We propose a technique for enhancing the achievable spectral efficiency of multiuser direct-sequence code-division multiple-access (DS-CDMA) fading channels in the presence of additive white Gaussian noise (AWGN) and multiple access interference (MAI). The proposed scheme involves the joint optimization of outer loop power control (OLPC) and rate control using variable spreading factors (VSFs). The optimality is in the sense of average spectral efficiency maximization. The optimum outer loop target signal-to-noise ratio (SNR-target) and the corresponding spreading factor are derived jointly, in terms of the number of active users. Along with transmit power adaptation in the inner loop, this leads to maximized average spectral efficiency. Total and truncated channel inversion strategies are used in the inner loop. The average spectral efficiency of the joint optimization scheme is derived for the conventional matched-filter and the multiuser decorrelating detectors. Average transmit power and instantaneous bit error rate (BER) constraints are considered and the performance is evaluated over Nakagami-m flat-fading channels. In low SNRs, the proposed scheme can provide a considerable gain in bits/s/Hz for either of the detectors, compared to a VSF-assisted system that does not exploit OLPC and thus the optimum SNR-target.
Published in: IEEE Transactions on Wireless Communications ( Volume: 8, Issue: 1, January 2009)
Page(s): 186 – 195
Date of Publication: 20 February 2009
Introduction
Adaptive transmission allows efficient utilization of the available resources to take advantage of favorable channel conditions. In code division multiple access (CDMA) systems, adaptive transmission involves varying either one or a combination of transmission power [1] [2], modulation level [3]–[5], processing gain [6]–[9], chip rate [10], number of spreading codes [11] [12], and coding scheme or rate [13], in order to use the available bandwidth more efficiently, resulting in higher spectral efficiency. In CDMA systems, uplink power control consists of closed loop power control and open loop power control. Closed loop power control contains outer loop power control (OLPC) that determines the target value of signal-to-noise ratio (SNR) at the receiver, and the fast inner loop power control (ILPC) that aims to achieve this SNR-target in order to maintain the desired communication quality dictated by the system design. The open loop power control is used for initial power setting at the beginning of a connection. In this paper, only closed loop power control aspects are studied.
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