OVSF codes assignment and reassignment at the forward link of W-CDMA 3G systems

We examine the problem of assigning and reassigning orthogonal variable spreading factor (OVSF) codes at the forward link in 3G mobile communications systems employing W-CDMA. Efficient channelization code management results in high code utilization and increased system capacity. We introduce a new code selection scheme, which, by means of a very simple measure, attempts to mitigate the problem of scattered codes that fragments the capacity of the OVSF code tree. The main advantage of this new scheme is its ability to combine effectively the merits of other previously proposed schemes (see Tseng, Y.-C. et al., Proc. IEEE GLOBECOM, vol.1, p.562-6,2001). Thus, our proposed scheme is as fast as the random or ordered strategies, while simulation results have shown that it is as efficient, in terms of capacity improvement, as the crowded-first scheme.