Radio Access Network VoIP Optimization and Performance

14.1

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 over Internet Protocol (VoIP) to provide voice service without circuit switched service. 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 Release 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 UL direction and High Speed Downlink Packet Access (HSDPA) [6] in DL direction, and was originally designed to carry high-bit rate delay tolerant non-real time (NRT) services like web browsing. Even though HSPA is not originally designed to support RT services, with careful design of the system, the RT services can be efficiently transported over HSPA, and number of new features have been introduced to 3GPP Release 6 and 7 to 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 topic 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 outlined and the performance impact of various aspects of the concept is addressed. The topic concludes with a system simulation-based performance analysis of the VoIP service, including a comparison between HSPA and LTE.


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