Code placement and replacement schemes for WCDMA Rotated-OVSF code tree management

Orthogonal variable spreading factor (OVSF) channelization codes are widely used to provide variable data rates for supporting different bandwidth requirements in wideband code division multiple access (WCDMA) systems. Many novel works in the literature have intensively investigated code placement and replacement schemes in OVSF code trees to reduce the code blocking probability and the code reassignment cost. In this paper, we introduce a new code tree structure, namely, a rotated-orthogonal variable spreading factor (ROVSF) code tree, whose code capacity is the same as that of the traditional OVSF code tree. This work addresses both code placement and replacement schemes in the ROVSF code tree system, where ROVSF codes can be used at the forward link of WCDMA systems. Some valuable properties of the developed ROVSF code tree are presented to develop code placement/replacement schemes with lower code blocking probability and less code reassignment cost. The main contribution of our scheme is to identify linear-code chains (LCCs) and nonlinear-code trees (NCTs) in the ROVSF code tree. This work exploits the unsequence property of linear-code chains to design a new code placement and replacement mechanism. Our code placement/ replacement schemes initially attempt to allocate request codes on LCCs and, then, to allocate them to NCTs. Using LCCs with the unsequence property allows us to efficiently reduce the code blocking probability and the code reassignment cost. Extensive simulations are conducted to illustrate that our code placement/replacement results based on the ROVSF code tree actually improve the code blocking probability and the code reassignment cost.