Performance analysis of packet scheduling strategies for multimedia traffic in WCDMA

This paper presents the performance analysis of different scheduling strategies suitable for transmission of real time and nonreal-time services in packetized form in the FDD mode of UMTS. A centralized demand assignment protocol is implemented to transmit over the dedicated physical channel. Given that in CDMA the capacity is not fixed and varies with the interference, the arrangement of transmissions is combined with a distributed power control that allows one to maximize the throughput by adjusting the transmitted power of terminals in order to minimize the interference. The scheduling strategies analyzed are based on the establishment of static priorities between classes of traffic combined with the assignment of dynamic priorities, associated with time stamps, to the individual packets belonging to a connection. Although scheduling based in time stamps offers differentiated quality of service to multimedia users, we have shown that the maintenance of the control channel (DPCCH) associated with data channel (DPDDH), when mobile stations MS are waiting their permit to transmit, is an important factor in the reduction of the capacity of system. So, this paper also considers that DPCCH are only present if mobiles are transmitting on DPDCH. In this case, the impact of imperfections in the power control mechanism is evaluated. Although results improve, simulations show that the mean and variance of the ratio between intercell and intracell interference grows, decreasing the global capacity. A new scheduling mechanism is proposed in order to improve the performance. The main idea is to limit the number of changes in the assignment of permits to transmit maintaining the permits to the same users as long as the delay tolerance of the waiting users allows it. A complementary priority is associated to the packets.