¡@¡@In recent year, orthogonal frequency division multiplexing ¡]OFDM¡^ technology has been widely used in high-speed communication systems. One primary reason for the popularity of OFDM is its ability to provide good performance in multi-path channels than the other systems through the use of Guard interval(GI). By using the Guard interval, it can convert these inter-symbol interference (ISI) channels into ISI-free channels. But Guard interval without any information will caused inter-channel interference. In order to solve this problem, a guard interval using cyclic prefix (CP) is inserted to avoid inter-symbol interference from the adjacent symbols and inter-channel interference from other sub-channels. However, using long cyclic prefix will decrease the transmission rate, reduce the spectral efficiency, and increase the signal-to-noise power ratio¡]SNR¡^loss. If we choose a shorter one, the channel length may be longer than the cyclic prefix. The interference caused by insufficient cyclic prefix can seriously degrade the performance of OFDM systems. In order to solve this problem, a time domain equalizer¡]TEQ¡^is usually used in the receiver to shorten the channel length of OFDM transmission system, and therefore minimize the ISI and ICI. However, because of its high complexity, the optimum design of TEQ is hard to realize. ¡@¡@So we use an iterative channel estimation technique between time domain and frequency domain to mitigate the ISI and ICI which is caused by insufficient cyclic prefix. The iterative technique can remove ISI and hold the circular convolution property. By utilizing the iterative technique we can improve the channel estimation performance as the OFDM symbol used sufficient cyclic prefix. ¡@¡@In the computer simulations, we consider the worst case that the OFDM symbol is transmitted without guard interval. The results show that our proposed method can effectively suppress residual ISI. The comparison between our proposed method including both proposed channel estimation¡]PCE¡^and proposed data demodulation¡]PDD¡^, decision feedback channel estimation¡]DFCE¡^, avoid ISI preamble channel estimation¡]AISIP¡^, and residual ISI cancellation¡]RISIC¡^algorithm are made in this thesis. Finally, the performance improvement of the proposed algorithm under several channel conditions is considered and compared with other algorithms.

Approved for public release, distribution is unlimited / Synchronization of orthogonal frequency-division multiplexed (OFDM) signals is significantly more difficult than synchronization of a single-carrier system. The recently approved IEEE Standard 802.11g specifies a packet-based OFDM system that provides a basis for the discussion of OFDM synchronization in a packet-based environment. Algorithms that synchronize the receiver carrier demodulation frequency and phase, the data frame, the OFDM symbol timing, and the data symbol timing are discussed and analyzed in an AWGN channel. System View simulation is used to implement the frame and carrier frequency synchronization algorithms, where the performance of these algorithms is analyzed and they are shown to be useful detection algorithms for Standard 802.11g signal reception. / Lieutenant Commander, United States Navy

Cette thèse porte sur l'optimisation des performances des systèmes de transmission multimédias MIMO-OFDM prenant conjointement en compte les imperfections de l'amplificateur de puissance et les distorsions du canal. Les fluctuations d'amplitude des signaux OFDM, caractérisées par un PAPR élevé, rendent la transmission vulnérable à la non-linéarité de l'amplificateur de puissance. On propose dans un premier temps une méthode permettant d'améliorer les performances de la méthode Tone Reservation en termes de gain de réduction du PAPR et de rapidité de convergence, en associant les échantillons de l'Intervalle de Garde aux Sous-Porteuses Nulles. Les simulations en présence d'un amplificateur de puissance à effets mémoire et d'un canal radio basé sur un modèle de propagation réaliste montrent que la méthode proposée offre de bonnes performances tout en respectant les spécifications fréquentielles, dans le cadre du standard IEEE 802.11a. Dans un second temps, on propose d'étudier l'impact de la non-linéarité dans un système MIMO-OFDM précodé dédié à la transmission d'images JPWL, respectant la norme IEEE 802.11n. On montre que la non-linéarité affecte la robustesse de transmission contre les erreurs de transmission et dégrade considérablement la qualité visuelle des images reçues. Enfin, on propose une stratégie de précodage originale prenant conjointement en compte l'amplificateur de puissance, le canal de transmission et le contenu de l'image à transmettre. Cette stratégie alloue successivement la puissance sur les sous-canaux SISO issus de la décomposition du canal MIMO afin de maximiser la qualité visuelle des images reçues tout en réduisant la puissance totale d'émission. Les résultats de simulation montrent que cette nouvelle stratégie qui considère un amplificateur et un canal réalistes, permet de garantir la robustesse de transmission et d'améliorer la qualité visuelle des images reçues. / This thesis focuses on the optimization of multimedia transmissions in MIMO-OFDM systems by jointly taking into account the power amplifier non-linearity and the wireless channel distortions. The OFDM modulation generates a high peak fluctuation, measured by the PAPR, which is affected by the RF non-linearity such as the power amplifier, reducing the transmission quality. We first propose a new method to improve the Tone Reservation method performances in terms of PAPR reduction gain and convergence speed, by including the samples of the Guard Interval Signal in the optimization algorithm. Simulations results using a power amplifier model with memory effects and a radio channel based on a realistic propagation model show that the new method offers the better performances, while respecting the IEEE 802.11a spectrum mask. Secondly, we propose to study the impact of power amplifier nonlinearity on the transmission of scalable image contents over a precoded Closed-Loop MIMO-OFDM system. The simulations in a realistic context, under the standard IEEE 802.11n standard show that the RF non-linearity affects the robustness against transmission errors and highly degrades the visual quality of the received JPWL images. Finally, we propose a new precoding strategy which jointly takes into account the power amplifier, the radio channel and the image content to be transmitted. This strategy successively allocates power between the SISO sub-channels obtained from the MIMO channel decomposition in order to maximize the visual quality of the received images, while reducing the total output power. Simulations with a realistic power amplifier model, associated with a realistic channel model show that this new strategy ensures a robust transmission and improves the visual quality of the received images.

Digital Video Broadcasting (standard DVB) is a system for transmission of the television signals in the digital form. There are used a various types of modulations in this system as QPSK modulation is used in the systems of satellite video broadcasting DVB-S (Satellite) and M-QAM modulations in the cable transmitting DVB-C (Cable). This paper mainly deals with system of the terrestrial digital video broadcasting DVB-T (Terrestrial), where OFDM modulation is used. This type of signal processing is more resistant to the distortions caused by multipath transmitting, which is main problem in the DVB-T. Matlab application, which can simulate digital modulation and demodulation of the transmission signals in the DVB-T, is created in this thesis. The models of the transmission channel is inserted between structures of modulator and demodulator. The user of this application can set the parameters of the broadcasting (e.g. constellation, OFDM mode, guard interval insertion) and the type of distortions (additive noise, reflected and delayed signals). By calculation of the channel bit error rate (BER) user can study influences of broadcasting parameters to the quality of transmission.

Approved for public release, distribution is unlimited / Orthogonal frequency division multiplexing (OFDM) is being successfully used in numerous applications. It was chosen for IEEE 802.11a wireless local area network (WLAN) standard, and it is being considered for the fourthgeneration mobile communication systems. Along with its many attractive features, OFDM has some principal drawbacks. Sensitivity to frequency errors is the most dominant of these drawbacks. In this thesis, the frequency offset and phase noise effects on OFDM based communication systems are investigated under a variety of channel conditions covering both indoor and outdoor environments. The simulation performance results of the OFDM system for these channels are presented. / Lieutenant Junior Grade, Turkish Navy