A Scheme For Space-Time-Frequency Coding And Adaptive Multiple Antenna Selection For Improving The Performance Of Multiple-Input Single-Output Ofdm System

In this thesis, issues such as link adaptation that include adaptive spatial mode in the context of orthogonal frequency division multiplexing (OFDM) is addressed, particularly in a multiple-input single-output (MISO) configuration suitable for mobile communications. The performances of ST- and SF-OFDM incorporating channel coding are extensively investigated in the presence of channel fading and noise. Realizing that ST and SF behave differently in different environments, a new adaptive spatial mode (ASM)-OFDM is proposed. This scheme selects the mode of transmission based on the instantaneous signal to noise ratio (SNR) for a MISO system. A new adaptive multiple antenna selection (AdMAS) using second-order moment of the channel impulse response between multiple transmitting antennas and a single receiver MISO-OFDM configuration is also proposed as a major contribution. This scheme employs a space-time-frequency coding with a symbol rate of ¾, using orthogonal signals, to avoid high complexity in detection. In the proposed AdMAS scheme the worst faded channel(s) is(are) adaptively turned off based on the calculated second-order moment of the channel impulse response. Findings show that the proposed scheme outperforms other multiple antenna schemes such as the one based on the mean of the channel impulse response, especially at higher order modulation. Throughout the simulation, channel state information (CSI) is assumed to be known both by the transmitter and the receiver which may be achieved via a dedicated feedback channel or through the reciprocity of the channel such as that found in time division duplex (TDD) system. Minor contribution of this thesis includes a simplified sphere decoding algorithm for MIMO that forms part of the detection scheme at the receiver. A maximum achievable diversity order of the MISO-OFDM has also been derived analytically and proved through simulation. A diversity gain of more than 10 dB can be achieved with Binary Phase Shift Keying modulation when compared to Alamouti’s Space-Time Block Code (STBC) at a bit error rate (BER) of 10-3. This makes the proposed scheme a potential scheme to be considered for the future high data rate communication. The proposed schemes are tested through simulations in different types of practical channels such as Wireless Local Area Network (WLAN) IEEE 802.11, Rayleigh fading with various delay spreads, as well as the most recent Fixed Broadband Wireless Access (FBWA) IEEE 802.16 Stanford University Interim channels, or better known as the WiMAX channels.

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