Multicarrier spread -spectrum systems for multiple -access communications

In this thesis, we consider multicarrier spread-spectrum multiple access systems. In the first work, a subspace-based linear minimum mean-squared error (MMSE) multiuser detection scheme is proposed for a multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) system. Typically, a MC-DS-CDMA system employs a band-limited chip waveform. The band-limited nature of the chip waveform causes problem in applying standard subspace techniques because no non-null noise subspace can be formed. It is shown that channel and timing information needed for the construction of the linear MMSE detector can be identified by a multiple-signal-classification-like algorithm based on a finite-length truncation approximation of the chip waveform. In practice, since perturbed versions of the subspaces assumed in the finite-length truncation approximation are actually observed, and because of the band-limited property of the chip waveform, the accuracy of the channel estimation and, hence, the performance of the MMSE detector are degraded. This effect is investigated. In the second work, MC-DS-CDMA systems in frequency-selective fading channels are investigated. A consistent channel model is used for each system. The tapped delay line channel model with uniformly spaced, uncorrelated taps is investigated to model a complex Gaussian, wide-sense stationary, uncorrelated scattering channel. An approximate average bit error rate expression is obtained for the receiver with RAKE fingers on each subcarrier branch, whose outputs are combined according to the maximum signal-to-noise ratio criterion. In the third work, we consider multiuser communications in an additive white Gaussian noise channel in the presence of significant Doppler shifts. It is well-known that the time asynchrony between users in a DS-CDMA system allows for interference suppression. Similarly, the different Doppler shifts of received signals from a multi-user channel can be exploited in the frequency domain to achieve interference suppression. In this work, we show that this can be achieved with MC-CDMA. The relationship between the interference suppression capability and the Doppler shifts is studied in terms of signal-to-interference ratio