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Publication Open Access A hybrid architecture for federated and centralized learning(Institute of Electrical and Electronics Engineers (IEEE), 2022) Elbir, Ahmet M.; Papazafeiropoulos, Anastasios K.; Kourtessis, Pandelis; Chatzinotas, Symeon; Department of Electrical and Electronics Engineering; Ergen, Sinem Çöleri; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 7211Many of the machine learning tasks rely on centralized learning (CL), which requires the transmission of local datasets from the clients to a parameter server (PS) entailing huge communication overhead. To overcome this, federated learning (FL) has been suggested as a promising tool, wherein the clients send only the model updates to the PS instead of the whole dataset. However, FL demands powerful computational resources from the clients. In practice, not all the clients have sufficient computational resources to participate in training. To address this common scenario, we propose a more efficient approach called hybrid federated and centralized learning (HFCL), wherein only the clients with sufficient resources employ FL, while the remaining ones send their datasets to the PS, which computes the model on behalf of them. Then, the model parameters are aggregated at the PS. To improve the efficiency of dataset transmission, we propose two different techniques: i) increased computation-per-client and ii) sequential data transmission. Notably, the HFCL frameworks outperform FL with up to 20% improvement in the learning accuracy when only half of the clients perform FL while having 50% less communication overhead than CL since all the clients collaborate on the learning process with their datasets.Publication Open Access A low-SWaP, low-Cost transceiver for physically secure UAV communication with visible light(Springer, 2020) Department of Electrical and Electronics Engineering; Ergen, Sinem Çöleri; Soner, Burak; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 7211; N/AUnmanned aerial vehicles (UAV) are expected to utilize optical wireless technologies alongside radio frequency technologies for reliable, secure and high bandwidth communications. While terrestrial and atmospheric laser-based solutions in the past have achieved physically secure communication with very complex beam tracking/pointing mechanisms with large and costly telescopes, such systems are neither suitable nor necessary for medium-range (<100 m) commercial UAV communications. With the proliferation of low-cost solid-state lighting equipment and visible band photodetectors, visible light communications (VLC) offer a low-size-weight-and-power (SWaP) and low-cost solution. This paper presents a novel low-SWaP and low-cost transceiver for physically secure VLC in medium-range commercial UAV applications. Full implementation details for a proof-of-concept prototype built completely with off-the-shelf components are also reported.
Publication Metadata only A mechanical transduction-based molecular communication receiver for ınternet of nano things (IoNT)(Assoc Computing Machinery, 2021) N/A; Department of Electrical and Electronics Engineering; Aktaş, Dilara; Akan, Özgür Barış; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 6647Molecular conununication (MC) is one of the most promising technology to enable nanonetworks. Despite many aspects of MC have been investigated broadly, the physical design of the MC receiver has gained little interest. High-performance MC receivers based on bioFETs are proposed and extensively analyzed. However, they have some challenges such as limited detection with charged molecules, Debye screening, and the need for reference electrodes. To overcome these shortcomings, we propose a mechanical-based transducing scheme. In particular, we focus on a Flexure field-effect transistor (FET)-based MC receiver architecture, which provides exponentially high sensitivity by utilizing a nonlinear electromechanical coupling. In addition, the detection of neutral molecules with much simpler instrumentation is possible. In this paper, we analyze its fundamental performance metrics; sensitivity, noise power, signal-to-noise ratio, and the symbol error probability, from an MC theoretical perspective.Publication Open Access A new RIS architecture with a single power amplifier: energy efficiency and error performance analysis(Institute of Electrical and Electronics Engineers (IEEE), 2022) Alexandropoulos, George C.; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Taşçı, Recep Akif; Kılınç, Fatih; Faculty Member; Master Student; Researcher; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; 149116; N/A; N/AMany electrochemical devices are based on the fundamental process of ion migration and accumulation on surfaces. Complex interplay of molecular properties of ions and device dimensions control the entire process and define the overall dynamics of the system. Particularly, for ionic liquid-based electrolytes it is often not clear which property, and to what extent, contributes to the overall performance of the device. Herein we use X-ray photoelectron spectroscopy (XPS) while the device is under electrical bias. Such a procedure reveals localized electrical potential developments, through binding energy shifts of the atomic core levels, in a chemically specific fashion. Combining it with square-wave AC modulation, the information can also be extended to time domain, and