Advanced Techniques for High Speed Wireless Communications

This dissertation, "Advanced Techniques for High Speed Wireless Communications" by Yiqing, Zhou, 周一靑, 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 "Advanced Techniques for High Speed Wireless Communications" Submitted by Zhou Yiqing for the degree of Doctor of Philosophy at The University of Hong Kong in November 2003 Future wireless communication systems aim to provide extremely high speed data transmission. To improve the data rate in wireless communications, some advanced techniques are investigated in this thesis, including turbo-coded hybrid automatic-repeat-request (ARQ), variable-spreading-factor (VSF) orthogonal frequency and code division multiplexing (OFCDM) systems, and hybrid multi-code interference (MCI) cancellation and minimum mean square error (MMSE) detection. Turbo codes have been proposed in ARQ systems as the forward-error-correction (FEC) codes. Although sub-packet schemes have been proposed in ARQ systems, optimum sub-packet transmission is more effective to maximize throughput in a dynamic channel. Chapter 2 shows that turbo codes illustrate sparse and burst packet error patterns at moderate to high signal-to-noise ratio (SNR). These patterns are suitable for sub-packet schemes. Moreover, sub-packet schemes can provide additional error correction capability with iterative turbo decoding algorithms. An efficient method is proposed to estimate the optimum number of sub-packets, and adaptive sub-packet schemes (i.e. schemes that enable a system to employ different optimum numbers of sub-packets under variable conditions) are suggested to achieve the maximum throughput of the system. Simulations show that the adaptive sub-packet scheme is effective at moderate SNR (e.g. from 2.6dB to 5.0dB) and can provide higher throughput, reduced delay, and lower dropping rate than conventional packet schemes. The broadband VSF-OFCDM system with two-dimensional spreading (time and frequency domain spreading) is becoming a very attractive technique for high speed wireless communications. Assuming slow fading and ideal channel estimation, an analytical study is presented in Chapter 3 on the performance of the VSF-OFCDM system with hybrid MCI cancellation and MMSE detection. The weights of MMSE are derived and updated stage by stage of MCI cancellation. Defining N=NN TF as the total spreading factor, where N and N are the spreading factors in time and T F frequency domain respectively, various values of N and N are dealt with in a T F quadrature-phase-shift-keying(QPSK)-modulated VSF-OFCDM system. It is shown that the hybrid detection scheme performs much better than the pure MMSE. For the hybrid detection, a larger value of N is expected. With hybrid detection, the performance of VSF-OFCDM systems over a fast fading channel is analytically studied in Chapter 4. It is shown that in a fast fading channel, considerable improvement in bit-error-rate (BER) can be achieved by use of a novel code assignment approach. The optimum power ratio of pilot channel to one data channel is about 0.25K, where K is the total number of data channels. When the Doppler frequency f is less than 200Hz, the effect of the Doppler on the system performance is negligible. However, increasing f from 200Hz degrades the system performance rapidly. Although the gap in BER between the hybrid and pure MMSE detection decreases as f increases, the gap is still large as long as f is less than D D around 800Hz. For a wide range of Doppler shift, a larger N is expected, whereas a smaller N provides better performance w