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UYSAL, Murat

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Now showing 1 - 10 of 350
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    Outage performance of multi-hop hybrid FSO/RF communication systems
    (IEEE, 2015) Kazemi, H.; Uysal, Murat; Touati, F.; Haas, H.; Electrical & Electronics Engineering; UYSAL, Murat
    In this paper, five scenarios are considered for the realization of a multi-hop hybrid free-space optical/radio frequency (FSO/RF) communication system with decode-and-forward (DF) relaying, and analytical outage probability expressions are derived. The scenarios have a different number of FSO and RF links. The aim is to identify an optimum scenario to effectively improve the end-to-end reliability of the multi-hop hybrid FSO/RF system. Outage performance is investigated for the scenarios under various weather conditions through numerical examples. It is identified that for a given link distance the end-to-end outage probability is minimized when using a hybrid FSO/RF link between every two successive nodes of the multi-hop system, and this requires the maximum number of FSO and RF links to be deployed.
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    Performance analysis of cooperative diversity networks with imperfect channel estimation over Rician fading channels
    (IEEE, 2010) Ikki, S. S.; Feteiha, M.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
    In this paper, we examine the effect of channel estimation error on the error and outage probabilities of a multi-relay system with amplify-and-forward relaying over frequency-flat Rician fading channel. We consider orthogonal relaying and study both conventional cooperative systems (i.e., all relays participate in the relaying phase) and opportunistic cooperative systems (i.e., only the best relay participates in the relaying phase). Based on the derivation of effective signal-to-ratio (SNR) at the receiver taking into account channel estimation error, we obtain closed-form expressions for error and outage probabilities in high SNR regime. Such closed form solutions are highly desirable because they allow for rapid and efficient evaluation of system performance. Computer simulations are used to validate our analytical results.
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    Non-orthogonal multiple access (NOMA) for indoor visible light communications
    (IEEE, 2015) Kizilirmak, R. C.; Rowel, C. R.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
    Providing multiple access support to visible light communication (VLC) systems requires new networking architecture. Non-orthogonal multiple access (NOMA) is a proposed multiple access technique for future cellular systems. In this work, based on a realistic indoor channel conditions, we apply NOMA to indoor VLC channels and demonstrate its superior performance over orthogonal frequency division multiple access (OFDMA).
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    Broadcasting brake lights with MIMO-OFDM based vehicular VLC
    (IEEE, 2016) Turan, B.; Narmanlıoğlu, Ömer; Coleri Ergen, S.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Narmanlıoğlu, Ömer
    Inter-vehicular connectivity to enhance road safety and enable highly autonomous driving is increasingly becoming popular. Despite the prevalent works on radio-frequency (RF) based vehicular communication schemes, visible light communication (VLC) is considered to be a promising candidate for vehicular communications due to its low complexity and RF interference-free nature. This paper investigates applicability of VLC to enhance road safety based on real world measurements. Deployment of multiple light emitting diodes (LEDs) enables multiple-input multiple-output (MIMO) transmission in the context of vehicular VLC. We consider direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) based MIMO transmission scheme and evaluate the performances of different MIMO modes including repetition code (RC) and spatial multiplexing (SM), different modulation orders and different transmitter-receiver selection. The results reveal that selection of the closest transmitters to the receivers, provide better performance due to high signal-to-noise-ratio (SNR) requirements for RC mode. However, usage of all possible transmitters does not always yield better performance due to power division at the transmitter side. Moreover, SM suffers from channel correlation whereas the performance of RC shows more degradation on higher-order modulations that are required to yield the same throughput with SM.
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    A comparative evaluation of propagation characteristics of vehicular VLC and MMW channels
    (IEEE, 2023) Hosseinabadi, Fahimeh Aghaei; Eldeeb, H. B.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Hosseinabadi, Fahimeh Aghaei
    Vehicle-to-vehicle (V2V) communication is an underlying key technology to realize future intelligent transportation systems. Both millimeter wave (MMW) communication and visible light communication (VLC) are strong candidates to address V2V connectivity. Most of the earlier literature focuses on an individual technology. In an effort to better highlight the differences and relative advantages of these two competing technologies, we provide a comprehensive one-to-one comparison between vehicular VLC and MMW channels in this paper. For this purpose, we utilize ray tracing simulations which enable the consideration of three-dimensional modeling of the propagation environment and allow the study of various system parameters and road conditions in both line-of-sight and non-line-of-sight conditions. Under the same settings, we present the received signal strengths for both systems and investigate the channel characteristics for communication between two vehicles in the same lane as well as in different lanes with a lateral shift. We also analyze the impact of low, medium, and high density of neighbor vehicles as well as partial and complete blockage.