we investigate devices configured as a coplanar capacitor, with an ionic liquid as the electrolyte, in macro-dimensions. Our analysis reveals that a nonlinear voltage profile across the device emerges from spatially non-uniform electrical double layer formation on electrode surfaces. Interestingly the coplanar capacitor has an extremely slow time response which is particularly controlled by IL film thickness. XPS measurements can capture the ion dynamics in the tens of seconds to microseconds range, and reveal that ionic motion is all over the device, including on metallic electrode regions. This behavior can only be attributed to motion in more than one dimension. The ion dynamics can also be faithfully simulated by using a modified PNP equation, taking into account steric effects, and device dimensions. XPS measurements on two devices with different dimensions corroborated and validated the simulation results. The present results propose a new experimental approach and provide new insights into the dynamics of ions across electrochemical devices.Publication Open Access A novel NOMA solution with RIS partitioning(Institute of Electrical and Electronics Engineers (IEEE), 2022) Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Khaleel, Aymen; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 149116; N/AReconfigurable intelligent surface (RIS) empowered communications with non-orthogonal multiple access (NOMA) has recently become an appealing research direction for nextgeneration wireless communications. In this paper, we propose a novel NOMA solution with RIS partitioning, where we aim to enhance the spectrum efficiency by improving the ergodic rate of all users, and to maximize the user fairness. In the proposed system, we distribute the physical resources among users such that the base station (BS) and RIS are dedicated to serve different clusters of users. Furthermore, we formulate an RIS partitioning optimization problem to slice the RIS elements between the users such that the user fairness is maximized. The formulated problem is shown to be a non-convex and non-linear integer programming (NLIP) problem with a combinatorial feasible set, which is challenging to solve. Therefore, we exploit the structure of the problem to bound its feasible set and obtain a sub-optimal solution by sequentially applying three efficient search algorithms. Furthermore, we derive exact and asymptotic expressions for the outage probability. Simulation results clearly indicate the superiority of the proposed system over the considered benchmark systems in terms of ergodic sum-rate, outage probability, and user fairness performance.Publication Metadata only A novel orthogonal frequency division multiplexing with index modulation waveform with carrier frequency offset resistance and low peak-to-average power ratio(Wiley, 2022) Kucukyavuz, Defne; Onat, Furuzan Atay; N/A; Department of Electrical and Electronics Engineering; Gürol, İlter Erol; Başar, Ertuğrul; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 149116In this paper, we propose a novel orthogonal frequency division multiplexing (OFDM) scheme with high carrier frequency offset (CFO) resistance and low peak-to-average power ratio (PAPR). In this scheme, we consider a hybrid model with two subblock types, namely, pilot subblocks and standard subblocks. In pilot subblocks, active subcarriers are utilized for PAPR reduction while inactive carriers generated by the index modulation (IM) are utilized for the coarse CFO estimation. For standard subblocks, we consider unique subcarrier activation patterns (SAPs) with high-diversity IM to enhance the bit error performance of the overall system. Additionally, the inactive data tones in standard subblocks are utilized for fine CFO estimation, which enhances the CFO estimation quite significantly. Furthermore, in this paper, we show that proposed hybrid OFDM-IM (H-OFDM-IM) scheme can outperform conventional OFDM-IM and classical OFDM both in CFO estimation and PAPR reduction without requiring transmission of any side information. Finally, we show both mathematically and through computer simulations that proposed H-OFDM-IM can achieve a satisfactory bit error rate (BER) performance under high CFO scenarios.Publication Metadata only A novel reconfigurable intelligent surface-supported code index modulation-based receive spatial modulation system(IEEE-Institute of Electrical and Electronics Engineers, 2024) Ozden, Burak Ahmet; Cogen, Fatih; Aydin, Erdogan; Ilhan, Haci; Wen, Miaowen; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; College of EngineeringToday's wireless communication networks have many requirements such as high data rate, high reliability, low latency, low error data transmission, and high energy efficiency. High-performance index modulation (IM) techniques and reconfigurable intelligent surface (RIS) technology, which has recently attracted the attention of researchers, are strong candidates to meet these requirements. This paper introduces a novel RIS-supported code IM-based receive spatial modulation (RIS-CIM-RSM) system. The proposed RIS-CIM-RSM system uses quadrature amplitude modulation (QAM) symbols, receive antenna indices, and spreading code indices for wireless data transmission. In the proposed system, an RIS applies a phase rotation that maximizes signal-to-noise ratio (SNR) to the signals coming to the reflecting elements and directs them to the selected receive