Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Conference Object Citation - Scopus: 1Yüksek İrtifa Platform İstasyonları için Korelasyon Matrisinin Yaklaşımı(Institute of Electrical and Electronics Engineers Inc., 2021) Cengiz, Aybüke; Tedik Başaran, Semiha; Karabulut Kurt, Güneş; Özbek, Berna; Yanıkömeroğlu, HalimA High Altitude Platform Station (HAPS) is a new and promising technology with different uses like wireless communication services, traffic monitoring, navigation applications and Internet of Things (IoT) applications. Since it is positioned much higher than ground-level users, line-of-sight (LOS) is more common in HAPS systems. Especially in HAPS systems that include multiple antennas, the effect of correlation between antennas is an important factor limiting the performance. The effect of correlation is strongly influenced by the spacing between the antenna elements and environmental factors. This effect can be reduced by the effect of rich scattering environment in terrestrial Multi-Input/Multi-Output (MIMO) systems, yet this observation is not applicable to HAPS systems. In this study, a tight bound expression for the correlation matrix which does not have an expression in closed form and has a significant place in the performance of HAPS systems has been obtained. Comprehensive numerical values have been obtained under different system parameters to demonstrate the convergence performance of the approximate expression to the real value, and the results have been compared. © 2021 IEEE.Conference Object Ofdm Sistemleri için Pilot Tabanlı Em Kanal Kestirim Başarımı(Institute of Electrical and Electronics Engineers Inc., 2007) Baştürk, İlhan; Özbek, BernaIn this work, pilot-based Expectation-Maximization (EM) channel estimation for OFDM systems are studied by considering different pilot placements. In the first method, the pilot symbols are distributed at certain locations in the OFDM time-frequency lattices to find appropriate initial values. In the second method, the pilot symbols are placed in the first OFDM symbol and then a sequential channel estimation is applied. These two methods are compared and the performances are studied in terns of mean square error and bit error rate. Moreover, the number of iterations used in the algorithm are given to compare the complexity of the methods.Conference Object Citation - Scopus: 2Genetik Algoritma ile Çok Kullanıcı Dfbç Sistemleri için Toplam Veri Hızı Enbüyüklemeli Uyarlanır Alttaşıyıcı ve Bit Yerleşimi(Institute of Electrical and Electronics Engineers Inc., 2008) Özbek, Berna; Le Ruyet, DidierIn this work, we perform the adaptive subcarrier and bit allocation to maximize the sum data rate for multiuser OFDM systems using genetic algorithm. Since the convergence time of the genetic algorithm could be slow, we propose to use a different initial condition to accelerate it without increasing the complexity of genetic algorithm and sacrificing the performance of the system. We obtain the simulation results and compare them with the classical methods. We show that the genetic algorithm with proposed initial condition gives better performances than the classical ones for sum data rate maximized allocation.Conference Object Citation - WoS: 2Citation - Scopus: 2Gezgin Rölelerin Dfbçe Tabanlı Hücresel Ağ Performansına Etkisi(Institute of Electrical and Electronics Engineers Inc., 2014) Baştürk, İlhan; Özbek, BernaOFDMA-based mobile relay enhanced cellular networks which have lower infrastructure cost and larger coverage areas are important technologies for the next generation communication systems since they allow to communicate at higher data rates. However, it is required to develop efficient radio resource management algorithms in order to reveal these advantages. Thus, in this study, relay selection and resource allocation algorithms are proposed for the mobile relay enhanced cellular networks. It is revealed that the number of satisfied users and the data rate of the cell-edge users that are far from the base station and have lower channel gains are increased by using mobile relays with the proposed algorithms.Conference Object Energy-Efficient Resource Allocation for Multiuser Ofdm Wireless Networks(Institute of Electrical and Electronics Engineers Inc., 2015) Durbaş, Şükrü; Özbek, BernaIn the last decades, the energy consumption in wireless communication systems is increasing and becoming a major problem for cellular operators. As a result of that, solutions