Enhanced Techniques for Broadband Wireless Communications

This dissertation, "Enhanced Techniques for Broadband Wireless Communications" by Bin, Xia, 夏斌, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "Enhanced Techniques for Broadband Wireless Communications" Submitted by XIA Bin for the degree of Doctor of Philosophy at The University of Hong Kong in January 2004 This study investigates three enhanced techniques for broadband wireless communications, which aim to meet the challenge of high data rate services. They include (1) multi-level quadrature amplitude modulation (MQAM) with antenna diversity; (2) multicode code division multiple access (CDMA) with MQAM and interference cancellation and (3) orthogonal frequency and code division multiplexing (OFCDM) systems with Minimum Mean Squared Error (MMSE) or optimum detection. The first chapter of this thesis explains the challenges posed by high data rate services. In Chapter Two the performance of MQAM systems is studied. In particular, the effect of channel estimation error due to time varying fading and additive noise is analyzed. It is shown that with the antenna diversity technique, the bit error rate (BER) performance of MQAM systems improves significantly. By employing higher order QAM modulations with the help of antenna diversity, it is therefore possible to increase system throughput in low signal-to-noise ratio (SNR) cases. It is also observed that the inaccurate channel estimates limits the benefit of diversity. More accurate channel estimation can be achieved by increasing the length of the channel estimator and the amplitude of the pilot symbol, thereby improving the BER performance. In Chapter Three the performance of MQAM modulated multicode CDMA systems is analyzed. Two important factors (channel estimation error and additive interference) on system performance are considered. Numerical results show that with the help of interference cancellation, MQAM can be employed in high SNR cases to increase system throughput without extra bandwidth requirement. Furthermore, when using the interference cancellation technique, extra pilot power should be invested for more accurate channel estimation, so that better BER performance can be achieved. The variable spreading factor (VSF)-OFCDM has great potential value for future broadband wireless communications, which will be able to provide data services up to 100Mbps. In Chapter Four, the per-carrier MMSE (pcMMSE) and optimum detectors are analytically studied in slow fading channels. Numerical results show that although the MMSE detector cannot perform as well as the optimum detector, it can improve system performance significantly with lower complexity. Thus, if only a small number of code channels are involved, the optimum detector can be employed to achieve better performance, whereas the MMSE detector is better suited to a system with a large number of code channels. The MMSE detector is also more robust than the optimum detector to the different configurations of system parameters. In Chapter Five the effect of fast fading on system performance is evaluated, and an improved MMSE detector is presented. Numerical results demonstrate that the improved MMSE outperforms the pcMMSE. It is shown that better performance is obtained if the time domain spreading factor N of 8 is used for a large range of Doppler frequencies, and the frequency domain spreading factor is as large as possible; and that system performance is optimized if the power of the pilot channel is assigned at 20% and 25% of the total transmitted power in slow and fast fading respectively.