Blind Number of Active Users Estimation and Synchronization in CDMA Systems with Unequal Power's Signal on Flat Fading Channel

Blind estimation of number of active users and synchronization of those have important role in identification of system’s parameters, for the spectrum surveillance system design of multi-user direct sequence spread spectrum systems. For the number of active user estimation, signal’s eigenvalues of the received signal covariance matrix is distinguished from those of noise using an adaptive threshold. Analytical results and computer simulations show that signal of users with limited received power difference (less than 1.5 dB) can be synchronized using maximizing the Frobenious norm of the received signal covariance matrix. Moreover, this method is robust against of the carrier frequency offset which is due to Doppler shift in the carrier frequency of different users. By increasing difference among received powers of processed signals, performance of this method will be degraded. Hence, an iterative algorithm for interference cancellation of users with higher received power is proposed, which can be synchronize and estimate the number of active users with lower received power. The proposed method for number of active user estimation reduces computational complexity in comparison with the traditional methods for number of active user estimation, such as MDL and AIC, while keep the accuracy of estimation in low SNR regime. The performance of blind synchronization method in the flat fading channel is degraded; hence, a multi-antenna receiver is proposed to improve the probability of correct synchronization.