Space Time/Space Frequency/Space Time Frequency processing of MIMO-OFDM Systems

Abstract

The growing demand for multimedia applications and the growth of internet related content has increased interest for high speed communications in practical environments. To achieve these goals multiple transmit and receive antennas are employed to form multiple input multiple output (MIMO) channels. MIMO systems help to improve system capacity through spatial multiplexing, reliability through diversity and interference reduction in multiple users by exploiting spatial dimension. To further improve system capacity and mitigation against fading, MIMO systems are combined with orthogonal frequency division multiplexing (OFDM) to form MIMO-OFDM systems. These systems are adopted as a basis of 4G and beyond wireless communication standards. newline In this thesis, the degree of freedom present in the MIMO channels is exploited by transmitting the signal through multiple independent fading paths in time, frequency or space domains. Such codes are called space time (ST), space frequency (SF) and space time frequency (STF) codes. Initially, a general performance analysis for MIMO-OFDM systems is presented with well known Alamouti ST, SF and STF codes. The analysis is done in severely faded channel environments, where channel coefficients are close to signal amplitudes and the signal is severely faded. It is extremely difficult to retrieve signal in such channel scenario. In this work, the signal is recovered by simply increasing the diversity order instead of employing precoding as done in the literature. Analytical and simulation results show that the signal is retrieved successfully without diversity loss. The increase in diversity leads to increase in decoder complexity that can be reduced by using sphere decoder.