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UMTS (Universal Mobile Telecommunications System) uses WCDMA (wideband code division multiple access) as its radio interface. The main advantage of this is its flexibility in resource management. The paper investigates the performance of two main call admission control schemes that exploit this flexibility - the wideband power based (WPB) scheme and the throughput based (TB) scheme. It is observed that the WPB scheme works well when the UMTS network carries more voice traffic than data traffic and the TB scheme performs better when more data traffic is prevalent. Based on these results, the paper introduces a new scheme called the 'adaptive call admission control' (ACAC) based scheme. All three schemes are applied to a UMTS network and two quality of service (QoS) parameters are observed, the blocking and dropping probabilities. The system is modeled as a multi-rate, multi-class, priority based system. Confidence intervals are obtained that show that the ACAC scheme is the best in a heterogeneous UMTS environment.