الفهرس | Only 14 pages are availabe for public view |
Abstract With the rapid growth of digital communication in recent years, the need for high speed data transmission is increased. Moreover, future wireless systems are expected to support a wide range of services which includes video, data and voice. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for achieving high data rates in mobile environment, due to its resistance to Inter-Symbol Interference (ISI), which is a common problem found in high speed data communication. Moreover, the main problem in the design of a communication system over a wireless link is to deal with multipath fading (specially deep fading channel), which causes a significant degradation in terms of both the reliability of the link and the data rate even though those systems exhibit efficient bandwidth utilization. Since the errors caused by the mobile wireless channels are bursty in nature, interleaving is a must in mobile communication systems. Several interleaver schemes have been proposed. The simplest and most popular of such schemes is the block interleaver scheme. In spite of the success of this scheme to achieve a good performance in wireless communication systems ,it cannot deal with two dimensions (2-D) errors, there is a need for a much powerful scheme for severe channel degradation cases to overcome 2-D errors. In OFDM, the modulation may be coherent or non-coherent. When using non-coherent data detection (such as differential modulation) which performs data detection by processing multiple symbols without the knowledge of the Channel Impulse Response (CIR) but its performance is inferior than the coherent system. In the contrast, the coherent modulation requires the channel estimation which gives better performance. Our objective in this thesis is to work on channel estimation which can extract the CIR using just one block of the received OFDM data. This thesis presents a new interleaving scheme for efficient data transmission with OFDM over fading channels. Chaotic interleaving generate permuted versions from the sample sequences to be transmitted, with low correlation among their samples, and hence a better performance can be achieved. Moreover, a comparison between the proposed chaotic interleaving and the conventional block interleaving is performed. The proposed chaotic interleaving approach adds a degree of encryption to the transmitted data. The performance of the proposed approach is tested on the conventional Fast Fourier Transform OFDM (FFT-OFDM), the DiscreteWavelet Transform OFDM (DWT-OFDM) and the Discrete Cosine Transform OFDM (DCT-OFDM) with and without chaotic interleaving. The proposed system comprises Frequency-Domain Equalization (FDE) to obtain high diversity gains over frequency selective multipath fading channels. The simulation results show that the FFT-OFDM, DWT-OFDM and DCT-OFDM systems with the new interleaving scheme provide better Symbol Error Rate (SER) performance on different types of channels. Additionally, the CIR must be known to coherently detect the transmitted data. Expectation Maximization (EM) algorithm is also proposed to efficiently estimate the CIR of such a system operating on a channel with multipath fading. Starting from the Maximum Likelihood (ML) principle, we derive an iterative estimation algorithm based on the EM algorithm. This algorithms are capable of improving the channel estimate. In the initialization phase of this iterative algorithm, the initial channel estimate is based on the observation of the pilot carriers only. The soft information resulting from the detector can be iteratively exploited to improve the estimation process. To reduce the complexity of the proposed algorithm, a sub-optimal estimation scheme is also introduced. The resulting SER essentially coincides with the case of the perfectly known channel. By simulations, the efficiency of these algorithms can be investigated with simulation and the results of estimation will come to a comparison. This thesis also presents, a Peak to Average Power Ration (PAPR) mitigation technique, which improves system performance. This technique is based on Constant envelope OFDM (CE-OFDM). The fundamental aspects of CE-OFDM modulation are studied. The constant envelope signal can be efficiently amplified with nonlinear power amplifiers thus achieving greater power efficiency. We proposed Continuous Phase Modulation (CPM) based (FFT/DWT/DCT)- OFDM systems with and without chaotic interleaver. Performance is evaluated over AWGN and multipath fading channels with different modulation indices. It is shown that CPM-OFDM outperforms conventional OFDM. |