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    Deep reinforcement based power allocation for the max-min optimization in non-orthogonal multiple access
    (IEEE, 2020) Siddiqi, U. F.; Sait, S. M.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
    NOMA is a radio access technique that multiplexes several users over the frequency resource and provides high throughput and fairness among different users. The maximization of the minimum the data-rate, also known as max-min, is a popular approach to ensure fairness among the users. NOMA optimizes the transmission power (or power-coefficients) of the users to perform max-min. The problem is a constrained non-convex optimization for users greater than two. We propose to solve this problem using the Double Deep Q Learning (DDQL) technique, a popular method of reinforcement learning. The DDQL technique employs a Deep Q- Network to learn to choose optimal actions to optimize users' power-coefficients. The model of the Markov Decision Process (MDP) is critical to the success of the DDQL method, and helps the DQN to learn to take better actions. An MDP model is proposed in which the state consists of the power-coefficients values, data-rate of users, and vectors indicating which of the power-coefficients can be increased or decreased. An action simultaneously increases the power-coefficient of one user and reduces another user's power-coefficient by the same amount. The amount of change can be small or large. The action-space contains all possible ways to alter the values of any two users at a time. DQN consists of a convolutional layer and fully connected layers. We compared the proposed method with the sequential least squares programming and trust-region constrained algorithms and found that the proposed method can produce competitive results.
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    Capacity of hybrid cognitive radio networks with distributed VAAs
    (IEEE, 2010-09) Hong, X.; Wang, C.-X.; Uysal, Murat; Ge, X.; Ouyang, S.; Electrical & Electronics Engineering; UYSAL, Murat
    A cooperative hybrid cognitive radio (CR) network is proposed to simultaneously operate on a dedicated licensed band and a secondary band. The licensed band is used for communications between a base station (BS) and mobile CR users, whereas the secondary band is used to facilitate the licensed band communication by coordinating multiple CR users to form distributed virtual antenna arrays (VAAs). The capacity of the proposed CR network is studied at both the link and system levels. At the link level (single VAA case), we present an amplify-and-forward-based cooperative signaling scheme that employs power control to prevent harmful noise propagation. The resulting virtual multiple-input-multiple-output (MIMO) link capacity is derived and compared with the real MIMO system. At the system level (multiple VAAs case), the system capacity is derived as a function of multiple parameters, including the primary user density, CR user density, primary exclusion region radius, and VAA radius. Under an average interference power constraint, the maximum system capacity is further calculated by solving an optimization problem with adjustable system parameters. Numerical studies reveal that the proposed cooperative hybrid CR network has a fundamental advantage over a pure CR network by being insensitive to the characteristics of the coexisting primary network. This merit, however, relies on a high CR user density and a wide bandwidth of the secondary band.
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    Vehicular VLC system with selection combining
    (IEEE, 2022-11) Mohamed, Bassam Aly Abdelrahman; Elamassie, Mohammed; Uysal, Murat; Electrical & Electronics Engineering; ELAMASSIE, Mohammed; UYSAL, Murat; Mohamed, Bassam Aly Abdelrahman
    The availability of LED-based headlights and taillights makes possible the use of visible light communication (VLC) as a wireless access technology for vehicle-to-vehicle (V2V) communications. A critical concern to establish a reliable VLC link between two moving vehicles is the number of photodetectors and their locations on the destination vehicle. In this paper, we experimentally investigate the usage of dual photodetectors for a vehicular VLC system with selection combining to enhance signal reception in mobile conditions. We measure the electrical receive signal-to-noise ratio (SNR) for each individual photodetector as well as at the output of selection combiner in two different tracks involving straight and curved roads. Through data fitting to experimental data, we obtain a probability density function (PDF) to describe the instantaneous SNR. Based on the PDF, we derive a closed-form expression for the average bit error rate (BER) in the form of a finite summation and compare it with the measurements.
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    Performance of eU-OFDM based relay-assisted visible light communications
    (IEEE, 2016) Djengomemgoto, Gerard; Narmanlioglu, Omer; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Djengomemgoto, Gerard; Narmanlioglu, Omer
    In this paper, we consider relay-assisted visible light communication (VLC) system where intermediate light source cooperates with main light source. Following IEEE 802.15.7r VLC reference channel model, we assume presence of two difference light sources in office space. First one is information source employed on ceiling and second one is desk lamp. Our system builds upon enhanced unipolar optical orthogonal frequency division multiplexing (eU-OFDM) where task light performs amplify-and-forward relaying to assist the communication and operates in half-duplex mode. We investigate bit error rate performance of relay-assisted VLC system and demonstrate that cooperation brings significant performance gains over direct transmission.
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    EM-based joint time delay and channel estimation for cooperative communications
    (IEEE, 2011) Ak, S.; Odabaşıoğlu, N.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
    In this paper, we study joint time delay and channel estimation for a cooperative communication system with amplify-and-forward (AF) relaying. The proposed joint estimator is based on Expectation-Maximization (EM) algorithm and presents a low-complexity solution for iterative maximum likelihood (ML) estimation. Our simulation results demonstrate that the performance of proposed cooperative system lies within 0.5dB of the genie bound (i.e., perfect knowledge of time delays and fading coefficients) at a target bit error rate of 10-4 assuming the deployment of 4 pilot symbols and 3 iterations.