VoLTE Leads the Way to 4G Voice

Since 2013, operators all over the world have been busy deploying voice over LTE (VoLTE). Infonetics Research predicts that there will be 12 commercial VoLTE networks and 8 million VoLTE subscribers worldwide by early 2014.

VoLTE is based on IMS and supports voice and multimedia services. Both GSMA and NGMN have specified VoLTE as the only end-to-end solution for voice over LTE. To ensure voice continuity, operators need to provide full LTE coverage or deploy single radio voice call continuity (SRVCC), which enables ongoing voice calls to be smoothly handed over to 2G/3G networks.

Driving Force Behind VoLTE

Voice and SMS are still the basic communication needs of users. It is estimated that voice revenue will account for about 65% of total operator revenue by 2015. Therefore, LTE voice solutions are important and significantly affect overall user experience.

 VoLTE deployment involves:

●    relinquishing the circuit-switched (CS) domain for LTE air interfaces. In their preliminary research on LTE R8 standards, 3GPP and NGMN defined a next-generation mobile network as an all-IP network that supports real-time multimedia. VoLTE will replace traditional CS services, and traditional CS domain networks will be ended.

●    preventing traffic being offloaded by OTT providers. Mobile internet applications such as Google Voice and Skype siphon off a great amount of revenue from operators. VoLTE prevents voice being offloaded by OTT services. With rich communication-suite enhanced (RCSe), VoLTE creates a new service experience. It enables users to keep their original numbers without notifying others that they have changed number. VoLTE is based on IMS architecture and provides mechanisms that facilitate roaming, interoperability, bandwidth, QoS, and service continuity.

●   improving air interface efficiency. Utilization of VoLTE air interfaces is three times higher than utilization of R99 UTRAN air interfaces. If CS services are migrated to VoLTE architecture, more high-value, low-frequency spectrum is released.

●    converging with the core network. VoLTE easily supports innovative fixed-mobile convergence (FMC) services. Subscribers can access VoLTE from different networks and have the same service experience.

 Current Deployment of VoLTE

In the initial stage of LTE deployment, operators enhance data experience. Voice services are still provided by the CS domain, and circuit-switched fallback (CSFB) is used in the interim. LTE only provides data services: when a voice call is made or received, it fall backs to the CS domain. An operator only needs to upgrade the MSC instead of deploying the IMS in order to provide services quickly. However, the drawback is a greater call setup delay.

Some operators already offer VoLTE HD voice and video within LTE coverage areas. Outside these areas, the same services are provided using traditional CS domains.

 Key issues in deploying VoLTE include:

●    wireless coverage and service continuity. As with 3G network construction, LTE network construction also involves first providing hot-spot coverage and then progressing to full network coverage. When a subscriber moves out of an LTE area, the IP voice call is interrupted. SRVCC smoothly hands over an IP voice call in the IMS domain to the CS domain of a 2G/3G network. The R8 SRVCC architecture has drawbacks such as complex signaling flows and long handover time. ZTE has joined hands with China Mobile and come up with an enhanced SRVCC (eSRVCC) solution using the 3GPP standard. This solution significantly shortens voice handover time to less than 300 ms and improves user experience.

●    immature terminal industry chain. Mainstream vendors have launched LTE terminals that support CSFB. However, only about ten types of LTE terminal support VoLTE. Therefore, operators need to consider selecting, testing, and recommending more VoLTE terminals.

●    network evolution. In terms of user roaming, operators must ensure their network devices are compatible with CSFB and VoLTE. ZTE provides solutions for CSFB/SRVCC proxy and CS/IMS convergence. These solutions have fewer requirements in terms of equipment upgrade, and they enable CS subscribers to smoothly evolve to VoLTE.

●    operating and billing. Operators need to consider how to make billing and business-acceptance systems compatible with both CS and VoLTE business systems and how to ensure consistency of CS and VoLTE businesses.

RTCWeb: The Future of VoLTE

Real-time communication in Web browsers (RTCWeb) is designed for real-time video and audio communication. RTCWeb integrates audio/video processing, network transmission, and session control into a browser so that third-party application developers can easily provide real-time audio and video with a simple JavaScript API. If the browser network is based on LTE, RTCWeb will probably be a future VoLTE trend.

RTCWeb creates new opportunities for operators. If an operator creates IMS network capacities in API mode, browser-oriented applications can provide unified user authentication, unified accounting, QoS guarantee, and compatibility with conventional networks. Browser-oriented applications can be rapidly deployed; they have a large user base; and they are not costly to maintain. They will boost operator IMS businesses.

Global VoLTE Deployment and Prospects

According to GSA’s latest report, 21 operators worldwide, including SK Telecom, DoCoMo, T-Mobile, AT&T, Verizon and CSL, have deployed or are deploying VoLTE.

SK Telecom and U Plus, both in South Korea, have launched commercial VoLTE HD voice, video, and RCS services. In North America, MetroPCS has also rolled out VoLTE services.

In December 2013, CSL and ZTE demonstrated VoLTE HD voice and video calls on a 4G network in Hong Kong. This service also supports eSRVCC. CSL has evolved from 3G to 4G, and is ready to deploy VoLTE/SRVCC over its commercial 4G networks. Once VoLTE/SRVCC terminals have been put into commercial use, CSL will be the first to offer VoLTE/SRVCC in Hong Kong. China’s 4G licenses were formally issued in early December 2013. How will China Mobile, China Telecom and China Unicom launch their voice services over LTE after they have deployed 4G networks? All three operators have wide 2G/3G CS coverage, high-capacity equipment, and large subscriber bases and need to do much work within a short period of time to upgrade their existing networks to VoLTE. In their early stage of LTE deployment, they will focus on data services to enhance mobile broadband experience, and they will use CSFB for voice calls. Operators may launch VoLTE HD voice and video first in order to compete with OTT services. Both CSFB and VoLTE solutions can co-exist on a network. As LTE networks expand and commercial LTE terminals mature, operators will gradually introduce SRVCC for better user experience.

Milestones in ZTE’s VoLTE Development

●    December 2008. ZTE introduced new-generation ETCA hardware platform. It is a unified, converged platform on which the IMS+CS solutions are based.

●    2010 GSMA Mobile World Congress. ZTE demonstrated the world’s first VoLTE call over a commercial LTE network.

●    September 2011. ZTE attended the MultiService Forum organized by GSMA and participated in the LTE/EPC interoperability test (IOT). The test scenarios included basic service interoperability, global roaming, and VoLTE interconnection.

●    November 2011. CSL put ZTE’s CSFB solution into operation.

●    Third quarter 2012. ZTE provided a full set of mature VoLTE and voice continuity solutions.  MMTel AS has a built-in SCC AS and can smoothly evolve to SRVCC after CSFB has been deployed and LTE coverage is complete.

●    February 2013. ZTE’s IMS-based eSRVCC+HD voice solution passed a field test conducted by CSL Hong Kong. This solution considerably shortens call setup and handover time for better user experience.

●    December 2013. ZTE and CSL jointly launched a 4G VoLTE network, the first of its kind in Hong Kong and the second in Asia-Pacific. ZTE and China Mobile also jointly implemented eSRVCC 2G/4G interoperability in a hybrid TDD network architecture, which was the first time this had been achieved.

ZTE works with operators and related parties worldwide to explore VoLTE deployment and operation in the 4G era. ZTE provides efficient and flexible solutions to reduce capex and opex, enhance an operator’s core competitiveness, and deliver crystal clear voice calls to users through 4G networks.