Orthogonal Frequency Division Multiplexing (orthogonal frequency division multiplexing, OFDM) has been widely applied in today's wireless communications. One major reason for the popularity of OFDM systems is that almost half of bandwidth can be saved from traditional systems with subcarriers' orthogonality. The transmitted signals may suffer Carrier Frequency Offsets (CFO), caused by Doppler effects and misadjustment of transmit and receive oscillators, and the orthogonality will be destroyed. The CFO will result Intercarrier Interference (ICI) and degrade the system performance. Therefor, CFO must be estimated accurately to remove the ICI. In 1998, H. Liu and U. Tureli proposed a CFO estimation algorithm based on Multiple Signal Classification (MUSIC)[ 11 ]. This method is blind and can provide hight accuracy. However, it only works in cyclic prefix (CP) OFDM system. In this thesis, we propose a MUSIC CFO estimation which can be used in zero padding (ZP) OFDM. After appropriate superposition of ZP-OFDM received signals, a CP-like signal can be obtained. Such that the MUSIC algorithm can be applied to ZP-OFDM.A modified singular value decomposition (SVD) channel estimation for ZP-OFDM with full-loaded subcarriers is also proposed in this thesis. From simulation results, the MUSIC CFO estimation for ZP-OFDM is workable. The performance of CFO estimation depends on the searching interval of the minimization process. Smaller searching results better. In SVD channel estimation, ZP-OFDM with null subcarriers can provide better root-mean-square error performance. The performance of SVD channel estimation is only related to the length of channel path.

A blind channel estimation based on subspace method for orthogonal frequency division multiplexing (OFDM) systems is proposed in this thesis. Conjugate symmetric property of the OFDM symbols in the frequency-domain, which will result real time-domain symbol, is used in the proposed method to build the noise subspace. When the performance of the proposed method is compared with that of conventional method using virtual carriers, more OFDM symbols are needed in the conventional method than those needed in the proposed method. In addition, the computational complexity of the conventional method is also larger than that of the proposed method. Although the proposed method has better performance and less computational complexity compared with the conventional method, it would lose half of the transmission data rate. From the simulation results, our proposed method outperforms the conventional method.

Ulm, Univ., Diss., 2008.

Ulm, Univ., Diss., 2007.

In this dissertation, the problem of synchronization for OFDM-based wireless communication systems is studied. In the first part of this dissertation, the sensitivity of both single input single output (SISO) OFDM and multiple input multiple output (MIMO) OFDM receivers to carrier and timing synchronization errors are analyzed. Analytical expressions and numerical results for the power of inter-carrier interference (ICI) are presented. It is shown that the OFDM-based receivers are quite sensitive to residual synchronization errors. In wide-sense stationary uncorrelated scattering (WSSUS) frequency-selective fading channels, the sampling clock timing offset results in rotation of the subcarrier constellation, while carrier frequency offsets and phase jitter cause inter-carrier interference. The overall system performance in terms of symbol error rate is limited by the inter-carrier interference. For a reliable information reception, compensatory measures must be taken. The second part of this dissertation deals with the impact of spatial diversity (usage of multiple transmit/receive antennas) on synchronization. It is found that with multiple transmit and receive antennas, MIMO-OFDM systems can take advantage of the spatial diversity to combat carrier and timing synchronization imperfections. Diversity can favorably improve the synchronization performance. Data-aided and non-data-aided maximum likelihood symbol timing estimators for MIMO-OFDM systems are introduced. Computer simulations show that, by exploiting the spatial diversity, synchronization performance of MIMO-OFDM systems in terms of mean squared error (MSE) of residual timing offset becomes significantly more reliable when compared to conventional SISO OFDM systems. Therefore, spatial diversity is a useful technique to be exploited in the deployment of MIMO-OFDM communication systems. In MIMO systems with synchronization sequences, timing synchronization is treated as a multiple hypotheses testing problem. Generalized likelihood ratio test (GLRT) statistics are developed for MIMO systems in frequency flat channels and MIMO-OFDM systems in frequency selective fading environments. The asymptotic performance of the GLRT without nuisance parameters is carried out. It is shown that the asymptotic performance of the GLRT can serve as an upper bound for the detection probability in the presence of a limited number of observations as well as a benchmark for comparing the performances of different timing synchronizers.

