Time-varying signal processing techniques for multiple access wireless communication systems

In this dissertation, several problems regarding wireless communication systems using time-varying signal processing techniques are addressed. Specifically, new methods are investigated to increase the performance of time-varying wireless channels suffering from multiple distortions such as interference between users, multipath and Doppler spread and jamming interference. Also, modulation schemes using time-varying wideband signals are proposed to reduce multiple access interference (MAI), to achieve multipath diversity and to estimate unknown channel information.; In direct sequence code division multiple access (DS-CDMA) systems, a recently proposed time-frequency RAKE receiver was shown to achieve higher diversity levels for fast fading channels than the conventional RAKE receiver. Thus, a minimum mean square estimation using orthonormal basis projections is proposed that combines these higher diversity levels with improved multiuser detection.; The highly localized matched signal transform (MST) is proposed as a simple suppression technique of wideband time-varying jamming interference. The good performance of the MST-based interference suppression is demonstrated for DS-CDMA systems in noisy and time-frequency selective fading channels and is compared to quadratic suppression techniques.; Wideband time-varying signals such as linear frequency-modulated (FM) chirps have inherent properties that can be exploited in different applications. An orthogonal linear chirp modulation scheme with a time-varying amplitude is proposed that is based on assigning different users with optimally designed FM rates in older to reduce MAI. Moreover, A simple condition is derived on the FM rates to achieve multipath diversity. For unknown time-varying wireless channels, a nonlinear chirp pilot based estimation method using the modified matching pursuit technique is proposed to estimate multipath, Doppler and fading channel parameters. This method is compared with the estimation scheme that uses two linear chirp pilots and the reassigned spectrogram.; The linear chirp modulation scheme is applied to the frequency-hopping CDMA (FH-CDMA) system by assigning different FM rates to different users. It is demonstrated that the new multi-chirp rate FH-CDMA provides better performance than the traditional FH-CDMA with multipath diversity and low multiple access interference. For unknown and fast varying channels, the nonlinear chirp pilot estimation can be successfully integrated in the multi-chirp rate FH-CDMA system.