ZTE Ready for LTE Voice Services

Long Term Evolution (LTE) is the major step in mobile radio communications. The advantages of LTE such as high throughput, lower latency, plug & play capability and superior end-user experience have pushed mobile operators and cell phone manufacturers to embrace LTE as the next generation technology. But LTE involves all-IP network, and data in LTE remains the only service offered. In the move to the LTE, with migration away from the Circuit Switched (CS)-domain, the industry faces problems of interoperability and fragmentation in voice services which will be unable to maintain the freedom of roaming and capability to make off-net and no-SIM emergency call we have now.

To solve the problem, telecom companies involving AT&T, Orange, Telefonica, Teliasonera, and Verizon have jointly developed a technical profile for LTE voice and SMS services, known as the One Voice Initiative. “IP Multimedia Subsystem (IMS) based solution, as defined by 3GPP, is the most applicable approach to meeting the consumer’s expectations for service quality, reliability and availability when moving form existing circuit switched telephony service to IP- based services.” In 2010, GSMA also announced to adopt One Voice Initiative to drive the global mobile industry towards a standard way of delivering voice and messaging services for LTE using IMS.

IMS is not the only solution to the problem of carrying voice over LTE, and two alternative techniques have been proposed. One is Circuit-switched fallback (CSFB), pushed by 3GPP, using some initial signaling over the LTE RAN, actually “fall back” to the 2G/3G RAN to establish calls. The other is a proprietary system backed by T-mobile, called Voice over LTE via Generic Access (VoLGA), which involves feeding existing voice and text services into LTE IP-centric system.

As a leading provider of telecommunications equipment and network solutions, ZTE has invested large resources to this research and plans to deliver these functions in the near roadmap. Next we will give detailed explanation for these three solutions.

IMS based SRVCC

SRVCC (Single Radio Voice Call Continuity) refers to continuity between Internet Protocol (IP) Multimedia Subsystem (IMS)-over-Packet Switched (PS) access and Circuit Switch (CS) calls that are anchored in IMS when the UE is capable of transmitting /receiving on only one of those access networks at a given time. IMS is a framework that is intended tointegrate the data, speech and network technology into a single platform. As IMS can serve fixed broadband and LTE wireless services simultaneously, the use of One Voice will also “open the path to service convergence”. IMS also provides mobile operators with the ability to offer services that can integrate voice calls with enhanced, rich features delivered in an interoperable and multi-operator environment.

Figure1. Voice over IMS in LTE network

By deploying one converged IMS network, handover among fixed-line terminal, 2G/3G and LTE comes to reality. LTE VoIP service is provided as an additional application without any impact on the existing access network. IMS VoIP continuity between different RATs can be implemented by using SRVCC.

SRVCC is aimed to provide seamless handover for voice call from IMS VoIP over LTE access to CS voice over 3G/2G access. Please note that the current architecture does not define the handover from the current CS access to LTE considering the almost omnipresence of the legacy CS coverage.

SRVCC MSC server needs to be enhanced as shown in the figure below.

Figure 2. SRVCC architecture for E-UTRAN to 3GPP UTRAN/GERAN

Though IMS has emerged as the preferred path for voice over LTE, it is far from ready for deployment. The problems with IMS are as follows:

1. There are still no mature IMS systems out there today;

2. IMS standard is getting complicated;

3. Power consumption, VoIP calls cause a much higher processor load during a call.

Therefore, the movement to IMS will involve substantial financial and infrastructure commitments from operators. Its deployment will require operators to realign their operational and business support system for this new IP environment. So carriers not wishing to move to IMS at an early stage can choose interim solutions: CS fallback and VoLGA.

CS Fallback

CS Fallback is a 3GPP-defined standard solution that requires terminals to provide either dual-mode single-standby or dual-mode dual-standby capabilities. With CS fallback approach, LTE handsets drop the existing LTE connection to revert, or “fall back” to the 2G or 3G radio network whenever the user needs to make or receive a call. Upon ending the call the device will re-associate and register with the LTE network.

This function is only available in case E-UTRAN coverage is overlapped by 2G/3G networks and CS fallback to UTRAN/GERAN is realized by using the SGs interface mechanism between the MSC Server and the MME.

Figure 3.Reference architecture for CS fallback to UTRAN/GERAN

The movement towards LTE has been fuelled by users’ requirements for increased capabilities, higher throughput and lower latency. With CS fallback, for every voice call made or received, the LTE handsets must perform a fallback to the 2G/3G network, attach to that network to conduct the call. The delay caused by this fall-back and following re-alignment with the LTE network would decrease rather than enhance the user experience.

Despite its less satisfactory performance, it requires little network changes. Therefore, CS fallback has received some publicity, it is seen within the industry as a transitional solution during initial LTE construction with an investment in backward-looking technology providing no logical path towards IMS. 

VoLGA

VoLGA supports existing circuit services, as well as IMS and other combinational services. Unlike CS fallback, VoLGA offers a clear roadmap towards LTE and does not require further costly investment in legacy technologies.

As shown in the following figure, GANC is inserted between E-UTRAN and the core network. The GAN provides an access between UE and CS without requiring specific enhancements or supporting in the network UE traverses. VoLGA only supports access to CS services. Thus, non-voice PS services employ E-UTRAN directly without any additional entities or functions related to VoLGA. VoLGA requires UE to support both LTE and another RAT radio access in single mode.

Figure 4. Voice over LTE via Generic Access architecture

VoLGA, however, faces some challenges. For example, this solution is not available standard in 3GPP, and its evolution to IMS solution is difficult.

Conclusions

Considering market requirements, terminal and chipset status, ZTE assumes that IMS based SRVCC and CS Fallback are two widely accepted voice solutions in the LTE industry.  At the initial LTE deployment stage, users’ services are concentrated on wideband data services in hot areas. CS Fallback solution can be adopted by upgrading the legacy 2G/3G networks, maximally making use of existing infrastructure. When LTE gets large-scale commercial application, and VoLTE service has been widely delivered, IMS based SRVCC technology can guarantee smooth voice handover to 2G/3G network to ensure the voice continuity in the area beyond LTE coverage.

ZTE has always been concerned about LTE/EPC and IMS development and contributes many patents on voice solutions. For VoIP over LTE technology, ZTE will keep its undisputed leadership, and continue its unwavering commitment to the LTE development, endeavoring to provide superior voice experience to the LTE subscribers in China and elsewhere in the world in the near future.