In this paper, we derive a new expression for the signal-to-interference ratio (SIR) at the output of a DS-CDMA receiver, and employ the expression to simulate network power control in a dynamic multipath environment. In the theoretical analysis, we incorporate all relevant time-varying components of the mobile cellular system into the channel model. We use this theoretical framework along with an extensive simulation model to study and analyze the performance of the power control algorithm and its impact on capacity and outage probability for a multiple-cell DS-CDMA system. We evaluate the capacity of the system for 144 Kbps data rate under a vehicular environment. Simulation results indicate that there is a reduction in the capacity of the multiple-cell system by 25% compared to the case in which fast fading is not included in the system simulation.