Wideband communications over time and frequency spread fading channels

In this thesis, we provide a comprehensive study on some fundamental issues related to the physical layer of wireless communications. We begin our research by studying data transmissions over narrow-band Nakagami fading channels. We propose a new model for investigating the impact of complex imperfect channel state information (CSI) over Nakagami-m fading channels. The phase difference between two correlated complex Nakagami random variables is proposed and shown to accurately approximate that of the Ricean. Then, we analyze the impact of imperfect CSI on the bit error rate (BER) of uncoded and coded modulation and show that imperfect phase estimation is the major cause of the error floor.;Then, we shift our analysis to wideband communications. We focus on both theoretical and practical aspects of wideband communications. For theoretical analysis, we derive bounds to the channel capacity of orthogonal frequency division multiplexing (OFDM) systems over the underwater acoustic (UWA) fading channel as a function of the distance between the transmitter and receiver. Results from the model show a gap between the upper and lower bounds which depends not only on the ranges and shape of the scattering function of the UWA channel but also the distance between the transmitter and receiver. For practical analysis, we develop the bit-interleaved frequency-domain turbo equalization (Bi-TFDE) with spatial diversity and investigate its BER performance over experimental UWA channels. Then, we investigate the performance of the Bi-TFDE over frequency-selective fading channels with the residual inter-symbol interference (ISI). We focus on the impact of the residual ISI on the convergence signal-to-noise ratio threshold using the extrinsic information transfer (EXIT) chart. We demonstrate the accuracy of the EXIT chart in predicting the transfer function of the equalizer. Hence the convergence behavior of the TFDE, with the residual ISI, is parameterized by the product of the symbol block length and Doppler spread. Finally, we extend the EXIT chart analysis to Bi-TFDE using the subblock processing. We show that EXIT chart analysis can be used as a design guideline to determine if the subblock processing improves or deteriorates the convergence behavior of the Bi-TFDE with subblock processing.