Adaptive Power and Rate Transmission with Single and Multi-User Detection

We propose a technique for enhancing the achievable spectral efficiency of multiuser direct-sequence code-division multiple-access (DS-CDMA) channels in the presence of additive white Gaussian noise (AWGN) and multiple access interference (MAI). The proposed scheme is the joint optimization of outer loop power control and variable spreading factors (VSFs). The optimum SNR-target, the SNR that results in optimal spectral efficiency, is set in the outer loop. Accordingly, appropriate spreading factor is selected, and along with the transmit power adaptation in the inner loop, it leads to maximized spectral efficiency. Total and truncated channel inversion strategies are used in the inner loop. Analytical expressions are derived for the spectral efficiency of the joint optimization scheme for the conventional matched-filter and the multiuser decorrelating detectors. Average transmit power and instantaneous BER constraints are considered and the performance is evaluated over Nakagami-m flat fading channels. We show that our scheme achieves higher spectral efficiency than a system that uses VSF transmission but without outer loop optimal SNR-target. The gain is realized at no transmit power cost and without sacrificing the BER.