Quality-Based OVSF Code Assignment and Reassignment Strategies for WCDMA Systems

In third-generation code division multiple access (3G CDMA) mobile cellular systems, orthogonal variable spreading factor (OVSF) codes are applied in downlink to maintain the orthogonality between different channels. Since the number of available codes is limited, efficiently utilizing OVSF codes becomes an important issue. The code assignment and reassignment problem focuses on the subject of minimizing the number of code relocations. Previous works were based on some assumptions, including fixed service data rates and a code-limited system capacity. In this paper, we investigate this issue from a different perspective, by applying variable service data rates and an interference-limited system capacity. We have proposed three OVSF code assignment and reassignment strategies for this scenario, including sparse-first, sparse-first/rightmost, and modified sparse-first/rightmost, with a view to achieving the best service quality. The proposed strategies are well evaluated and compared with other strategies, based on the number of code reassignments, service quality, and throughput, via simulation. It is found that our proposed strategies outperform others under the scenario with variable service data rates and an interference-limited system capacity.