Some issues in enabling technologies for high data rate reliable wireless communications: OFDM and adaptive ARQ (2001)

Generation by generation, wireless communication has advanced in various ways and&#13;\nprovided reliable communication services at higher and higher data rates to the needs&#13;\nof more and more advanced wireless applications. Two main issues towards future&#13;\nwireless communications are high-speed transmission technique to provide high data&#13;\nrate services and reliable communication to ensure the required performance. This&#13;\ndissertation focuses on these two issues. Since orthogonal frequency division multiplexing&#13;\n(OFDM) has emerged as an enabling technique for high-speed transmission&#13;\nin dispersive environments, major and fundamental issues in OFDM, namely, synchronization,&#13;\nchannel estimation, and peak-to-average power ratio (PAPR) reduction&#13;\nare addressed. For the required reliability, automatic repeat request (ARQ) schemes&#13;\nmust be applied. Due to large potential performance improvement, adaptive ARQ&#13;\nschemes have recently attracted much attention and are also addressed here.&#13;\nWe propose two improved OFDM timing synchronization methods which overcome&#13;\nthe drawbacks of existing methods. We present a time-domain-based OFDM channel&#13;\nestimation which outperforms the existing time-domain-based approach and has a&#13;\nsimilar performance to the linear minimum mean square error estimator but with&#13;\nless complexity. For OFDM systems with transmit diversity, we present a reduced&#13;\ncomplexity channel estimation which has a comparable performance to the existing&#13;\nmethod for channels with relatively small delay spreads, but achieves much complexity&#13;\nsaving. An approach to find the number of most significant channel taps is described&#13;\nfor diverse channel environments. We analyze the effect of non-sample-spaced channel&#13;\npath on the channel estimation and propose a modification for further improvement.&#13;\nTiming synchronization, frequency synchronization and channel estimation are&#13;\nusually addressed separately. Since they can affect each other, the idea of jointly addressing&#13;\nall of them together is much desirable and pursued here. This joint approach&#13;\nreflects the actual performance and gives an opportunity to exploit some information&#13;\nobtained from one task in another, hence promising more improvement. The proposed&#13;\ntraining preamble-based joint timing and frequency synchronization utilizes some information&#13;\nfrom the channel estimation. The sync detection is also considered. We&#13;\ndesign the training symbol to achieve a better coarse timing synchronization. Methods&#13;\nto suppress or circumvent the interference in the frequency estimation caused by&#13;\ntiming errors are presented. A new performance measure for OFDM timing synchronization&#13;\nis proposed which leads to obtaining optimal timing estimation setting. Next,&#13;\nwe present a joint timing synchronization, frequency synchronization and channel estimation&#13;\nbased on training preamble based maximum likelihood realization. Further&#13;\ncomplexity reduction by an adaptive scheme is also proposed.&#13;\nWe address some fundamental questions on peak factors, sampling theorem and&#13;\nsampling series. We present several bounds of bandlimited functions and peak factor&#13;\nratio bound of a continuous signal and its sampled signal. Some discussion on the requirements&#13;\nof sampling theorems and related aspects on sampling series are presented.&#13;\nWe study PAPR behavior of some Reed-Muller codes in OFDM systems in an attempt&#13;\nto find a code with good error correction, low PAPR, efficient encoding/decoding and&#13;\nreasonable code rate. Some regularities of the second and third-order cosets of first order&#13;\nReed-Muller codes with low PAPR are presented which indicates possibility of&#13;\nfinding such code for OFDM.&#13;\nThe main issues in adaptive ARQ schemes are how to design the adaptive system&#13;\nparameters and how to effectively sense the channel conditions. We present an&#13;\napproach for designing the adaptive ARQ system parameters based on the through put&#13;\ncalculation and optimization. An alternative approach which avoids the tedious&#13;\nthroughput calculation is also presented. An effective channel sensing algorithm which&#13;\nutilizes the error correcting capability is proposed. Incorporation of the adaptive frequency&#13;\nhopping concept into the ARQ scheme with adaptive error control is introduced&#13;\nwhich has a significant throughput improvement in slow fading channels.