How to Veritably Determine the Accomplishments of Static and Dynamic OVSF Code Assignment Algorithms

In this paper reference is made to some allocation techniques suitable for the assignment of OVSF (orthogonal variable spreading factor) codes on the downlink of a WCDMA system. Yet, in contrast to what has been previously assumed in literature, this work proposes a criterion for the admission of a new user that explicitly takes into account transmission requirements. In detail, two distinct conditions are to be fulfilled to let a new call enter the system: given a code allocation strategy is selected, the corresponding rules are first applied with the goal of finding a free code to assign the user, fitting the user´s data rate needs; additionally, it is required that the signal-to-noise ratio - computed accounting for the presence of the new user - takes on values higher than a proper threshold with a probability sufficiently high (higher than a predefined admission value) for all ongoing connections. Equivalently, system load can be increased, but only up to the point where the assignment of a new code within the tree does not degrade transmission quality below an acceptable level. An accurate description of the underlying physical layer is employed, and it is demonstrated how markedly the requisite of adequate transmission conditions affects the number of admitted users. As it is the interference level to ultimately drive the OVSF code assignment, realistic limits on the performance that some popular code allocation techniques can achieve in a wideband CDMA system are clearly evidenced