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Yayın Hızlı sönümlemeli kanallarda yeni bir uzay-zaman-frekans kodlamalı OFDM sistem tasarımı(IEEE, 2004) Oğuz, Onur; Aygölü, Hasan Ümit; Panayırcı, ErdalBu çalışmada, frekans seçici hızlı sönümlemeli kanallarda telsiz iletişim için bir uzay-zaman-frekans çeşitlemesi yöntemi önerilmiştir. Önerilen yöntem, OFDM tekniğini kullanarak frekans seçici kanalı uygun hale getirdikten sonra sırasıyla dik uzay-frekans ve dik uzay-zaman blok kodlama uygulayarak uzay-zaman-frekans çeşitlemesi sağlamaktadır. Elde edilen sistemin başarımını arttırmak üzere sisteme uygun bu kafes kodlama tekniği araştırılmış ve uygun kafes yapısının oluşturulması için yeni kriterler ortaya çıkarılmıştır. Oluşan sistemin başarımını, bilgisayar benzetimleriyle incelenmiş ve var olan yöntemlerle karşılaştırılmıştır.Yayın Analysis of self noise in a clock recovery systems with a high-order nonlinearity(IEEE, 1998) Panayırcı, ErdalThis paper presents a new technique to compute efficiently the I and Q spectra, and the I - Q cross-spectrum of the self noise appearing at the output of the zero-memory, high order nonlinear device employed in a clock recovery system. It is known that these spectra play an important role in the phase jitter performance of the clock regenerator. The results are very general and applicable to many cases of practical interest. A numerical example provided at the end of the paper shows that the new approach yields very accurate results and is much faster that the usual computer simulation method.Yayın Channel estimation for space-time block coded OFDM systems in the presence of multipath fading(IEEE, 2002) Panayırcı, Erdal; Çırpan, Hakan AliIn this paper, a computationally efficient, non-data-aided maximum a posterlori(MAP) channel estimation algorithm is proposed for orthogonal frequency division multiplexing (OFDM) systems with transmitter diversity using space-time block coding. The Alamouti's transmit diversity scheme with two transmit antennas is employed here and generalized for OFDM systems. The algorithm requires a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve orthogonal expansion and estimates the complex channel parameters of each subcarriers iteratively using the Expectation Maximization(EM) method, which converges to the true MAP estimation of the unknown channel. Ananalytical expression is derived for the Modified Cramer-Rao. lower bound of the proposed MAP channel estimator. The performance is presented in terms of the mean-square error for a system employing QPS signaling.Yayın Analysis of self noise in a clock recovery systems with a high-order nonlinearity(Bogazici University Bebek, 1999) Panayırcı, ErdalThis paper presents a new technique to compute efficiently the I and Q spectra, and the I - Q cross-spectrum of the self noise appearing at the output of the zero-memory, high order nonlinear device employed in a clock recovery system. It is known that these spectra play an important role in the phase jitter performance of the clock regenerator. The results are very general and applicable to many cases of practical interest. A numerical example provided at the end of the paper shows that the new approach yields very accurate results and is much faster that the usual computer simulation method.Yayın Blind channel estimation for space-time coding systems with Baum-Welch algorithm(IEEE, 2002) Çırpan, Hakan Ali; Panayırcı, ErdalIn recent years, space-time coding is proposed to provide significant capacity gains over the traditional communication systems in fading wireless channels. In this paper, we consider the problem of blind estimation of the channel parameters along with space-time coded signals. Our proposed approach exploits the finite alphabet property of the space-time coded signals and is based on the unconditional signal model by treating the information sequence as stochastic I.I.D. sequences. The iterative Baum-Welch algorithm is then adapted to solve resulting unconditional ML optimization cost function. Finally, some simulation results are presented.Yayın A new algorithm for joint time and phase synchronization in OFDM systems(IEEE, 2000) Panayırcı, ErdalIn this paper, a computationally efficient algorithm is presented for joint maximum likelihood (ML) timing and carrier phase synchronization of OFDM systems employing M-PSK modulation scheme with additive Gaussian noise, based on the Expectation-Maximization (EM) method. A nondata-aided(NDA) scheme is considered for the joint timing and phase synchronizer which maximizes the low SNR limit of the likelihood function averaged over the MPSK signal constellation. For this, an EM algorithm is derived which estimates the timing offset and the phase rotations of each subcarrier iteratively and which converges to the true ML estimation of the unknown timing and phase. It is shown that the algorithm becomes independent of the signal-to-noise ratio for