Transmission of embedded VBR multimode encoded speech on UMTS common packet channels

Being a W-CDMA system, UMTS is intrinsically able to support variable bit-rate dedicated channels. For more bursty traffic sources, UMTS adopts a packet-switched approach, which tailors particularly time-unconstrained data services. This article investigates the performance of UMTS uplink common packet channels (CPCH) when dealing with real-time variable bit-rate traffic. In the presented case study, UMTS shared channels convey the lower priority bit stream produced by a multimode speech encoder in combination with dedicated channels devoted to transport the higher priority core-coding flow. The shared channels are requested on demand following a MAC protocol similar to slotted-Aloha. Simulation results highlight the critical aspects of the original protocol in the unbounded interference rise and in the large access delay. Better performance are obtained when the power control commands exchanged on the dedicated channel are exploited to rule the transmitting power level in the CPCH access procedure