o improve the efficiency of low-bit rate delay sensitive applications like VoIP. It has also been shown that VoIP can provide better capacity on HSPA than CS voice [7]. Long-term evolution (LTE) of 3GPP [8] work (targeted to 3GPP Release 8) has been defining a new packet-only wideband radio-access technology with a flat architecture, aiming to develop a framework for a high-data-rate, low-latency, and packet-optimized radio access technology called E-UTRAN. As E-UTRAN is purely a PS radio access technology, it does not support CS voice, which stresses the importance of efficient VoIP traffic support in E-UTRAN. This chapter provides an overview of the challenges faced in implementing VoIP service over PS cellular networks, in particular 3GPP HSPA and LTE. Generally accepted solutions by 3GPP leading to an efficient overall VoIP concept are out- lined and the performance impact of various aspects of the concept is addressed. The chapter concludes with a system simulation-based performance analysis of the VoIP service, including a comparison between HSPA and LTE. 10.2 System Description 10.2.1 High-Speed Packet Access, a 3GPP standardized evolution of Wideband Code Division Multiple Access (WCDMA), has become a huge success as the world's leading third-generation mobile standard. HSPA consists of HSUPA and HSDPA © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology 3.5G+ HSPA+ bis HSUPA HSPA+ WCDMA HSDPA 28 Mbps DL 384 kbps DL 14.4 Mbps DL 5.7 Mbps UL RTT - 50 ms 11 Mbps UL optimization 384 kbps UL RTT - 100 ms RTT ~ 30 ms extensions RTT ~ 150 ms CS/PS 3GPP Rel.99/4 Release 5 Release 6 Release 7 Release 8 Figure 10.1 WCDMA/HSPA key performance indicators. which enhance WCDMA in the uplink and the downlink separately. Several key techniques such as HARQ, fast base station-controlled scheduling, and the shorter frame size were introduced into HSPA targeting the higher data rates with more efficient spectrum usage and lower transmission delay (see Figure 10.1). In HSDPA, a h