Power allocation and spectral efficiency of DS-CDMA systems in fading channels with fixed QoS-part I: single-rate case

Power allocation and the resulting spectral efficiency are considered for DS-CDMA systems operating over fading channels with a fixed single quality of service (QoS) requirement. For systems both with and without channel state information at the transmitter, the decoding scheme of successive interference cancellation combined with minimum mean square error (SIC+MMSE) multiuser detection is shown to minimize the total transmitting power under certain conditions. Both upper and lower bounds on the required receive power are obtained for SIC+MMSE systems with channel state information at transmitters (CSIT) and an analytical expression for the required transmit power is obtained for SIC+MMSE systems without CSIT. The parameters of spectral efficiency in both low and high energy regions are obtained. The good performance of SIC+MMSE in non-elastic traffic systems is also demonstrated.