Abstract
Third generation (3G) WCDMA systems evolve from second-generation wireless CDMA systems such as IS-95 and adopt orthogonal variable spreading factor (OVSF) codes. However, OVSFCDMA systems (with a single code per user) has two severe problems which limit the bandwidth utilization and efficiency. The two problems are the code blocking problem (also known as external fragmentation problem) and the coarsely quantized data rate problem (also known as the internal fragmentation problem). To solve the external fragmentation problem, we propose two innovative region division algorithms (called DRDA and VRDA). Both algorithms completely eliminate the external fragmentation problem without causing a fairness problem. In addition, VRDA is the first mechanism that not only optimizes the number of OVSF codes that must be reassigned to support a new call but also takes future potential code blocking possibility into consideration. On the other hand, to solve the internal fragmentation problem, we introduce a shared code mechanism which lets multiple users share OVSF codes on a time division basis. Our simulation results demonstrate that VRDA performs superiorly in reducing income/spectrum loss and has less number of reassignments than other schemes proposed in the literature.