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

In this paper reference is made to some allocation techniquessuitable for the assignment of OVSF (Orthogonal VariableSpreading Factor) codes on the downlink of a WCDMA system.Yet, in contrast to what has been previously assumed inliterature, this work proposes a criterion for the admission of anew user that explicitly takes into account transmission requirements.In detail, two distinct conditions are to be fulfilled to leta new call enter the system: given a code allocation strategyis selected, the corresponding rules are first applied with thegoal of finding a free code to assign the user, fitting the user’sdata rate needs; additionally, it is required that the signal-tonoiseratio – computed accounting for the presence of the newuser – takes on values higher than a proper threshold with aprobability sufficiently high (higher than a predefined admissionvalue) for all ongoing connections. Equivalently, systemload can be increased, but only up to the point where the assignmentof a new code within the tree does not degrade transmissionquality below an acceptable level. An accurate descriptionof the underlying physical layer is employed, and it is demonstratedhow markedly the requisite of adequate transmissionconditions affects the number of admitted users. As it is the interferencelevel to ultimately drive the OVSF code assignment,realistic limits on the performance that some popular code allocationtechniques can achieve in a Wideband CDMA systemare clearly evidenced.