Code placement and replacement strategies for wideband CDMA OVSF code tree management

Abstract

The use of OVSF codes in WCDMA systems has offered opportunities to provide variable data rates to flexibly support applications with different bandwidth requirements. Two rarely addressed issues in such environments are the code placement problem and code replacement problem. The former may have significant impact on code utilization and thus code blocking probability, while the latter may affect the code reassignment cost if dynamic code assignment is to be conducted. The general objective is to make the OVSF code tree as compact as possible in order to support new calls, either with less blocking or with less reassignment cost. This paper is perhaps the first one which addresses these issues in WCDMA. Three simple strategies, which can be adopted by both code placement and code replacement, are proposed: random, leftmost, and crowded-first. Among them the crowded-first strategy looks most promising, which is shown to be able to reduce, for example, the code blocking probability by 77% and the number of reassignments by 81% as opposed to the random strategy when the system is 80% fully loaded and the max SF=256.