In mobile radio systems, the bandwidth of spread spectrum signals for CDMA system is very wide. When the bandwidth exceeds the coherence bandwidth of the channel, the effect of multi-path fading become serious and the transmission signals are severely distorted. In OFDM systems, the available bandwidth is divided into a lot of small parts. By well selecting the number of sub-carriers according to the characteristics of the mobile channel will let the bandwidth of transmission signals smaller than the coherence bandwidth of channel. Then the multi-path fading problem can be overcome. In future mobile communications, the demand for multimedia will grow spectacularly. For example, the need of voice transmission for mobile phone and the need of high data rate transmission for wireless local area network (WLAN). Therefore, multi-rate system was developed in the recent years. Yet, the characteristics of mobile channel vary with the moving velocity of the systems and the environment of the mobile channel. Therefore no single transmission mode owns the best performance in terms of spectrum efficiency, bit error rate, and transmission power. In this thesis, we study coded multi-rate OFDM systems in different mobile environment and evaluate their performance by means of computer simulation. Hoping to find better transmission modes in different mobile channel.

This thesis analyzes and quantifes the performance of adaptive orthogonal frequency division multiplexing (OFDM) systems in conjunction with single and multi antenna systems operating over frequency selective Rayleigh and Rician fading environments. We introduce a generalized transceiver model for adaptive cyclic prefixed (CP)-OFDM and isotropic orthogonal transform algorithm (IOTA)-OFDM. Fundamental issues concerning the BER performance of CP-OFDM and IOTA-OFDM are investigated under typical system imperfections. Furthermore, a throughput comparison of these two types of adaptive systems is given. Next, the peak-to-average power ratio (PAPR) problem of adaptive OFDM is considered. Focusing on wideband channel variations in the frequency domain, we have developed a novel statistical analysis for adaptive multiple input multiple output (MIMO)-OFDM systems. In addition, a central limit theorem (CLT) was developed for a wide range of block-based performance metrics. This thesis aims to present a systematic study of channel variation with a statistical analysis of the MIMO-OFDM channel and system performance. In particular, we focus on the behavior of block based performance measurements by considering the correlation across the frequency bins of the OFDM block. In addition, we investigate the eigenvalue variation of MIMO-OFDM systems across frequency. We show that eigenvalue and link gain changes in frequency can be analyzed and have presented novel results on the distributions and moments of such changes. We have also given expressions for the autocorrelationfunctions (ACFs) of the maximum eigenvalue and the link gain. Of particular interest is the very simple approximation to the ACF of the maximum eigenvalue. This leads to accurate closed-form approximations to the variance and CDF of the eigenvalue differences. We consider three types of adaptive MIMO-OFDM systems; one is based on (diversity mode) maximal ratio transmission-maximal ratio combining (MRT- MRC), while the others are spatial multiplexing techniques using singular value decomposition (SVD) or minimum mean square error (MMSE) receivers with linear precoding. We derive closed-form expressions for the joint cumulative distribution function (CDF) of arbitrarily selected eigenvalues in the same bin and in different bins. Furthermore, for MIMO with MRT-MRC, and MIMO- SVD, the exact mean and variance of the number of bits transmitted per OFDM block has been computed analytically and veried with Monte Carlo simulations.

Berlin, Technische University, Diss., 2006.

Berlin, Techn. Univ., Diss., 2003. / Computerdatei im Fernzugriff.

Berlin, Techn. Univ., Diss., 2003. / Computerdatei im Fernzugriff.

München, Techn. Universiẗat, Diss., 2003.

Zugl.: Darmstadt, Techn. Universiẗat, Diss., 2008.

A ground-penetrating radar is being designed to find subterranean structures. This is difficult to do because of varying mediums. Having more bandwidth can help mitigate this problem. Because the frequency spectrum is so cluttered, one method to do this is to use non-contiguous orthogonal frequency division multiplexing (NC-OFDM) to occupy several free areas of the spectrum. An NC-OFDM waveform was designed and optimized with respect to peak-to-average-power ratio, orthogonality, spectral leakage and autocorrelation sidelobes. Techniques such as the use of a Zadoff-Chu sequence and a gap filling algorithm were implemented to do this. The waveform was tested in simulation to show that while computationally expensive, this may be a viable waveform for ground-penetrating radar.

Two novel structures of training symbols are proposed for both timing offset and frequency offset estimations in orthogonal frequency division multiplexing (OFDM) systems. The first proposed training symbol has repeated sample blocks and a sign pattern on each block. With the proposed structure, system synchronization can be obtained with relatively low complexity. In particular, the frequency synchronization is accomplished using two successive stages to obtain both the fractional and integral parts of frequency offset with an estimation range of sub-carrier spacing. Simulation experiments demonstrate that the performance of the proposed scheme is substantially superior to the traditional schemes, or has negligible difference. The second proposed training symbol has the form of repeat and conjugate symmetric blocks and a sign pattern on each block. Then, a weighted timing metric is proposed to estimate the timing offset. Simulation results demonstrate that the proposed timing metric substantially improves the traditional schemes.

In this thesis, the superimposed training scheme is applied to the orthogonal frequency division multiplexing (OFDM) systems. To optimize power amplifier efficiency, the optimal value of pilot power allocation is derived to maximize data power. Peak-to-average power ratio (PAPR) is one of the major drawbacks for OFDM systems. A large PAPR will push the power amplifier into the saturation area, leading to nonlinear distortion, and reduce power efficiency. By using the superimposed training schemes, the system can not only improve the bandwidth efficiency, but also reduce the PAPR. In this thesis, the relation between PAPR and power efficiency is analyzed. In addition, the close form formula is derived for determining the pilot power allocation so that data power is maximized.