Masters Research - Masters of Engineering / Mobile telephony, wireless networks and wireless telemetry systems have gone from simple single-input single-output wireless architectures with low data transmission rates to complex systems employing multiple antennas and forward error correction algorithms capable of high data transmission rates over wireless channels. Claude Shannon provided the fundamental capacity limits for a communications system and it can be shown that the capacity for a single-input single-output systems is limited in it’s capability to provide for modern wireless applications. The introduction of multiple-input multiple-output systems employing multiple antenna elements and orthogonal coding structures proved beneficial and could provide the capacities required for modern wireless applications. This thesis begins with an introduction and overview of space-time coding and the codes of Tarokh, Jafarkhani and Alamouti. Further, this thesis provides an introduction and overview to the family of forward error correction codes known as low-density parity-check (LDPC) codes. LDPC codes, when employed over Gaussian channels, provide near-Shannon limit performance and the question is posed as to their suitability for a wireless multiple-input multiple-output system employing multiple antennas and space-time coding. This question is answered by the use and demonstration of LDPC codes as outer codes to a MIMO system employing space-time block codes and a modified maximum-likelihood decoder. By modifying the space-time block-code decoder to provide a soft-information output, iterative decoders such as the sum-product algorithm can be employed to provide significant performance gains over a Rayleigh flat-fading channel. Further the use of design tools such as EXIT charts can then be used to design codes. The key to allowing the use of EXIT charts is the observation that a MIMO system employing orthogonal transmissions in a Rayleigh flat-fading channel is the equivalent to a SISO channel employing Nakagami-m fading coefficients. The seemingly complex MIMO system can now be analyzed in the form of a simpler SISO equivalent allowing the use of techniques such as EXIT charts to be employed in order to design codes with known and predictable performance haracteristics. This thesis demonstrates this technique and shows by example the performance gains that can be achieved for MIMO systems and opens some further questions for future research.

Masters Research - Masters of Engineering / Mobile telephony, wireless networks and wireless telemetry systems have gone from simple single-input single-output wireless architectures with low data transmission rates to complex systems employing multiple antennas and forward error correction algorithms capable of high data transmission rates over wireless channels. Claude Shannon provided the fundamental capacity limits for a communications system and it can be shown that the capacity for a single-input single-output systems is limited in it’s capability to provide for modern wireless applications. The introduction of multiple-input multiple-output systems employing multiple antenna elements and orthogonal coding structures proved beneficial and could provide the capacities required for modern wireless applications. This thesis begins with an introduction and overview of space-time coding and the codes of Tarokh, Jafarkhani and Alamouti. Further, this thesis provides an introduction and overview to the family of forward error correction codes known as low-density parity-check (LDPC) codes. LDPC codes, when employed over Gaussian channels, provide near-Shannon limit performance and the question is posed as to their suitability for a wireless multiple-input multiple-output system employing multiple antennas and space-time coding. This question is answered by the use and demonstration of LDPC codes as outer codes to a MIMO system employing space-time block codes and a modified maximum-likelihood decoder. By modifying the space-time block-code decoder to provide a soft-information output, iterative decoders such as the sum-product algorithm can be employed to provide significant performance gains over a Rayleigh flat-fading channel. Further the use of design tools such as EXIT charts can then be used to design codes. The key to allowing the use of EXIT charts is the observation that a MIMO system employing orthogonal transmissions in a Rayleigh flat-fading channel is the equivalent to a SISO channel employing Nakagami-m fading coefficients. The seemingly complex MIMO system can now be analyzed in the form of a simpler SISO equivalent allowing the use of techniques such as EXIT charts to be employed in order to design codes with known and predictable performance haracteristics. This thesis demonstrates this technique and shows by example the performance gains that can be achieved for MIMO systems and opens some further questions for future research.

