Performance of UMTS Packet Data Services over dedicated Channels (DCH)

The provision of high speed packet data services in an efficient way is probably the most important challenge for the UniversalMobile Telecommunications System (UMTS) since this can give an advantage over second generation systems. The efficiency depends mainly on the radio interface due to its great flexibility. It is of utmost importance to investigate the effect of different configurations on the system performance. In this paper we evaluate the performance of packet data services over downlink dedicated channels (DCH) by means of detailed simulations. Due to the traffic variability, Automatic Repeat reQuest (ARQ) mechanism and closed loop power control the system behavior is quite complex and not so easily predictable as with constant rate services such as voice. We study the effect of several parameters (spreading factor, code rate, channel setup delay, etc.) on the system capacity by means of the delay-throughput curves. We show that the setting of parameters may be critical for system capacity and stability. For this reason we propose a flow control mechanism which is able to guarantee stability and enable the system capacity to be almost independent of channel rates.