A smart antenna array for CDMA systems with noncoherent M-ary orthogonal modulation
Ming Lu, Titus K. Y. Lo, John Litva · 2002
DS/CDMA cellular systems are plagued with the near-far problem which can result in considerable reduction in capacity. Adaptive antenna arrays are used to increase link capacity, improve coverage and permit less stringent power control. Adaptive beamforming algorithms are crucial to the success of the smart antennas. The existing beamforming methods are found to be far from satisfactory for DS/CDMA in the urban multipath environment where user signals are very dissimilar in strength and perfect power control is hardly possible. This paper proposes a new decision-directed beamforming algorithm for CDMA with noncoherent M-ary orthogonal modulation which performs very well in typical CDMA environments. We introduce a quality measure of a symbol decision and a criterion for picking good weights and rejecting bad weights during the process of adaptation. With incorporating the measure into the least-squares algorithm, we can select weights in a positive manner such that the beamformer is forced to keep track of its optimum weights. The advantages of the proposed algorithm include: (1) an amount of immunity to the near-far effect; (2) the ability of tracking the optimum weight vector in the context of time-variant channels; (3) insensitivity to decision errors; (4) good convergence property; (5) the ability of working in low SNR scenarios; (6) tolerance to timing errors between the antennas; (7) mild synchronization requirement, and (8) computational simplicity. The simulation results are given in this paper to demonstrate the effectiveness of the proposed algorithm.