail distribution and incorporating confidence intervals (CIs) into the estimations to cope with the availability of limited data. Finally, the system reliability is assessed by utilizing the outage probability metric. Through analysis of the data obtained from the engine compartment of the Fiat Linea, our study showcases the effectiveness of the proposed methodology in surpassing traditional extrapolation-based approaches. This innovative method not only achieves a higher transmission rate, but also effectively addresses the stringent requirements of ultra-reliability. The findings indicate that the proposed rate selection framework offers a viable solution for achieving a desired target error probability by employing a higher transmission rate and reducing the amount of training data compared to the conventional rate selection methods. Authors