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.
Date of Conference: 11-15 March 2007
Date Added to IEEE Xplore: 04 June 2007
Conference Location: Hong Kong, China
Introduction
The advantages of direct-sequence code-division multiple access (DS-CDMA) compared to other multiple-access techniques, such as robustness to multipath distortion, low probability of interception, immunity to jamming, and simple random access to the network has established it as the multiple access protocol for the third generation (3G) wireless communication systems [1] and its derivatives such as multi-carrier DS-CDMA are envisaged as important techniques for the fourth generation (4G) wireless systems [2]. A major limitation to the capacity of the CDMA systems is the multiple access interference (MAl) produced by other co-channel mobile users. To that end, various power control and adaptive transmission schemes have been proposed with the intention of suppressing the MAI and boosting the performance [4]–[7].
Read More : https://ieeexplore.ieee.org/document/4224569
