An Efficient Dynamic Orthogonal Variable Spreading Factor Code Allocation Approach in WCDMA through Modified Adaptive Genetic Algorithm

P Kavipriya,C Gomathy

doi:10.19026/rjaset.7.565

Abstract

Orthogonal Variable Spreading Factor (OVSF) codes would give variable data rate transmissions for different bandwidth supplies in Wideband CDMA (WCDMA) networks. These OVSF codes are used for the channelization of codes in WCDMA. In WCDMA, effective utilization of OVSF codes has become an active area of research as the number of codes is very limited. It is a fact that the successor and predecessor codes of OVSF cannot be used simultaneously when a specific code is used in OVSF as their encoded sequences become indistinguishable. Consequently, OVSF code tree has inadequate number of available codes. Thus, this research study uses Adaptive Genetic Algorithm (AGA) based approach for dynamic OVSF code assignment in WCDMA networks. Different from existing Conventional Code Assignment (CCA) and dynamic code assignment schemes, population is adaptively constructed according to existing traffic density in the OVSF code-tree. In existing technique in order to improve the ability of the GA, ＂dominance and diploidy＂ structure is employed to adapt to changing traffic conditions. Because in SGA algorithm cannot convergence if the new user is included into the existing OVSF code tree while SGA is running to find optimum OVSF code tree, SGA cannot adapt its structure to this unexpected variation. This problem can be overcome by the Modified Adaptive Genetic Algorithm (MAGA). Performance of the proposed MAGA approach is evaluated in terms of blocking probability and spectral efficiency and is compared with SGA, D&D GA.

Highlights

In Code Division Multiple Access (CDMA) mobile cellular systems, all downlink channels transmitted from the same Base Station (BS) are spread by different orthogonal codes to maintain orthogonality (Razavizadeh, 2008)
This study presents an efficient GA called Modified Adaptive Genetic Algorithms (MAGA) algorithm for the purpose of Orthogonal Variable Spreading Factor (OVSF) code assignment, which could adjust the parameters adaptively based on the value of individual fitness and dispersion degree of population
Proposed dynamic OVSF code allocation using modified adaptive genetic algorithm: In this research study, in order to overcome the drawbacks of the SGA and AGA, MAGA is presented in this approach to have better code blocking probability

Summary

INTRODUCTION

In Code Division Multiple Access (CDMA) mobile cellular systems, all downlink channels transmitted from the same Base Station (BS) are spread by different orthogonal codes to maintain orthogonality (Razavizadeh, 2008). Proposed dynamic OVSF code allocation using modified adaptive genetic algorithm: In this research study, in order to overcome the drawbacks of the SGA and AGA, MAGA is presented in this approach to have better code blocking probability. The OVSF code tree which is input to the MAGA block is called as initial chromosome (Chini) and this chromosome is denoted with the index number which belongs to active users in the given code tree (Chini = (6 9 14 16 21)) In this approach the integer value is taken from the index numbers 1 to SF-1, allocated from root code which is indicated as index 1. It is clear that iteration number of optimal solution is depends on population size (n), users’ data bit rates (H (i)) and their location in the code tree. The fitness value for each chromosome of population is evaluated according to fitness function, which is defined specially for OVSF code assignment-reassignment problem. The stopping criterion for this process is either run-on until to assign the requested data bit rate to a user or until the end of predetermined loop counter

EXPERIMENTAL RESULTS AND EVALUATION

Results

CONCLUSION