both low and high SNR cases. The algorithm is applied to the QPSK modulated OFDM systems and-it is concluded that for SNR values greater than 10 dB the convergence is achieved in first iteration and for SNR values less than 10 dB, at most in three iterations. It is also concluded that the convergence is independent of the initial starting points.Yayın EM-Based sequence estimation for wireless systems with orthogonal transmit diversity(IEEE, 2003) Panayırcı, Erdal; Aygölü, Hasan Ümit; Pusane, Ali EmreIn this paper, an optimum sequence estimation algorithm for wireless systems with Alamouti's two transmitter diversity in the presence of multipath fading is proposed. The algorithm is based on a jointly iterative channel and sequence estimation according to the maximum likelihood (ML) criterion, using the Expectation-Maximization (EM) algorithm employing M-PSK modulation scheme with additive Gaussian noise. The discrete multipath channel is represented in terms of the channel gains from each transmit antenna to the receive antenna. EM algorithm derived estimates jointly the complex channel parameters of each channel And the data sequence transmitted, iteratively, which converges to the true ML solution. The channel estimation is achieved in a simple way through the iterative equations by decoupling of the signals transmitted from different antennas. The algorithm is applied to the trellis coded modulation systems and efficiency of the algorithm proposed has been shown by the computer simulations. Simulation results show that the EM algorithm converges quickly for fast fading channels. The performance of the EM-based decoder approaches that of the ML receiver which has perfect knowledge of the channel.Yayın Joint ML timing and phase estimation in OFDM systems using the EM algorithm(IEEE, 2000) Panayırcı, Erdal; Georghiades, Costas N.In this paper, a computationally efficient algorithm is presented for joint maximum likelihood (ML) timing and carrier phase estimation of OFDM systems employing M-PSK modulation scheme with additive Gauissian noise, based on the Expectation-Maximization (EM) algorithm. A nondata-aided(NDA) scheme is considered for the joint timing and phase synchronizer which maximizes the low SNR limit of the likelihood function averaged over the M-PSK signal costellation. For this, an Ehl algorithm is derived which estimates the timing offset and the phase rotations of each subcarrier iteratively and which converges to the true ML estimation of the unknown timing and phase. It is shown that the algorithm becomes independent of the signal-to-noise ratio for both low and high SNR cases. The algorithm is applied to the QPSK modulated OFDM systems and it is concluded that for SNR values greater than 10 dB the convergence is achieved in first iteration and for SNR values less than 10 dB, at most in three iterations. It is also concluded that the convergence is independent of the initial starting points.Yayın Pilot-aided bayesian MMSE channel estimation for OFDM systems: Algorithm and performance analysis(IEEE, 2004) Şenol, Habib; Çırpan, Hakan Ali; Panayırcı, ErdalThis paper proposes a computationally efficient, pilot-aided minimum mean square error (MMSE) channel estimation algorithm for OFDM systems. The proposed approach employs a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve (KL) orthogonal expansion and estimates uncorrelated series expansion coefficients. Moreover, optimal rank reduction is achieved in the proposed approach by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the estimation algorithm. The performance of the proposed approach is studied through analytical and experimental results. We first consider the stochastic Cramer-Rao bound and derive the closed-form expression for the random KL coefficients. We then exploit the performance of the MMSE channel estimator based on the evaluation of minimum Bayesian MSE.Yayın Non-data aided EM-Based channel estimation for OFDM systems with time-varying fading channels(Kluwer Academic Publishers, 2002) Panayırcı, Erdal; Çırpan, Hakan AliIn this paper, a computationally efficient algorithm is presented for maximum a posteriori (MAP) channel estimation for OFDM systems employing M-PSK modulation scheme with additive Gaussian noise, based on the Expectation Maximization (EM) method. A non-data-aided scheme is considered for the estimation of a multipath time-varying channel by averaging over the M-PSK signal constellation. For this, an EM algorithm is derived which estimates the complex channel parameters of each subcarriers iteratively in frequency domain and which converges to the true MAP estimation of the Unknown channel, The algorithm requires a convenient representation of die discrete multipath fading channel based on the Karhumen-Loeve orthogonal expansion. The algorithm is applied to the QPSK modulated OFDM systems and efficiency of the method proposed is shown by the computer simulations.
