Study On The Algorithm Of Blind Multi-user Detection In The CDMA Communication System

Code Division Multiple Access(CDMA)as the main access mean in the third generation mobile communication system,which is based on the spread spectrum signal,use the different code to transfer the signal of multi-user.So the transferred signal is overlapped both in the time domain and sequence domain.The advantages of CDMA communication system are increased capacity,soft capacity, soft handover,higher spectral efficiency and the ability to resist the effects of multi-path fading,and so on.However,CDMA is self-interfered system,where multiple access interference(MAI)and Near-far effect are existing as the main factors restricting CDMA system capacity.At present,CDMA communication system mostly adopts power control technique to deal with MAI problem.The technique only mitigates MAI.The 3G mobile communication system regards multi-user detection(MUD)as an effective method to overcome MAI.MUD is an efficient technique for interference suppression that reduces the MAI and that solves the Near-far problem and improves system capacity. Traditional multi-user detector makes good use of all signals which result in multiple access interference so that it provides optimum MAI resistance. Nevertheless,there are some defects as follows:it assumes that the receiver can acquire the signature waveform and timing of desired user and the interfering users; it has no ability to suppress intercell multiple access interference;it can not be applied in downlink channels.Based on the traditional multi-user detection, adaptive multi-user detector eliminates the need to know the signature waveforms and the timing of the interferers and has to need training data sequences for every active user.But the frequent use of training sequence is certainly a waste of channel bandwidth.The blind adaptation schemes(BMUD)does not require training sequences and the interference user's information.It is an especially attractive research hotspot to the downlinks of CDMA lately because it only needs the prior knowledge of the signature waveform and the timing of the desired user, that is,the same knowledge as the conventional receiver. This thesis mainly focuses on the blind multi-user detection algorithm of CDMA systems.At first In the paper the basis theory of multi-user detection is introduced,which include the basic thought of the multi-user detection and the basic structure of the multi-user detecter.Then the mathematical model of the multi-user detection is build in the CDMA system.The vectors of DD detector and MMSE detector is given.Then we study three classic algorithm deeply.The Least Mean Square algorithm(LMS)takes mean output power of the detector as the cost function,which can gets vector iteratively.The CMA algorithm uses constant model to get the detector.and the LS-CMA is better than CMA algorithm.The vector of the Kalman filter detector is describes state variable equation,which borrow Gram-Schmidt orthogonalization.The principle of subspace based blind multi-user detection algorithm is the emphasis of this paper.The principle of subspace based blind multi-user detection algorithm is that the weight vectors of detector are expressed by signal subspace parameters,paper[19]first applied the subspace conception to the multi-user detection technology,proposed a subspace based structure of blind multi-user detection.The vectors of detector are calculated with adaptive subspace tracking algorithm,and the signal of the desired user can be obtained.When multi-user detector is adapted in blind mode,it usually adopts eigenvalue decomposition or singular value decomposition of received sample correlation matrix and tracking algorithms,which result in high computational complexity.The PASTd algorithm with less computation complexity is widely used in the multi-user detection, however,and it can not guarantee the orthogonality of eigenvectors which induces the slow convergence rate.To overcome this problem,an improved PASTd subspace tracking algorithm was proposed,which enhances the convergence rate and reduces calculation of the algorithm.Simulation results demonstrate the effectiveness and feasibility of the proposed algorithm.The blind multi-user detection algorithm based on Chebyshev approximation method is a novel algorithm,which in order to solve a equation iteratively whose solution is the optimal tap-weight vector of blind multi-user detector.Simulation results show that this new detector has fairly faster convergence and much better performance than RLS algorithm,while its computation complexity stays just the same as RLS.In this paper,we proposed a subspace based Chebyshev approximation blind multi-user detection algorithm.We combine the Chebyshev approximation method and the subspace tracking Simulation results this new algorithm has fairly faster convergence and much better performance.