 Results and Analysis 10.7.3 VoIP Performance Comparison between HSPA and LTE Summary and Conclusion References Introduction Circuit switched (CS) voice used to be the only way to provide voice service in the cellular networks, but during the past few years there has been growing interest to use cellular networks for real-time (RT) packet-switched (PS) services such as Voice © 2009 by Taylor & Francis Group, LLC Radio Access Network VolP Optimization over Internet Protocol (VoIP) to provide voice service without circuit switched ser- vice. The main motivation for the operators to use VoIP instead of CS voice are the savings that could be achieved when the CS-related part of the network would not be needed anymore. It is also expected that VoIP can bring better capacity than CS voice due to more efficient utilization of resources. Supporting VoIP in any radio access technology faces certain challenges due to VoIP traffic characteristics (strict delay requirements, small packet sizes), which make the efficient exploitation of radio interface capacity difficult due to control channel constraints. The solutions to these challenges vary for different technologies. The introduction of 3G networks with integrated IP infrastructure [1] included in WCDMA Release 99 made it possible to run VoIP over cellular networks with reasonable quality, although with lower spectral efficiency than the circuit switched voice [2]. 3GPP Releases 5 and 6 have brought High Speed Packet Access (HSPA) [3] to WCDMA downlink (DL) and uplink (UL). HSPA consists of High- Speed Uplink Packet Access (HSUPA) [4,5] in the UL direction and High Speed Downlink Packet Access (HSDPA) [6] in the DL direction and was originally designed to carry high-bit rate delay tolerant non-real time (NRT) services like web browsing. Even though HSPA was not originally designed to support RT services, with careful design of the system, the RT services can be efficiently transported over HSPA, and a number of new features have been introduced to 3GPP Releases 6 and 7 t