Quasi-Orthogonal Space-Time Block-Coded OFDM Wireless Mobile Communication Systems using Array-Processing Approach

Abstract

The phenomenon of multipath fading constitutes a fundamental problem in the wireless communication. Researchers have proposed many methods to improve the reliability of communication over wireless communication channels in the presence of fading. MIMO is one of the important techniques used to achieve the diversity gain or multiplexing gain. There are various diversity techniques, in which transmit diversity has the advantage of power and bandwidth efficiency. OFDM is another technique used to combat the effect of frequency-selectivity of multipath fading channel. OFDM also removes the ISI (intersymbol interference). OFDM converts frequency-selective channel into frequency flat fading channel and hence single tape equalizer is required at the receiver end. MIMO-OFDM is an important technique to improve SER performance in the high data transmission rate communication systems. In this thesis report, QO-STBC designs are used to achieve diversity and coding gain. Orthogonal designs are the optimum choice, which provides full diversity gain and can be decoded by linear processing unit. But unfortunately, orthogonal design with full diversity gain & full code rate does not exist for more than two transmitting antennas for the complex signal constellation. To increase data rate, the quasi- orthogonal design is used, but we have to sacrifice the diversity gain. Computational complexity in QO-STBC design increases and results in more transmission delay. To reduce computational complexity, the array-processing technique is used, in which the received data is converted into parallel streams. Each parallel data stream is decoded simultaneously by the separate OSTBC decoder. We assume perfect CSI (channel state information) estimation and perfect synchronization. Hence, the effect of channel estimation does not affect the SER performance. Simulation results are investigated for Rayleigh and Nakagami-m fading channels. Multipath fading channel is considered to be time-variant frequency-selective and it is assumed that the channel gain remains constant over K consecutive OFDM symbols (number of symbols used in STBC code design).