The impact of code allocation on the multiple access interference in WCDMA systems

Code allocation has a significant impact on the performance of Code Division Multiple Access (CDMA) systems. For the uplink direction, in 3G Wideband CDMA (WCDMA) cellular systems, orthogonal codes are employed to differentiate the physical channels of one transmitter and Pseudo Noise (PN) sequences are used to provide mutual randomness between the users. Since the received data spread by different codes are not perfectly orthogonal, the emerged Multiple Access Interference (MAI) is a major limitation of the system capacity. In this paper, we propose a new code allocation scheme with the aim of reducing the interference by decreasing the number of non-orthogonal spreading codes used in a cell. Consequently, the new proposed scheme improves the system capacity for the uplink direction in term of accepted calls in one cell. The idea behind is to assign one scrambling code for each class of service in a cell instead of assigning one scrambling code per mobile station as in 3G WCDMA-based systems. Note that the traffic belonging to one class of service may be originated from different mobile stations. We also develop a new power control algorithm associated with the new code allocation scheme. In order to evaluate the performance, we develop analytical models for the code allocation scheme currently used in UMTS (Universal Mobile Telecommunication Systems) WCDMA-based system considered as a benchmark and for our proposed scheme. Numerical and simulation results are showing that our proposed scheme offers significant gain for the uplink capacity compared with the current scheme used in UMTS.