Code reservation schemes at the forward link in WCDMA

We examine resource reservation schemes for the management of orthogonal variable spreading factor (OVSF) codes at the forward link of 3G mobile communications systems employing WCDMA. Like in every multi-service network, calls with different rate requirements will perceive very dissimilar system performance at the forward link in 3G systems if no measures are taken and the channelization code tree is treated as a common pool of resources. Assuming that the traffic level for each class is known in advance, we introduce complete sharing (CS), complete partitioning (CP) and hybrid partitioning (HP) policies to manage the code tree. At the resource reservation level, we develop an efficient method to partition the available codes based on the offered traffic load of each class of calls and the size of the tree. The resulting partition is optimal in the sense that the maximum blocking probability of the different rate classes is minimized. At the call level, we use a real-time scheme to assign free codes to incoming requests, and evaluate its performance in terms of blocking probability per traffic class and utilization of codes in conjunction with the partitioning method used. It turns out that code blocking, which is encountered on this type of systems, further deteriorates the unfairness conditions at the forward link. Our simulation results show that fair access to codes by different rate calls is assured more by CP and less by HP schemes, at the expense of slightly lower code utilization at medium to high loads, compared to the CS scheme. Also, hybrid schemes absorb small traffic deviations more efficiently than CP, which is generally optimized for certain traffic mixes.