POWER CONTROL FOR CELLULAR COMMUNICATIONS WITH TIME-VARYING CHANNEL UNCERTAINTIES

Power control in a code-division multiple access (CDMA) based cellular network is a challenging problem because the communication channels change rapidly because of multipath fading. These rapid fluctuations cause detrimental effects on the control efforts required to regulate the signal-to-interference plus noise ratios (SINRs) to the desired level. Thus, there is a need for power-control algorithms that can adapt to rapid changes in the channel gain caused by multipath fading. Much of the previous work has either neglected the effects of fast fading, assumed that the fading is known, or assumed that all the link gains are known. In this paper, we model the effects of fast fading and develop practical strategies for robust power control based on SINR measurements in the presence of the fading. We develop a controller for the reverse link of a CDMA cellular system, and use a Lyapunov-based analysis to prove that the SINR error is globally uniformly ultimately bounded. We also utilize a linear prediction filter that utilizes local SINR measurements and estimates of the Doppler frequency that can be derived from local SINR measurements to improve the estimate of the channel fading used in the controller. The power-control algorithm is simulated for a cellular network with multiple cells, and the results indicate that the controller regulates the SINRs of all the mobile terminals (MTs) with low outage probability. In addition, a pulse-code-modulation technique is applied to allow the control command to be quantized for feedback to the transmitter. Simulation results indicate that the outage probabilities of all the MTs are still within the acceptable range if at least 3-bit quantization is employed. Comparisons to a standard algorithm illustrate the improved performance of the predictive controller.