In the master thesis, technologies for voice transmission in LTE networks are presented. Unlike previous technologies, which are circuit-switched, the LTE network is packet-switched, meaning that this is all-IP network. VoLTE technology is based on IP Multimedia Subsystem (IMS), which, apart from voice services, also allows many other multimedia services.
In the first part, LTE network is presented, which consists of access and core network. Due to a completely new design that LTE network has, it contains other elements as there are in the GSM/UMTS technologies. In the access network, multiple access is used that is different from the ones from earlier generations of mobile networks. After that, the development processes of VoLTE technology are shown. Mobile operators are developing the VoLTE technology in parallel with LTE network. In the initial stage, when the coverage with LTE technology is sparse, a circuit switched fallback (CSFB) procedure for the voice transmission is used. At the next stage of development, the voice transmission is already taking place through the LTE network, using IP Multimedia Subsystem, but due to sparse coverage, operators use single radio voice call continuity (SRVCC) procedure to enable voice call handover from LTE network to GSM/UMTS network. In the final stage of VoLTE technology development, the coverage is already very good, which allows continuous using of the technology.
The main part of the thesis includes a detailed description of these procedures. The circuit switched fallback allows the use of GSM/UMTS technologies for the voice transmission in the development stage of VoLTE technology and is described with the mobile terminating and mobile originating call, when the mobile terminal is attached to LTE network. When the mobile terminal performs or receives a call, it is done through SGs interface, which connects the LTE network with the GSM/UMTS network. In the next part the architectures of LTE network and IMS core are presented in more detail. After that the mobile terminal LTE attachment and IMS registration procedures are presented. A procedure of voice transmission over LTE network is described with the voice call establishment and the course of the call for the both sides of voice call. Then the voice call clearing procedure follows. Procedures which allow the voice call handover are presented at the end of the main part. The first one of them is a single radio voice call continuity procedure for enabling voice call handover from LTE network to GSM/UMTS network, when mobile terminal leaves the coverage with LTE technology. Next procedure describes a voice call handover between base stations within LTE network.
In the last part, the main advantages of VoLTE technology are presented, which allow good quality of experience and offer competitiveness to mobile operators. Then a comparison of CSFB and SRVCC procedures is presented.