Ce mémoire est scindé en deux parties. La première partie traite de l'estimation de l'optimum combiner. Traditionnellement, l'optimum combiner est estimé à travers l'estimation du canal du signal désiré et la matrice de covariance de l'interférence plus bruit. Dans cette première partie, on propose d'estimer l'optimum combiner à travers l'estimation de deux filtres séparés, à savoir un filtre qui suppose que la transmission est sans bruit (ce filtre prend en compte l'interférence mais pas le bruit) et un filtre qui suppose que la transmission est sans interférences (ce filtre prend en compte le bruit mais pas l'interférence). Néanmoins ce type d'estimation reste optimal seulement dans le cas où le récepteur est composé de deux antennes et/ou la transmission n'est perturbée que par une seule source d'interférence, indépendamment du nombre d'antennes. Le cas d'une transmission avec un récepteur à deux antennes et une source d'interférence est simulé puis implémenté sur une cible type DSP en virgule fixe au format 16 bits.La deuxième partie est dédiée à l'estimation aveugle du canal où deux méthodes sont proposées. Ces deux méthodes sont basées sur les moments d'ordre supérieurs. La première méthode est une extension de l'algorithme de Viterbi & Viterbi avec résolution de l'ambiguïté inter sous-porteuses, pour le cas d'une transmission basée sur une forme d'onde OFDM. La deuxième méthode est construite autour du principe de l'auto déconvolution. On propose aussi dans cette partie une extension aux systèmes MIMO par l'introduction d'un précodage et d'un postcodage spatio-temporel adapté à la méthode d'estimation du canal et au type de la transmission. Enfin une étude de cas d'utilisation de cette extension MIMO est donnée pour un système de transmission basé sur le standard IEEE 802.11. / This thesis is split into two parts. The first part deals with the estimation of the optimum combiner. Traditionally, the optimum combiner is estimated through the estimation of both the desired signal channel and the covariance matrix of interference plus noise.In this first part, we propose to estimate the optimum combiner by estimating two separate filters, namely a filter which assumes that the transmission is noiseless (taking into account the interference but not the noise) and a filter which assumes that the transmission is no interfered (taking into account the noise but not the interference). However this method of estimation is optimal only in the case where the receiver has two antennas and/or transmission undergone a single source of interference regardless of the number of antennas. The case of a transmission with a receiver equipped with two antennas and interfered by a single source of interference, is simulated and implemented on a fixed-point DSP target in 16 bit format.The second part is dedicated to the blind channel estimation where two methods are proposed. Both methods are based on the higher order moments. The first method can be viewed as an extension of the Viterbi & Viterbi algorithm, with inter subcarrier ambiguity solving, for the case of an OFDM waveform. The second method is built around the principle of self deconvolution. In this section, an extension to MIMO systems based on a space-time pre-coding and postcoding is introduced. Finally a case of application of this extension, for a MIMO transmission system based on the IEEE 802.11 standard, is analysed and simulated.

ITC/USA 2009 Conference Proceedings / The Forty-Fifth Annual International Telemetering Conference and Technical Exhibition / October 26-29, 2009 / Riviera Hotel & Convention Center, Las Vegas, Nevada / Orthogonal frequency division multiplexing (OFDM) is a new technique for data transmission. Conforming to the final draft of OFDM-based DVB-T (ETSI EN 300 744 V1.6.1), which is intended for digital terrestrial television broadcasting, a DVB-T baseband system is designed. The system performance is simulated in MATLAB using Simulink. Then it is implemented on Field Programmable Gate Array (FPGA) with the help of System Generator software. The result shows that OFDM is robust against multipath effect and convenient for implementation as well, thus owning a quite promising future.

Les travaux de recherche présentés dans cette thèse portent dans un premier temps sur la fiabilisation de la technologie CPL et dans un second temps sur le diagnostic de réseaux filaires en utilisant le CPL dans le contexte aéronautique. L’objectif de l’étude est de fiabiliser le lien CPL en respectant les contraintes de temps de latence imposées par le domaine aéronautique. Nous avons introduit des erreurs de fréquence d'échantillonnage afin d’estimer les effets d'une désynchronisation sur le taux d'erreurs binaires. Ensuite une optimisation d'un algorithme d'estimation de l’erreur a été étudiée. La seconde partie de la thèse est consacrée à la surveillance des réseaux filaires embarqués, l'objectif étant d'utiliser la communication CPL comme moyen de détection de défaut. Nous avons centré notre étude sur la détection d’un défaut suite à un arc électrique, modélisé par une résistance parallèle, ou d’un défaut dans une connectique, modélisé par une résistance série sur un fil de la bifilaire. Afin de disposer des caractéristiques de canaux avec et sans défaut, un banc de test représentatif d'un harnais multibranches a été conçu avec différents échantillons de défauts résistifs. Nous avons proposé plusieurs coefficients de détection de défauts basés sur la corrélation, et la distance euclidienne ainsi qu’un algorithme aveugle basé sur les symboles reçus. Afin d'éviter des cas de fausse alarme ou d’absence de détection, des seuils de détection ont été discutés. Les différents coefficients de détection et les seuils ont été comparés et éprouvés en présence de bruit blanc en termes de la puissance émise, et du taux de détection. / The research presented in this thesis focuses firstly on the reliability of PLC technology and secondly on the diagnosis of wired networks using PLC in the aeronautical sector. First of all, we studied the OFDM synchronization in a PLC communication chain for data transmission on avionic systems. The objective of the study is to make the PLC link more reliable by respecting the latency constraints set in the aeronautical domain. We introduced sampling frequency errors in order to analyze the effects of sampling frequency desynchronization on the bit error rate. We then proposed an optimized algorithm for error estimation. The second contribution of the thesis is devoted to the monitoring of embedded wired networks and the objective being to use PLC communication as a mean of detecting a fault. We focused our study on the detection of a fault following an electric arc, modeled by a parallel resistance, or a connection fault, modeled by a series resistance on a wire of the two-wired cable. In order to have the channel characteristics with and without faults, a test bench of a multi-branch harness has been designed with different resistive fault samples. We proposed several correlation-based fault detection coefficients, Euclidean distance, and a blind algorithm based on received symbols. In order to avoid false alarms or no detection, detection thresholds were discussed. The different detection coefficients and the thresholds were compared and tested in the presence of white noise in terms of the transmitted power and the detection rate.

