Multi-code placement and replacement schemes for W-CDMA rotated-OVSF code tree

Orthogonal variable spreading factor (OVSF) channelization codes are widely used to provide variable data rates for supporting different bandwidth requirements in WCDMA systems. Much work in the literature has intensively investigated code placement/replacement schemes in OVSF code trees to reduce the code blocking probability and reassignment code. This work introduces a new code tree structure, namely an ROVSF (rotated-OVSF) code tree, whose the code capability is the same as that of the traditional OVSF code tree. Using the single code placement and replacement schemes usually lead to the internal and external fragmentation problems in the traditional OVSF or ROVSF code trees. The internal and external fragmentation problems can be efficiently improved by using the multi-code placement and replacement schemes. Therefore, this work mainly investigates both multi-code placement and replacement problems in a ROVSF code tree system. In this paper, we developed new multi-code placement and replacement schemes in a ROVSF code tree to offer the results of the less code blocking probability and less reassignment cost. Finally, the simulation results illustrate that our multi-code placement/replacement results based on the ROVSF code tree can actually improve the code blocking probability and the code reassignment cost.