1 Successive Blind Detection of DS / CDMA Signals

This paper provides an investigation into the performance of a multi-stages constant-modulus filter/ signal canceller (CMA/SC) for blind detection of direct sequence/code code division multiple-access (DS/CDMA) signals over additive white Gaussian noise (AWGN) channels. A simple stochastic gradient algorithm for implementing the scheme is presented and the convergence properties of the algorithms are analyzed. Simulation examples are given to demonstrate the robustness of the performance of the proposed scheme. The Direct sequence-code division multiple access (DS/CDMA) system implemented by direct-sequence spread spectrum (DS/SS) technique is the most promising multiplexing technology for cellular telecommunications services, such as personal communications, mobile telephony, and indoor wireless networks. The advantages of DS/SS for these services include superior operation in multi-path environments, flexibility in the allocation of channels, the ability to operate asynchronously, privacy, and increased capacity in burst or fading channels [1]. It is well known that the processing gain of a spread spectrum system will provide the system with a sufficient capability of interference rejection [2], [3]. Multiple access interference (MAI), due to many simultaneous users, constitutes the main limitation on the performance of DS/CDMA systems. Multiuser detection techniques can efficiently suppress MAI and substantially increase the capacity of CDMA systems [4], [5]. Various multiuser detection schemes for DS/CDMA systems have been developed over the past decade. More recently, blind adaptive multiuser detection, which requires the prior knowledge of only the spreading waveform and timing of the desired user, has received considerable attention [6]. The main motivation for employing a blind scheme is to avoid the need for a training sequence, to thus offer better spectrum efficiency. The most representative methods of the blind multiuser detection include the minimum output energy (MOE) [7] and subspace approach [8]. However these methods assume a knowledge of the spreading waveform, which is not available in some applications like electronic intelligence (ELINT). The constant-modulus (CM) receiver can perform almost as well as the non-blind/trained receiver except that the CMA captures an arbitrary signal from the combined received CDMA signals, which may be of no interest. In applications like ELINT it is necessary to detect all the active users, either in a parallel or a serial manner. This paper describes a mechanism for the successive blind detection for all the active users in a DS/CDMA system. The principle of operation is based on utilizing identical successive stages for detection. Each stage, CMA detects an arbitrary signal, which is …