Use of multiple antennas at both ends of a communication link, known as multiple-input multiple-output (MIMO), increases the reliability and/or capacity of that link. Orthogonal frequency division multiplexing (OFDM) is an effective technique for high data rate transmission over frequency selective channels. At this time MIMO-OFDM has been proposed for many emerging standards and seems to be a promising solution for future high data rate wireless communications. In the first part of this thesis, a novel sub-optimum detection method for spatially multiplexed multicarrier code division multiplexing (SM-MC-CDM) transmission is proposed. It is shown that compared to the spatially multiplexed OFDM (SM-OFDM), the frequency domain spreading in SM-MC-CDM systems results in an additional diversity gain. To take advantage of diversity and multiplexing while mitigating the interference, a low complexity efficient detector employing unified successive interference cancellation (U-SIC) is designed. Analytical results for the performance and capacity of zero-forcing (ZF) U-SIC are provided. Further performance improvement is achieved by adopting an iterative subcarrier reconstruction-detection algorithm in conjunction with the U-SIC. The results demonstrate significant performance improvement over other existing methods of comparable complexity. Performance of turbo-coded SM-MC-CDM transmission is also investigated. In the next part of the thesis, multiuser MIMO downlink is considered. Efficient transmission schemes based on zero-forcing (ZF) linear receiver processing, eigenmode transmission and partial channel state information are proposed. The proposed schemes utilize a handshaking procedure between the BS and the users to select (schedule) a subset of users and determine the precoding matrix at the base station (BS). The advantage of the proposed limited feedback schemes lies in their relatively low complexity scheduling algorithms and high sum rate throughput, even for a small pool of users. For large user pools and when the number of antennas at each user terminal is at least equal to the number of antennas at the BS, we show that the proposed scheme is asymptotically optimal in the sense that it achieves the same sum rate as the optimum scheme as the number of users approaches infinity. Next, net throughput is used as a benchmark to compare several MIMO-OFDM downlink transmission schemes with complete CSIT and also with limited feedback. Considering limited feedback per chunk user scheduling for MIMO-OFDM downlink, it is shown that there exists a chunk size which maximizes the average net throughput. It is shown that the net throughput maximizing chunk size depends on the number of users in the system and the communication channel's characteristics. Finally, future directions for possible research are given.

Orthogonal frequency-division multiplexing (OFDM) has been employed in several current and future 4-th generation (4G) wireless standards. Frequency offsets in OFDM introduce intercarrier interference (ICI). Channel estimations are also required. This thesis focuses on the channel and frequency offset estimation for OFDM-based systems. For cooperative-relay OFDM with frequency offsets, where inter-relay interference (IRI) exists, channel estimation is developed. Optimal pilot designs are proposed by minimizing the IRI in the mean square error (MSE) of the least square (LS) channel estimation. The impact of frequency offset on the channel estimation accuracy is derived. The pairwise error probability (PEP) with both the frequency offset and channel estimation errors is evaluated. The power allocation is discussed. For multiple-input multiple-output (MIMO) OFDM systems, channel and frequency offset estimation errors are investigated. The signal-to-interference-and-noise ratio (SINR) is first analyzed given channel and frequency offset estimation errors. The bit error rate (BER) is then approximated for multiple-antenna reception with maximal ratio combing (MRC) and equal gain combining (EGC). For orthogonal frequency-division multiplexing access (OFDMA) systems, the variance of the frequency offset estimation is derived as a function of SINR and signal-to- noise ratio (SNR). This variance information is exploited to improve the accuracy of frequency offset estimators. A successive interference cancellation (SIC)-based frequency offset estimator is also developed. The accuracy of frequency offset estimation of the OFDMA uplink can also be improved by using the cooperative relaying. Both conventional amplify-and-forward (AF) relays and new decode-and-compensate-and-forward (DcF) relays are studied. The frequency offset estimate is derived from combining different link estimates. In addition, when CSI is available, a scheme is proposed to adaptively switch between the cooperative and conventional (no relaying) transmissions to optimize the frequency offset estimation. / Wireless Communications