Blind adaptive decorrelating detectors for DS-CDMA systems

Abstract

Code Division Multiple Access (CDMA) systems suffer from the near-far effect, because of non-orthogonality of users```` signature sequences. Multiuser detection can be used to overcome the near-far problem by exploiting the known structure of multiple access interference (MAI) to effectively demodulate non-orthogonal signals of users. It is shown that the optimal multiuser detector has a computational complexity which increases exponentially with the number of active users. Several suboptimum detectors, including the decorrelating detector, have been proposed to achieve a performance comparable to that of the optimum detector while keeping the complexity low. The decorrelating detector eliminates the multiple access interference totally if the signature sequences of users are linearly independent at the cost of enhancing the additive white Gaussian noise (AWGN). The decorrelator is centralized and non-iterative. The construction of the decorrelator for a certain user requires the knowledge of the signature sequences of active users. In addition, the construction of the decorrelator requires inversion of the cross-correlation matrix of the signature sequences. Adaptive algorithms are desirable to overcome the need for the knowledge about the parameters of interfering users and to avoid the matrix inversion which may have a large computational complexity. In this dissertation, we propose two adaptive algorithms for synchronous DS-CDMA systems over an AWGN channel to achieve the performance of the decorrelator: one is the centralized multiuser detection scheme and the other is the single user detector. First, the centralized scheme is proposed for the uplink and convergence analysis of it is given. This detector is operating directly on the received signal instead of the matched filter output, thus it needs less computational power than the conventional decorrelator. We show that the performance of the proposed detector is identical to that of the decorrelator. ...