Electrical - Electronic Engineering / Elektrik - Elektronik Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/11
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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 Ofdm Sistemler için Pilot Tabanlı Yinelemeli Kanal Kestirimi(IEEE, 2008) Baştürk, İlhan; Özbek, BernaIn this study, pilot-based iterative channel estimation, which use virtual pilots, is proposed for OFDM systems In order to find final channel estimates, instead of averaging the group estimates, which are found by virtual pilots, it is proposed to combine them by taking into account their reliabilities that are calculated by using probability density function. Then the proposed method is compared to Expectation-Maximization (EM) algorithm with respect to performance and computational complexity. The performance results are given in terms of Bit Error Rate (BER) and Mean Squared Error (MSE). ©2008 IEEE.Conference Object Stbc-ofdm Sistemler için Em Tabanlı Kanal Kestirim Başarımı(Institute of Electrical and Electronics Engineers Inc., 2009) Baştürk, İlhan; Özbek, BernaBu çalışmada Uzay-zaman blok kodlu dikgen frekans bölmeli çoğullama sistemleri için Beklenti- Enbüyükleme algoritması yardımıyla frekans alanında her bir alttaşıyıcı için ayrı ayrı kanal katsayılarının kestirildiği bir algoritma önerilmiştir. Başlangıç kanal kestirimi olarak pilot sembollerden yararlanılmış ve bant genişliğinin verimli kullanılması için az sayda pilotla ardışıl kanal kestirimi yapılmıştır. Önerilen algoritma zaman alanında kanal katsayılarını Beklenti-Enbüyükleme algoritmasıyla kestiren ve hesaplama sırasında matris tersi işlemine ihtiyaç duyan algoritma ile karşılaştırılmış ve herhangi bir performans kaybı olmadan karmaşıklığı azalttığı gözlenmiştir. Her iki algoritma için bilgisayar benzetimleri yapılarak, başarımları bit hata oranı, en küçük karesel hata ve Beklenti-Enbüyükleme algoritmasında kullandıkları döngü sayıları ile incelenmiştir.Conference Object Citation - Scopus: 4Çoklu Antenli Bilişsel Radyo Sistemlerinde Enerji Algılamasına Dayanan Spektrum Sezimi(Institute of Electrical and Electronics Engineers Inc., 2009) Üstok, Refik Fatih; Özbek, BernaDünyanın tükenmeye yüz tutmuş sınırlı kaynakları olabilecek en etkili Şekilde kullanılması problemi; bugünkü toplumlarm üstesinden gelmeye çalıştığı en önemli problemlerden biridir. Doğal frekans spektrumu da sınırlıdır, bu nedenle verimli kullanılması gereklidir. Şu anda kullanılan durağan frekans tahsis etme yöntemleri, hızla artan miktardaki daha yüksek veri hızlı cihazların taleplerini karşılamada yetersiz kalmaktadır [1]. Bu nedenle de frekans spektrumunun kullanılabilirliğinin artırılması için spektrumun dinamik kullanımının, durağan kullanımdan ayırt edilmesi gerekmektedir. Spektrumun dinamik kullanılmasının ayırt edilmesi için; bilişsel radyo sistemleri, verimli spektrum kullanımı gibi kullanıcı gerekliliklerini uygun olarak yerine getiren yeni bir teknoloji olarak önerilmektedir. Bilişsel radyo sistemleri çevresini dinleyerek, işlemlerini hedeflenen başarımı sağlamak için ayarlayabilmektedir. Diğer bir deyişle bilişsel radyo sistemleri, ilk önce spektral çevresini sezinleyerek daha sonra parametrelerini, spektral verimliliği en büyükleyerek ortama göre değiştirebilirConference Object Citation - WoS: 1Citation - Scopus: 3Iterative Em-Based Channel Estimation for Stbc-Ofdm(Institute of Electrical and Electronics Engineers Inc., 2009) Baştürk, İlhan; Özbek, BernaIn this paper, an iterative EM based channel estimation algorithm is studied for STBC-OFDM systems. Compared to the time domain EM based channel estimation algorithm which needs matrix inversion, a frequency domain EM based channel estimation algorithm is proposed by estimating the channel coefficients for each subcarrier. The proposed channel estimation algorithm decreased the complexity without sacrificing the performance. The time domain and proposed frequency domain EM based channel estimation algorithms are compared in terms of bit error rate (BER), mean square error (MSE) and the number of iterations used in the EM algorithm.Conference