SNR maximizing linear filters with interference suppression capabilities for DS-CDMA

This thesis considers the design of receivers for multi-user communication systems based on asynchronous direct-sequence code-division multiple-access (DS-CDMA). Its primary aim is to improve current receiver designs which exhibit shortcomings in systems using long sequence spreading (where the spreading sequence changes with each consecutive bit). The conventional matched filter (MF) and noise-whit MF (NWMF) receivers incur a degradation in performance under power-imbalance (near-far like) conditions. Similar to that of the conventional MF (CMF), the near-far resistance of the NWMF is shown to be zero. Other receivers, including multiuser receivers, suffer from either incompatibility or excessive complexity under long sequence spreading.Motivated by the need for low-complexity receivers compatible with long sequence spreading, the thesis investigates the design of linear time-invariant (LTI) filters which maximize the signal-to-noise ratio (SNR) for bit symbol detection. It develops the chip-delay locked MF (CLMF) which requires the knowledge of interferer chip delays and signal powers; knowledge of their spreading sequences is unnecessary. Moreover, it takes advantage of the observation that multiple-access interference (MAI) is generally neither white nor stationary, but cyclostationary. Analysis of the CLMF demonstrates that it can deliver non-zero near-far resistance along with performance beyond that of the CMF and NWMF. Furthermore, the computation of the filter response is required only when an interferer signal parameter changes. The complexity of the computation grows linearly with the number of interferers. In addition, insight and directions are presented for the development of adaptive versions of the CLMF which eliminate the need for interferer signal parameters altogether.Based on the same approach used to develop the CLMF, the thesis presents a general framework from which other one-shot linear detectors can be derived. With increasing knowledge of interferer signal parameters, this approach can synthesize the NWMF, the one-shot linear minimum mean squared error (MMSE) detector and the one-shot decorrelator. Furthermore, the limiting forms of the NWMF and CLMF in the absence of background additive white Gaussian noise (AWGN) are shown to be, respectively, an inverse chip filter followed by a despreading filter (or correlator) and a decorrelator-type CLMF. The thesis also examines how additional knowledge of interferer phase-offsets, the presence of intersymbol interference (ISI) and the presence of both unlocked and chip-delay locked interferers affect filter design