| Digital communication techniques have been developed and improved to provide high rate data transmission as well as transmission reliability. High rate data transmission necessitates communication over broadband frequency selective channels. Multi-carrier modulation systems such as orthogonal frequency division multiplexing (OFDM) have become very popular for broadband communication due in part to computationally efficient multi-carrier generation using the fast Fourier transform (FFT) and simple one-tap equalization.; Recently, transmit diversity technique such as the Alamouti space-time block code involving multiple transmit antennas and possible multiple receive antennas has emerged as a promising method to improve transmission reliability. OFDM combined with transmit diversity is a promising means of achieving high rate data transmission with transmission reliability. A main assumption of OFDM systems with space-time block codes such as the Alamouti code is a quasi-static channel, i.e., wireless channels do not change over two consecutive OFDM symbol periods. Unfortunately, wireless channels become time-variant due to Doppler spread when a mobile receiver moves, resulting in performance degradation.; This dissertation proposes three novel signal detection methods for OFDM systems with transmit diversity when the channels are time-variant: a decision directed receiver, a sequence estimation receiver, and a frequency domain block linear filtering receiver. Computer simulations demonstrate the computational efficiency of the proposed methods, comparing with previous approaches.; This dissertation also proposes a OFDM transmission method without cyclic prefix. Since cyclic prefix in OFDM systems is redundancy, bandwidth efficiency increases as the redundancy is eliminated. It is also shown that the equalization technique for OFDM systems without cyclic prefix can also be used for timing error correction in non-synchronized sampling discrete multi-tone (DMT) systems, a wireline counterpart of OFDM. |