Object Citation - Scopus: 3Reduced Feedback Designs for Sdma-Ofdma Systems(Institute of Electrical and Electronics Engineers Inc., 2009) Özbek, Berna; Le Ruyet, DidierIn SDMA-OFDMA wireless communication systems, the feedback load increases with the number of users, subcarriers and antennas in the cell. In this paper, we propose two efficient reduced feedback algorithms by selecting the clusters at the user side. For each cluster, we select the users according to their norm and their orthogonality. We evaluate the performance of the user selection algorithms considering the quantization effect. We also design a specific codebook design to quantize CSI for the proposed criterion.Conference Object Citation - WoS: 3Citation - Scopus: 3Adaptive Resource Allocation for Sdma-Ofdma Systems With Genetic Algorithm(Institute of Electrical and Electronics Engineers Inc., 2009) Özbek, Berna; Le Ruyet, DidierIn this paper, we propose an adaptive resource allocation algorithm to maximize the sum data rate for spatial division multiplexing access (SDMA) orthogonal division multiple access (OFDMA) systems using genetic algorithm. We perform Zero forcing (ZF) beamforming at the transmitter constructing single and multiple beams for OFDMA systems with multiple transmit antennas. In order to reduce the complexity and convergence time of genetic algorithm (GA), we present an initial condition obtained by allocating the user to the subcarrier with maximum precoding gain. The proposed algorithm reduces the complexity without sacrificing the performance. We obtain the simulation results and compare them with the classical static and adaptive allocation methods. We show that the GA with the proposed initial condition increases the sum data rate compared to classical GA and reduces the convergence time significantly for SDMA-OFDMA systems.Article Citation - WoS: 7Citation - Scopus: 8Performance Evaluation of Multicast Miso-Ofdm Systems(Springer Verlag, 2008) Özbek, Berna; Le Ruyet, Didier; Khanfir, HajerIn this paper, we analyze the performance of multicast orthogonal frequency division multiplexing (OFDM) systems with single and multiple transmit antennas. We show that the resource allocation that includes the subcarrier allocation, bit loading, and the precoding vector selection in the multiple-input single-output (MISO) case is a difficult optimization problem. Consequently, we propose suboptimal algorithms based on the maximization of the sum data rate and the maximization of the minimum user data rate criteria. For practical application, we consider a complete transmission chain by combining powerful erasure codes with the proposed algorithms. Using this scheme, we guarantee that each user receives the same amount of information to decode the same data. Simulation results show that, for both single-input single-output (SISO)-OFDM and MISO-OFDM cases, the proposed multicast OFDM systems achieve gains over the worst user case algorithm.Conference Object Semi-Orthogonal User Selection for Miso Systems With Quantized Feedback(Institute of Electrical and Electronics Engineers Inc., 2008) Khanfir, Hajer; Le Ruyet, Didier; Özbek, BernaFor MISO multi-user downlink wireless communication system with precoding at the transmission, the channel state information at the transmitter can provide tremendous capacity gains. However, the amount of feedback data increases with the number of users in the cell and the number of transmit antennas. In this paper, we study on different algorithms and criteria in order to significantly reduce the amount of feedback data. We associate the classical norm criterion with a criterion based on the orthogonality between the user channels. Without cooperation between the users, we only allow users that are semi-orthogonal to feedback their channel information (CQI and CDI) to the base station. The feedback CDI is quantized using a local grassmanian packing. We show that the proposed combined criterion with a finite feedback rate gives better performance compared to the norm criterion. Furthermore we show that the performance is not affected by CQI quantization.