In the radio environment, transmitted signals arrive at the receiver via a direct, unobstructed path or via multiple paths from the reflection, diffraction and scattering of surrounding objects such as buildings and trees. This multipath propagation causes the signal at the receiver to distort and fade significantly, leading to inter-symbol interference (ISI). Spread spectrum mobile communication systems use RAKE receivers to minimize these communication errors resulting from multipath effects.
The aim of this thesis is to develop a MATLAB model of a spread spectrum system using RAKE receiver and evaluating its performance in a time varying channel. A DS-SS system with a BPSK modulation is first introduced and evaluated in AWGN and multipath fading channels with and without Rake receiver. This was performed under different conditions like using different spreading codes and using different number of rake fingers. Ideally, the number of correlators in the RAKE receiver should match the number of multipath signals. The effect of multipath delay spread is then investigated by using different combinations of predefined delays for the multipath channel. The next step was to provide channel estimation using LMS filter for a stationary channel and testing different pilot signals with different lengths. Finally, a complete system was introduced with a time varying channel and tested at different rates of change. The performance of the simulated system is evaluated through the BER calculation by Monte Carlo analysis and theoretical analysis supported by MATLAB (v.7.8).
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