Chip-Level DS-CDMA Downlink Interference Suppression With Optimized Finger Placement

In direct-sequence code-division multiple-access (DS-CDMA) downlink, multiaccess interference arises due to the loss of orthogonality of the spreading codes for each user in frequency-selective channels. We analyze the receiver structure where a linear combiner with a RAKE structure is used. A generic model is developed to account for arbitrary finger placement, which allows many CDMA downlink linear receivers to be treated as special cases. For fixed finger placement, we derive the optimal weights for the receiver under both chip-level and symbol-level criteria. We show that the weights designed to minimize the chip mean-square error (MSE) also maximizes the output symbol signal-to-interference-noise ratio (SINR) and differs only by a scaling factor from the weights that minimizes the symbol MSE. The performance of the receiver depends heavily on the finger placement scheme. We propose two strategies for finger placement design. The first one is analytical and is based on the sampling design theory. It gives the optimal finger placement in an asymptotic sense as the number of fingers becomes large and sheds important light on how the performance of the system depends on the number of fingers and the smoothness of the underlying chip pulse waveform. The other design is numerical and is based on a greedy search algorithm. An efficient recursive algorithm is developed to reduce the computational complexity associated with the search. For Rayleigh-fading channels, numerical results show that both design schemes achieve significantly better performance than uniformly spaced finger placement with the same number of fingers, or achieve performance similar to that of uniformly spaced finger placement but with a much smaller number of fingers