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Abstract A major technical challenge in wireless communications is to improve the error performance of the system while maintaining low implementation complexity without increasing the bandwidth. Wireless communication channel generally offers multiple sources of diversity such as space, frequency, time, channel propagation state, polarization. etc. However, most existing diversity techniques exploit only one of the various diversity resources of wireless channel. Multiple antenna communication systems provide high data rates and improved performance without increasing the bandwidth or transmitted power, Multiple Input Multiple Output (MIMO) has much larger capacity in fading channels than standard wireless systems. Practically, for frequency selective channels, a combination of MIMOOFDM is promising. With the advent of next generation (40) broadband wireless communications, the combination of MIMO wireless technology with OFDM has been recognized as one of the most promising techniques to support high data rate and high performance. In particular, the appropriate use of space time codes (STC) allows achieving large capacities , coding across the space, time, and frequency domains provided by MIMO-OFDM will enable a much more reliable and robust transmission over the harsh wireless environment. Maximum Likelihood Decoding (MLD) is the optimum decoding algorithm that is used for MIMO systems. Space time coding is a powerful scheme that combines coding with transmit diversity to achieve high diversity performance in wireless systems. Such a coding scheme can in general be classified into two major classes: Space Time Block Codes (STBC) which is diversity oriented scheme and Space Time Trellis Codes (SITC) which is multiplexing oriented scheme, a central issue in all these schemes is the exploitation of multi path effects in order to achieve high spectral efficiencies and performance gains. STBCs constructed from orthogonal designs achieve full diversity and have fast MLD at the receiver. The transmitted symbols can be decoded separately, not jointly. Thus, the decoding complexity increases linearly, not exponentially, with the code size. With the quasi-orthogonal structure, the MLD at the receiver can be done by searching groups of symbols. However, these codes do not achieve the full diversity. But has better performance at low SNR. This is due to the fact that the slope of the performance curve depends on the diversity order. Quasi-Orthogonal STBC (QOSTBC) structures for quasi-static channels provide rate-one and pair-wise MLD but fail to achieve full-diversity. Later on, improved QOSTBC was proposed through constellation rotation. A rotated QOSTBC provides full diversity, rate one and better performance compared to OSTBC. In this dissertation, a Space Time Frequency Block Code (STFBC) based on quasi-orthogonal designs is proposed over a frequency selective Rayleigh fading channel. The proposed code achieves rate-one and exploits all of the spatial, multipath and temporal diversity gains offered by the MIMO-OFDM channel. The designed block code over MIMO wireless systems is capable of jointly extracting multiple sources of diversity therefore improving the error performance. For quasi-static channel over adjacent OFDM symbol durations, as numerical results demonstrate, the proposed code introduces a significant performance gain over the existing STFBC codes in terms of bit error rate performance. By coding across the three dimensions of space, time and frequency with four frequency diversity branches, the proposed code has a four-wise maximum likelihood decoding. The system performance will be investigated under different delay spread of the channel. In addition, a performance comparison for different FFT size will be presented. Finally, some open topics are highlighted as the possible future research directions. |