Some issues in enabling technologies for high data rate reliable wireless communications: OFDM and adaptive ARQ

Two main issues towards future wireless communications are high-speed transmission technique to provide high data rate services and reliable communication to ensure the required performance. This dissertation focuses on these two issues. Since orthogonal frequency division multiplexing (OFDM) has emerged as an enabling technique for high-speed transmission in dispersive environments, major and fundamental issues in OFDM, namely, synchronization, channel estimation, and peak-to-average power ratio (PAPR) reduction are addressed. For the required reliability, automatic repeat request (ARQ) schemes must be applied. Due to large potential performance improvement, adaptive ARQ schemes have recently attracted much attention and are also addressed here.; We propose two improved OFDM timing synchronization methods which overcome the drawbacks of existing methods. We present a time-domain-based OFDM channel estimation which outperforms the existing time-domain-based approach and has a similar performance to the linear minimum mean square error estimator but with less complexity. For OFDM systems with transmit diversity, we present a reduced complexity channel estimation which has a comparable performance to the existing method for channels with relatively small delay spreads, but achieves much complexity saving. An approach to find the number of most significant channel taps is described for diverse channel environments. We analyze the effect of non-sample-spaced channel path on the channel estimation and propose a modification for further improvement.; The main issues in adaptive ARQ schemes are how to design the adaptive system parameters and how to effectively sense the channel conditions. We present an approach for designing the adaptive ARQ system parameters based on the through put calculation and optimization. An alternative approach which avoids the tedious throughput calculation is also presented. An effective channel sensing algorithm which utilizes the error correcting capability is proposed. Incorporation of the adaptive frequency hopping concept into the ARQ scheme with adaptive error control is introduced which has a significant throughput improvement in slow fading channels. (Abstract shortened by UMI.)...