Effect of Feedback Transmission Error on Fixed-Step and Variable-Step Closed Loop Power Control in CDMA Systems

This paper evaluates the effect of feedback transmission error on the performance of fixed-step and variable-step power control algorithms on the uplink of code division multiple access (CDMA) systems. The control mechanism is closed loop in that the algorithms are based on the feedback information of signal-tointerference ratio (SIR) measurement at the basestation. Under ideal conditions, variable-step algorithm outperforms fixed-step algorithm to alleviate the effect of Rayleigh fading because the variable-step algorithm can track the fading channel more accurately, with accuracy increasing proportionally to the number of power control bits for each power control interval. In practice, however, power control algorithms are imperfect due to the limitations of control parameters, such as SIR measurement error, feedback delay, power updating and fading rates, and feedback transmission error. In this work, the effect of feedback transmission error is evaluated by computer simulation. The result shows that when feedback information is subject to transmission error, the performance of variable-step algorithm degrades more significantly because it is more sensitive to command error than the fixed-step algorithm. Keywords— CDMA, feedback transmission error, power control.