Closed-loop power control in CDMA systems

A closed-loop power control system having both fixed-step and adaptive-step power control algorithms and incorporated with multitap RAKE receivers was proposed. Simulations were performed for wide-band radio channels having path loss, slow fading, and fast fading. As the measurement of the received power and the bit energy-to-interference power spectral density ratio play an important role in a CDMA power control system, a scheme to estimate these two parameters was proposed. For a Rayleigh fading channel and a Rician fading channel having a K factor of 6 dB, a three-tap RAKE receiver is desirable. The decrease in the standard deviation of the errors in E_b/I₀ due to both the two-tap and three-tap RAKE receivers were larger than 1.0 dB for Rayleigh fading channels. For Rician fading channels, the improvement in performance due to a RAKE receiver was still quite insignificant for a K of 6 dB, while only marginal for a K of 12 dB. As the number of taps in RAKE receivers increases, the dynamic range of the received power variation decreases, while the power variation becomes even more random. The performance of the adaptive-step-size power control scheme considered here was only slightly better than the fixed-step-size one for less than two taps. For a mobile´s speed of 96 km/h and a three-tap RAKE receiver, the standard deviation of power control errors in the bit energy-to-interference power spectral density ratio is 2.0 dB and 1.5 dB for Rayleigh and Rician fading channels having a K factor of 6 dB. The performance of the power control scheme is essentially unaffected for a control bit error rate of 10^-2