Blind Interference Suppression For CDMA Using Code-Constrained Constant Modulus Algorithm

In direct sequence/code division multiple access (DS/CDMA) communication system, different users employ distinct spreading codes but transmit at the same time and frequency. If the assigned spreading codes are orthogonal and signals are transmitted through a flat fading channel, there will be no multiple access interference (MAI). In practice, it is difficult to keep orthogonal signal constellation between different users. So there always exists MAI in DS/CDMA system, which limits system capacity.The matched filter detector that was used in conventional DS/CDMA treated MAI as Gaussian noise. Therefore, when the power level of certain users is significantly high, weak user with conventional matched detector may loose communication. This phenomenon is called near-far effect. Conventional matched filter detector cannot mitigate the near-far problem. However, multiuser detection techniques exploit the underlying structure induced by the spreading codes of active users in DS/CDMA system. It can efficiently suppress MAI and mitigate near-far effect, which dramatically improves the performance and increase the capacity of DS/CDMA system.More recently, the blind adaptive multiuser detectors, which do not require training sequences and the interference user's information, have received considerable attention.The Optimal detector and various suboptimal detectors are studied in this paper firstly. Their advantages and disadvantages are analyzed. For blind adaptive multiuser detection, two kinds of blind algorithms are discussed. One is the method based on minimum output energy (MOE), the other is the subspace approach.In the end, this paper considered detecting the desired user signal for uplink DS/CDMA system. In that case, we need to suppress both MAI and inter-chip interference (ICI) caused by multipath propagation. Constant modulus algorithm (CMA), which was used in blind equalization, is introduced in this paper. When applying CMA to multiuser communication system, proper initialization or adding some constraints are required due to the multi-modality of CM cost function. A code-constrained CMA initialized by delay-hypothesized matchedmatched filter based on minimum kurtosis criterion is proposed in this paper. Code-constrained CMA imposes constraint on the equalization coefficients. Thus, it can ensure no cancellation of the desired signal when minimizing the CM cost function to suppress interference. In this paper, delay-hypothesized matched filter based on minimum kurtosis criterion is used to initialize code-constrained CMA. The simulation results demonstrate that the algorithm proposed in this paper has better performance with lower mean squared error (MSE) and higner output signal to interference plus noise ratio (SENR).