Arrival distribution based region division for OVSF codes in 3G wireless networks

3G and beyond CDMA systems uses Orthogonal Variable Spreading Factor Codes (OVSF) for channel access by multiple users. OVSF codes suffer from the limitation of code blocking. Code blocking is due to the scattering of the vacant codes in the OVSF code tree. In this paper an efficient code assignment scheme is proposed to minimize future code scattering which gives minimum code blocking. The code assignment works in the three steps. In the first step, the code tree is divided into regions, where each region is for one class of users. The number of codes in a region is proportional to the arrival rate of users within the class. First step may not be able to find unique optimum code, for which second step is used. It considers the capacity of ancestors to resolve tie. If even the second step also does not lead to unique result, step 3 which uses the elapsed time of neighbors of the candidate vacant codes can be utilized. Simulation results are given to show the superiority of the proposed scheme with existing noble assignment schemes.