Multicode-based WCDMA for reducing waste rate and reassignments in mobile cellular communications

The third generation (3G) mobile communications are proposed to accomplish high data rate for personal multimedia communications. To achieve that, the UMTS standards adopt the orthogonal variable spreading factor (OVSF) code tree to assign a single channelization code for each accepted connection at the Wideband CDMA air interface. Nevertheless, there are two drawbacks within it, including code blocking and waste of the system bandwidth. One good solution for reducing waste rate is to adopt the mechanism of multiple codes assignment, in which each accepted connection can be assigned several codes for achieving that the summation rate of these codes is satisfied the required rate. Therefore, the waste rate can be reduced significantly; nevertheless, it brings some overhead, including large number of reassignments, long connection setup delay and computation complexity, and cost of using multiple rake combiners. Therefore, we propose herein an efficient multicode assignment algorithm to reduce the waste rate while reducing large number of reassignments. The motivation of this approach is to reserve high-rate codes for next arrival calls with concerning the current state of the OVSF code tree. Consequently, it makes the tree compact and thus obviously reduces the number of code reassignments. Since there is a trade-off between the waste rate and the number of rake combiners, the optimal number of rake combiners for multicode assignment is analyzed and thus minimizes the cost of rake combiner. Numerical results demonstrate that the proposed approach yield the least number of reassignments while reducing the waste rate significantly as compared to other approaches. (c) 2005 Elsevier B.V. All rights reserved.