Effect of erroneous power control on the performance of overloaded cellular UCDS-CDMA

Abstract

Channel overloading in Code Division Multiple Access (CDMA) paves the way to achieve more no of users (>N) in a space with dimension equal to the spreading factor (N). It puts its own priority over the conventional CDMA to provide broadband access in cellular wireless communication. A new overloading scheme for the uplink of cellular CDMA has been proposed in [7] using unequal chip delay spreading (UCDS). It divides all the active users into G groups with each group containing L number of users. Each user in a group is assigned the same orthogonal code, but with different chip durations (multiple of Tc/L). For spreading, all the L Users in a group use the assigned unique code with the insertion of predefined time delay. During spreading each user in a group has to go through unequal time delay between each successive spreading chip. Addition of suitable delays helps in maintaining the correct time alignment among all the users in a group. The receiver uses a very simple orthogonal successive interference cancellation (OSIC) multi user detector in order to recover the message bits of all the users. In this paper, the performance of this new overloading scheme (UCDS-CDMA) with an overloading factor (OF) of TWO has been evaluated with different levels of error in power control. The system model has been designed mainly for the uplink of the cellular CDMA over an additive white Gaussian noise (AWGN) channel. The effect of erroneous power control in the uplink of UCDS-CDMA costs a SNR of 1.1 dB, 1.3 dB and 1.6 dB as compared to the conventional perfectly power controlled underloaded CDMA (OF=1) with the power control error of 1 dB, 1.5 dB and 2 dB respectively at a BER of 10-3.