Error statistics of closed-loop power control in multirate DS-CDMA cellular systems

We investigate the error statistics of the close-loop power control scheme with variable spreading factors for the multi-rate services in the third generation wideband CDMA (WCDMA) system. We demonstrate that the long scrambling pseudo-noise code, besides its well known feature in differentiating users and base stations, can improve power control false command over a frequency-selective fading channel as well. Our computer simulation results indicate that the SIR measurement error can be reduced by 3 dB in a typical case, and the power control false command can be reduced from 27 % to 10 % in comparison to using pure short Walsh codes. It is shown that the close-loop power control error is a composite function of the spreading factor, target E_b / N_o and Doppler frequency. The statistics of closed loop power control false command can be easily characterized by a Bernoulli distribution with a parameter provided by our results.