Effect of feedback delay on fixed step and variable step power control algorithms in CDMA systems

Abstract

In this paper, we compare the performance of fixed step and variable step algorithms of a closed-loop power control (CLPC) scheme for the reverse link of a code division multiple access (CDMA) system. The CLPC under consideration is based on the signal-to-interference (SIR) measurement at the base station. Conceptually, variable step algorithms outperform fixed step algorithms because the variable step algorithm can track the fading channel more accurately, with accuracy increasing proportionally to the number of power control bits per power control interval. In practice, however, power control algorithms are subject to the limitations of power control parameters, particularly the effect of feedback delay. The performance of variable step algorithm with mode q=4 is found to be worse than that of a fixed step algorithm (q=1) under practical situations with feedback delay of two power control intervals. To overcome the effect of feedback delay, a channel predictor is devised using a recursive least square (RLS) method and is shown to. be very effective.