igh-speed downlink shared channel (HS-DSCH) is associated with a 2-ms frame size, in which the users share at most 15 fixed spreading factor 16 (SF 16) high-speed physical downlink shared channels (HS-PDSCHs) by code multiplexing. In this way, radio resources can be utilized in a more efficient manner than earlier dedicated resource allocation in WCDMA. Furthermore, HARQ in HSPA can provide physical layer (L1) retransmissions and soft combining, which reduce the amount of higher-layer ARQ transmissions and frame selections (i.e., hard combining). This also improves the efficiency of the lub interface between the base station and radio network controller (RNC) significantly. Rather than the conventional dedicated transport channel (DCH), the enhanced DCH (E-DCH) mapping on the enhanced dedicated physical data channel (E-DPDCH) is used in HSUPA to further improve the uplink data rate up to 5.76 Mbps. The 2-ms TTI frame length introduced into HSUPA can further reduce the transmission delay in the air inter-face: the minimum Round Trip Time (RTT) is decreased from about 150 ms in WCDMA to 50 ms in HSPA. Besides, base station controlled fast scheduling can select the user in a good channel condition for the transmission reaching the higher throughput due to the multiuser diversity gain. This is impossible in the WCDMA system, where the reaction of the RNC-controlled scheduling is too slow to follow the fast-changing channel conditions. This is because of the transmission delay between the UE and the RNC via the base station. 10.2.2 LTE To ensure competitiveness of 3GPP radio access technologies beyond HSDPA and HSUPA, a LTE of the 3GPP radio access was initiated at the end of 2004. As a result, the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) is now being specified as a part of the 3GPP Release 8. Important aspects of E-UTRAN © 2009 by Taylor & Francis Group, LLC Radio Access Network VolP Optimization MME/SAE Gateway MME/SAE Gateway (())) E-UTRAN ((q)()) Figure 10.2 E-UTRAN architecture [8]. include re