antenna. Performance analyses of the proposed RIS-CIM-RSM system such as data rate, throughput, and energy saving are obtained. The results obtained show that the proposed RIS-CIM-RSM system is superior to the counterpart RIS-based IM systems in the literature in terms of data rate, throughput, energy saving, and error performance.Publication Open Access A novel RIS-assisted modulation scheme(Institute of Electrical and Electronics Engineers (IEEE), 2021) Yang, Liang; Meng, Fanxu; Hasna, Mazen O.; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 149116In this work, in order to achieve higher spectrum efficiency, we propose a reconfigurable intelligent surface (RIS)-assisted multi-user communication uplink system. Different from previous work in which the RIS only optimizes the phase of the incident users’ signal, we propose the use of the RIS to create a virtual constellation diagram to transmit the data of an additional user. We focus on the two-user case and develop a tight approximation for the probability distribution function (PDF) of the minimum distance between constellation points of both users. Then, based on the proposed statistical distribution, we derive the analytical expressions of the average bit error rate of the considered two users. The letter also shows the trade off between the performance of two users as a function of the proposed phase shift at the RIS.Publication Metadata only A performance comparison of single-radio multi-channel medium access control protocols(Ieee, 2020) Ucar, Seyhan; N/A; Department of Electrical and Electronics Engineering; Kaytaz, Umuralp; Ergen, Sinem Çöleri; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 7211Single-radio multi-channel Medium Access Control (MAC) protocols aim to transmit in parallel on distinct channels while reducing the hardware cost. Although a variety of MAC protocols have been proposed in this context, no detailed classification and performance comparison is available. In this paper, we first classify previous efforts by their rendezvous characteristics as single- and multi-rendezvous protocols. Multirendezvous protocols have the capability of supporting simultaneous handshaking on different channels whereas with single-rendezvous protocols only asynchronous channel negotiations are allowed. Then, we further classify these protocols according to their spectrum decision mechanisms. We demonstrate the functionality of single- and multi-rendezvous protocols under different scenarios via extensive simulations. Our findings show that multi-rendezvous protocol performs better when the transmission range is low and less number of channels is available. Single-rendezvous protocols, on the other hand, are more suitable for networks with larger traffic loads due to their slot-based decision-making schemes. / Tek telsiz çok kanallI Ortam Erişim Kontrolü (MAC) protokolleri, donanIm maliyetini düşürürken farklI kanallarda paralel iletimi amaçlamaktadIr. Bu bağlamda çeşitli MAC protokolleri önerilmiş olmasIna rağmen, ayrIntIlI bir sInIflandIrma ve performans karşIlaştIrmasI mevcut değildir. Bu çalIşmamIzda, öncelikle mevcut protokolleri tek ve çok randevulu protokoller olarak buluşturma özelliklerine göre sInIflandIrmaktayIz. Çok randevulu protokoller, farklI kanallarda eşzamanlI el sIkIşmasInI destekleme yeteneğine sahipken, tek randevulu protokoller yalnIzca eşzamanlI olmayan kanal görüşmelerine izin vermektedir. Daha sonra, bu protokolleri spektrum karar mekanizmalarIna göre tekrar sInIflandIrmaktayIz. KapsamlI başarIm ölçümü aracIlIğIyla tek ve çok randevulu protokollerin işlevselliğini farklI senaryolar altInda göstermekteyiz. BulgularImIz, çok randevulu protokolün düşük iletim menzili ve az sayIda kanal bulunduğunda daha iyi performans sergilediğini göstermektedir. Diğer yandan, tek randevulu protokoller, sIra tabanlI karar planlamalarI nedeniyle büyük trafik yükü olan ağlar için daha uygun olmaktadIr.Publication Metadata only A performance comparison of single-radio multichannel medium access control protocols(Institute of Electrical and Electronics Engineers Inc., 2020) Department of Electrical and Electronics Engineering; N/A; N/A; Ergen, Sinem Çöleri; Uçar, Seyhan; Kaytaz, Umuralp; Faculty Member; PhD Student; PhD Student; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 7211; N/A; N/ASingle-radio multi-channel Medium Access Control (MAC) protocols aim to transmit in parallel on distinct channels while reducing the hardware cost. Although a variety of MAC protocols have been proposed in this context, no detailed classification and performance comparison is available. In this paper, we first classify previous efforts by their rendezvous characteristics as single- and multi-rendezvous protocols. Multirendezvous protocols have the capability of supporting simultaneous handshaking on different channels whereas with single-rendezvous protocols only asynchronous channel negotiations are allowed. Then, we further classify these protocols according to their spectrum decision mechanisms. We demonstrate the functionality of single- and multi-rendezvous protocols under different scenarios via extensive simulations. Our findings show that multi-rendezvous protocol performs better when the transmission range is low and less number of channels is available. Single-rendezvous protocols, on the other hand, are more suitable for networks with larger traffic loads due to their slot-based decision-making schemes.