to increase energy-efficiency (EE) in wireless networks are becoming more popular. Therefore, the resource allocation algorithms that provides EE for multiuser OFDM based wireless networks is presented in this paper. Then, the conventional spectrum-efficient (SE) designs are compared with EE based schemes and EE characteristics of the system has been analyzed. The simulation results show the advantages of EE scheme especially on high channel-gain-to-noise ratio and low number of users cases.Conference Object Citation - WoS: 2Citation - Scopus: 3Heterojen ağlarda iyileştirilmiş veri katarı seçimi ile girişim hizalama(IEEE, 2015) Le Ruyet, Didier; Aycan, Esra; Özbek, BernaIn this article, interference that occurs in heterogeneous networks is studied and a new stream selection approach with an interference alignment solution is presented. In the proposed algorithm, stream selection starts with the streams that belongs to the pico cell users and continues to select the strongest streams that increase the total sum rate. The power of each stream with the precoding and the postcoding vectors are calculated by performing singular value decomposition (SVD). Virtual channels that belong to the selected stream are calculated by using precoding and postcoding vectors. After the selection procedure, the interferences between the selected stream and the remaining streams are mitigated by orthogonally projecting the channel matrices of the remaining streams to the virtual channels of the selected stream.Conference Object Citation - Scopus: 1Kablosuz Ağlarda Hücre Seçim Algoritmalarının Başarımı(IEEE, 2012) Aycan, Esra; Özbek, BernaIn this paper, the performance of signal to interference and noise ratio (SINR) and distance based cell selection algorithms for wireless systems are examined considering different cells. The performance results are obtained by assuming the users are not moving within a setup includes macrocell, picocell and femtocells. The comparisons of the obtained results are shown in terms of SINR and load in the cells. © 2012 IEEE.Conference Object Çoklu Kullacılı Çgtç-dfbçe Sistemleri için Adil Kaynak Paylaşımı(IEEE, 2011) Baştürk, İlhan; Özbek, BernaIn this study, the user selection at the base station and resource allocation algorithms are presented for multiuser multipleinput single-output orthogonal frequency division multiple access (MU-MISO OFDMA) systems when the number of users in a cell is much more than the number of transmitter antennas. A fair resource allocation algorithm is proposed in order not to victimize the users far from the base stations. In general, Shannon capacity has been shown as the performance measure for MU-MISO-OFDMA systems. In this paper, the bit error rate (BER) is used as an performance metric instead of the Shannon capacity since, increasing the system capacity does not automatically yield the best performance in terms of error rates. It is also shown that the proposed algorithm provides fairness by calculating the fairness index. © 2011 IEEE.Conference Object Çoklu Antenli Ev Baz İstasyonlarının İç Mekan Kapasite Başarımı(IEEE, 2011) Aycan, Esra; Özbek, BernaIn this paper, indoor channel models for femtocells are examined and capacity performances are obtained. In order to evaluate the indoor channel models, the performance results are obtained employing the ray tracing based Wireless Insite model, the well known i.i.d model and the IEEE802.11n model. All the comparisons are shown for the single input single output (SISO) and the multiple input multiple output (MIMO) systems.1 © 2011 IEEE.Conference Object Citation - Scopus: 9Çoklu Antenli̇ Bili̇şsel Radyo Si̇stemleri̇nde Dönemli̇-duraǧan Özelli̇k Algılama Yöntemi̇ne Dayanan Spektrum Sezi̇mi̇(Institute of Electrical and Electronics Engineers, 2011) Üstok, Refik Fatih; Özbek, BernaCognitive radios aim to implement dynamic frequency usage which is proposed to increase the efficiency of the spectrum and can sense their environment, detect the idle bands and then allocate the secondary users into the detected idle bands without any interference to primary users' communications. Cyclostationary Feature Detection is one of the commonly used methods in the literature for spectrum sensing. This paper focuses on sensing the idle bands using cyclostationary feature detection method in Cognitive Radio based systems. The performance is improved by using the proposed multiple antenna technique and the simulation results are analyzed. © 2011 IEEE.