الفهرس | Only 14 pages are availabe for public view |
Abstract The Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multicarrier modulation (MCM), where a single data stream is transmitted over a number of lower rate subcarriers. It is worth mentioning here that OFDM can be seen as either a modulation technique or a multiplexing technique. One of the main reasons to use OFDM is to increase the robustness against frequency selective fading and narrowband interference. In a single carrier system, a single fade or interferer can cause the entire link to fail, but in a multicarrier system, only a small percentage of subcarriers will be affected. Error correction coding can then be used to correct the few erroneous sub-carriers. In OFDM systems, where the data symbols are transmitted in parallel on N different carriers, the length N of a symbol is extended with a factor ν. This extension of the symbol length causes the OFDM system to be less sensitive to channel dispersion than a single carrier system transmitting data symbols at the same data rate. However, at the edges of the OFDM symbol, the channel dispersion still causes distortion, and hence introduces interference between successive symbols (i.e. InterSymbol Interference, ISI) and interference between different carriers within the same symbol (i.e. Inter-Carrier Interference, ICI). To reduce the effect of the ISI, each symbol is extended with a guard interval. When the length of the guard interval is longer than the duration of the channel impulse response, ISI can completely be removed. However, as the transmission efficiency reduces with the insertion of the guard interval (during the guard interval, no new information can be transmitted); the guard interval must be chosen sufficiently small. The most commonly used guard interval is the cyclic prefix. Different guard interval techniques for the (OFDM) transmission are suggested to reduce the interference between successive symbols (i.e. ISI) and interference between different carriers within the same symbol (i.e. Inter-Carrier Interference, ICI). In this thesis, the codes have been written for data transmission using an OFDM technique over two types of channels. The effect of fading channel and additive white Gaussian noise (AWGN) channel is studied and suggested approaches are used to enhance the performance of the OFDM system. In this thesis, the impact of replacing the Cyclic Prefix (CP) by zero insertion (ZI) before the Inverse Fast Fourier Transform (IFFT) process on OFDM is studied. iv Associated with each OFDM symbol, the zeros will be added in the transmitter before the IFFT process. For symmetry of the OFDM symbol, the zeros will be inserted with the same length among the OFDM frames. The ZI acts as a buffer region where delayed information from the previous symbols can get stored. The ZI acts as a safe region of the high frequency component (details) of the signal where delayed information from the previous symbols freely corrupts the low frequency components, and the high frequency components are protected by giving it a high immunity (higher SNR) against the additive noise. The receiver has to exclude samples from the ZI which got corrupted by the previous symbol when choosing the samples for an OFDM symbol. The motivation of using the ZI instead of the cyclic prefix is the reduction in the transmission rate and the high performance in reducing of the channel distortion. The performance comparison among the proposed ZI technique, the CP, the Zero Padding (ZP), and the Known Symbol Padding (KSP) is introduced in this thesis. Simulation results show that the KSP has a slightly worse performance; however, the proposed ZI approach provides better performance than the other techniques and achieves a 20 % reduction in the rate of the transmitted signal relative to the CP-OFDM system. Another type of channels is the AWGN channel. The AWGN is a noise that affects the transmitted signal when it passes through the channel. It contains a uniform continuous frequency spectrum over a particular frequency band. The OFDM system has been built to reduce the effect of fading channel without enhancement the effect of the AWGN channel. In this work the approach of the wavelet thresholding denoising at the beginning of the receive r is used to denoise the received radio frequency (RF) signal. The simulation results prove that the signal power is enhanced by a 30 dBs relative to the SNR of the received signal. Finally, the proposed ZI approach in conjunction with the wavelet thresholding approach is used to modify the OFDM system and the overall system performance is tested. |