Guaranteeing quality of service to packetised voice over the UMTS air interface

A key feature of the air interface of the Universal Mobile Telecommunication System (UMTS) standardised by 3GPP is that its functionality can be tailored to the requirements of individual applications, In this paper we study how this flexibility can be exploited to meet the stringent delay and packet loss requirements of a voice-over-IP application (VoIP). We introduce an analytical model of the UMTS air interface that allows us to quantify the impact of the air interface parameters (power budget, interleaving span and channel coding rate) on the parameters that determine the perceived quality of VoIP calls (mouth-to-ear delay and packet loss probability). We use the model to explore the trade-offs that are inherent in the setting of the air interface parameters. The numerical results we present illustrate and quantify the trade-offs of bandwidth efficiency versus packet loss, and of packet loss versus packet delay. The latter trade-off is of particular importance for the feasibility of VoIP.