Minimum User Powers and Optimal Codewords in Uplink CDMA Systems

The paper presents a distributed algorithm that allocates the minimum user powers for the optimal codewords in the uplink of a code division multiple access (CDMA) system. The user cost function will be the user power written as a convex function of user codeword for a defined signal-to-interference plus-noise ratios (SINR) target. The optimization problem is treated as a constrained problem over a convex set, for which the optimal codewords are designed based on feasible direction approach and the optimal user powers are the minimum values of the user cost functions. Even if the optimal user powers are unique, the optimal codewords don´t correspond to a unique allocation, but rather to an entire class of codewords that can be related by unitary transformations. For the optimal point the user codewords maximize the uplink sum capacity (C_sum) for the minimum power allocation. The proposed algorithm is analyzed and illustrated with numerical examples obtained from simulations.