Adaptive fuzzy power control for CDMA mobile radio systems

Abstract

This paper introduces a new application of fuzzy-logic control (FLC) theory to the power control in a direct-sequence, code-division multiple-access (DS/CDMA) cellular system over the mobile fading radio channels. Power control is essential in DS/CDMA to compensate for the differing received powers due to both the slowly varying long-term and fast varying short-term fading processes and co-channel interference. The conventional feedback power control algorithms allow the base station to send a power command to either raise or lower each user transmitting signal power level by a fixed power step and then keep the received powers almost equal. The fixed-step approach is actually an integral control whose power increment is determined according to the bang-bang-like control policy. However, this control scheme suffers from poor system stability, large overshoot, and long rise time. To tackle this difficulty, a fuzzy proportional-plus-integral (PI) control, whose input variables are the received power error and error change, is introduced to determine each user's transmitting power in order to maintain simultaneously all users' signal power received at the base station nearly equal and to achieve better system stability and control performance. The derivation of the fuzzy PI control has been carried out by analyzing both the closed-loop steady state behavior and transient response of the system with a priori knowledge of the dynamics of the CDMA mobile fading channels. In fuzzy control, linguistic descriptions of actions in controlling a process are represented as fuzzy rules. This fuzzy-rule base is used by an inference mechanism in conjunction with some knowledge of the states of process in order to determine control actions. These control actions would lead to the fast rise time, minimum overshoot, and small root-mean-squared (rms) tracking error. Furthermore, the additional advantages of fuzzy PI control over conventional control theories are increased robustness despite interference and the ability to handle the time-delay process without system degradation since there is usually a latency between each user and base station. Simulation results show that the fuzzy PI power control provides much smaller rms tracking error and better traffic capacity performance compared with the fixed-step control, especially in poor co-channel interference conditions.