Distributed power control algorithms in the uplink of wireless code-division multiple-access systems

Power control is an important problem in today's wireless systems, which is related to battery utilisation in the mobile units. In the present study, this problem is addressed using a distributed approach. The uplink of a direct-sequence code-division multiple-access communication (DS-CDMA) system is studied, and through a proper selection of the tracking error function, the non-linear coupling among active users is transformed to individual linear loops, where equivalent power references incorporate the information of the remaining active users in the cell. It is concluded that the uplink channel variations do not destroy the stability of these feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. A linear quadratic (LQ)-optimal power control strategy is derived and compared to a dead-beat approach. In this sense, the dead-beat algorithm shows sensitivity to the roundtrip delays estimation, but the LQ-optimal control can be adjusted to be robust to this estimation error. In fact, there is a severe compromise between robustness and performance. But through an appropriate selection of the LQ parameters, a stable closed-loop system can be always guaranteed independently on the uncertainty in the estimation of the roundtrip delay. Simulation results are presented to compare the proposed control algorithms to a standard single-step power correction approach.