Master of Engineering by Research / As a multi-carrier modulation scheme, Orthogonal Frequency Division Multiplexing (OFDM) technique can achieve high data rate in frequency-selective fading channels by splitting a broadband signal into a number of narrowband signals over a number of subcarriers, where each subcarrier is more robust to multipath. The wireless communication system with multiple antennas at both the transmitter and receiver, known as multiple-input multiple-output (MIMO) system, achieves high capacity by transmitting independent information over different antennas simultaneously. The combination of OFDM with multiple antennas has been considered as one of most promising techniques for future wireless communication systems. The challenge in the detection of a space-time signal is to design a low-complexity detector, which can efficiently remove interference resulted from channel variations and approach the interference-free bound. The application of iterative parallel interference canceller (PIC) with joint detection and decoding has been a promising approach. However, the decision statistics of a linear PIC is biased toward the decision boundary after the first cancellation stage. In this thesis, we employ an iterative receiver with a decoder metric, which considerably reduces the bias effect in the second iteration, which is critical for the performance of the iterative algorithm. Channel state information is required in a MIMO-OFDM system signal detection at the receiver. Its accuracy directly affects the overall performance of MIMO-OFDM systems. In order to estimate the channel in high-delay-spread environments, pilot symbols should be inserted among subcarriers before transmission. To estimate the channel over all the subcarriers, various types of interpolators can be used. In this thesis, a linear interpolator and a trigonometric interpolator are compared. Then we propose a new interpolator called the multi-tap method, which has a much better system performance. In MIMO-OFDM systems, the time-varying fading channels can destroy the orthogonality of subcarriers. This causes serious intercarrier interference (ICI), thus leading to significant system performance degradation, which becomes more severe as the normalized Doppler frequency increases. In this thesis, we propose a low-complexity iterative receiver with joint frequency- domain ICI cancellation and pilot-assisted channel estimation to minimize the effect of time-varying fading channels. At the first stage of receiver, the interference between adjacent subcarriers is subtracted from received OFDM symbols. The parallel interference cancellation detection with decision statistics combining (DSC) is then performed to suppress the interference from other antennas. By restricting the interference to a limited number of neighboring subcarriers, the computational complexity of the proposed receiver can be significantly reduced. In order to construct the time variant channel matrix in the frequency domain, channel estimation is required. However, an accurate estimation requiring complete knowledge of channel time variations for each block, cannot be obtained. For time- varying frequency-selective fading channels, the placement of pilot tones also has a significant impact on the quality of the channel estimates. Under the assumption that channel variations can be approximated by a linear model, we can derive channel state information (CSI) in the frequency domain and estimate time-domain channel parameters. In this thesis, an iterative low-complexity channel estimation method is proposed to improve the system performance. Pilot symbols are inserted in the transmitted OFDM symbols to mitigate the effect of ICI and the channel estimates are used to update the results of both the frequency domain equalizer and the PICDSC detector in each iteration. The complexity of this algorithm can be reduced because the matrices are precalculated and stored in the receiver when the placement of pilots symbols is fixed in OFDM symbols before transmission. Finally, simulation results show that the proposed MIMO-OFDM iterative receiver can effectively mitigate the effect of ICI and approach the ICI-free performance over time-varying frequency-selective fading channels.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Canterbury, 2007. / Typescript (photocopy). "June 12, 2007." Includes bibliographical references (p. 153-171). Also available via the World Wide Web.

Thesis (Ph. D.)--Brigham Young University. Dept. of Electrical and Computer Engineering, 2006. / Includes bibliographical references (p. 101-106).

Thesis (M.E.)--University of Canterbury, 2007. / Typescript (photocopy). "March 2007." Includes bibliographical references (p. [83]-95). Also available via the World Wide Web.

Thesis (Ph. D.)--University of Hong Kong, 2008. / Also available in print.

Techn. University, Institut für Nachrichtentechnik, Habil-Schr. 2006--Hamburg-Harburg, 2006.

Thesis (M.S.E.C.E.)--University of Massachusetts Amherst, 2008. / Includes bibliographical references (p. 74-77).