Research On Adaptive MIMO/OFDM Systems For Broadband Communications

One challenge of the next-generation broadband wireless communication systems is how to provide high data rate with high QoS over hostile mobile environments with limited spectrum and ISI caused by multipath fading. OFDM combined with multiple antennas has been widely regarded as an effective way for mitigating the effects of ISI in a frequency selective fading channel and for providing reliable high-data transmission over wireless channels. This dissertation proposes dynamic resource allocation techniques in time, frequency, and space domains for OFDM systems combined with multiple antennas by investiging adaptive modulation, adaptive subchannel allocation, adapive power allocation, and adaptive beamforming interleaving.Firstly, we propose three adaptive modulation schemes including simple space-time block coding based MISO/OFDM, antenna selection diversity based MISO/OFDM, and adaptive antenna arrays based MISO/OFDM. We also investigate and compare their performances over multipath fading channels. The research results show that the proposed systems achieve significant performance improvement compared to the conventional MISO/OFDM systems, and the proposed adaptive antenna arrays based MISO/OFDM behaves best performance among the three proposed schemes.Secondly, we propose three different dynamic resource allocation algorithms with adaptive beamforming for MIMO/OFDM systems and investigate their performance over multipath fading channels under perfect and imperfect channel state information. The proposed dynamic spatial sub-channel allocation based MIMO/OFDM system adaptively selects the eigenvectors associated with the relatively large sub-channel eigenvalues to generate the beamforming weights at both the tranmsitter and receiver sides and then dynamically assigns the corresponding best sub-channels to transmit the OFDM block symbols. The proposed adaptive modulation based MIMO/OFDM system maximizes the optimal SNR on each sub-carrier obtained by adaptive beamforming under the constraint of fixed power and overall bit rate. The proposed adaptive frequency-domain power allocation based based MIMO/OFDM enhances the SNR on each sub-carrier by converting the deep faded sub-channels to AWGN-like sub-carriers. It is shown that significant performance improvement can be achieved by employing these proposed approaches in MIMO/OFDM systems compared to the conventional adaptive antenna arrays based OFDM systems. In order to reduce the realization cost and complexity of adaptive antenna arrays based MIMO/OFDM systems, we combine the benefits of the adaptive antenna arrays selection diversity methods and propose an adaptive interleaved beamforming approach for broadband MIMO/OFDM systems.Finally, we propose a space-time block coded adaptive transmit antenna arrays for broadband OFDM communication systems by utilizing partial channel side information. By employing space-time coding and adaptive power assigning between sub-channels obtained by applying an Eigenvalue Decomposition on conditional channel correlation matrix of each sub-carrier, the proposed system combines the benefits of both the space-time block coding and adaptive antenna arrays and is then robust to the channel imperfection.