Dead-Beat and LQ-optimal power control algorithms in the uplink of wireless systems

Power control is an important problem in today's wireless systems, which is related to the battery utilization in the mobile units. In this work, 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 error function, the nonlinear coupling among the active users is transformed to individual loops, where the power references incorporate the information of the remaining users in the cell. It is concluded that the uplink channel variations do not destroy the stability of the feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. An LQ-optimal power control strategy is derived and compared to a dead-beat approach. In this sense, the dead-beat algorithm is sensitive to the roundtrip delays estimation, but the LQ-optimal control can be adjusted to be robust to this estimation error. However, there is a severe compromise between robustness and performance. But through an appropriate selection of the LQ weight parameter, an 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 control algorithms using a standard single-step power correction approach.