Adaptive fuzzy power control for mobile radio systems

This paper investigates the 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. A fuzzy PI (proportional-plus-integral) control whose input variables are the received power error and error change is introduced to determine each user transmitting power in order to simultaneously maintain all users' signal power received at the base station nearly equal and achieve the 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 is 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 RMS tracking error. Furthermore, the additional advantages of fuzzy PI control over conventional control theories are: increased robustness in spite of interference and the ability to handle 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 cochannel interference conditions.