||In recent years, a lot of work has been done on the development of Third Generation (3G) mobile communications systems. Direct Sequence Code Division Multiple Access (DS-CDMA) as one of the accepted operation modes for 3G mobile communications systems, has attracted widespread research interest. However, most of the research schemes proposed so far have been focused on approaches related to the physical layer and power control. Issues related to the Quality of Service (QoS) at the call level have not received much attention yet. In this thesis, we propose an efficient adaptive call admission control scheme for DS-CDMA networks with multiple types of traffic. The scheme is based on one QoS metric, which is called the handoff QoS deteriorating probability, defined as the probability that a handoff call will violate the Signal-to-Interference-and-Noise-Ratio (SINR) requirements for other calls. In order to demonstrate the performance of our scheme, we compare the handoff deteriorating probability of our scheme with conventional schemes. The conventional admission scheme calculates the system capacity based on the boundary of outage probability, which called tier 1 admission control. In order to reduce the outage probability, our scheme proposes an extension of the tier 1 admission control called tier 2 admission control. The tier 2 admission control scheme reserves extra bandwidth for handoff calls to ensure that handoff calls will not violate the SINR requirements. The tier 2 admission control algorithm is executed only when a new arrival call is permitted by the tier 1 admission control. Admitted calls can liberally execute handoffs using the reserved bandwidth. With the adjustable QoS parameters, the tier 2 admission control scheme is adaptive to the traffic intensity and user mobility. Simulation results have shown that our proposed scheme provides satisfactory call performance.