VoLTE: The Best Voice Solution in the 4G Era

The latest GSA report forecasts that voice will still account for 65% of total revenue in the global telecom industry until 2016. In the LTE era, voice solutions still matter a lot to both operators and users.

ZTE provides standard circuited switched fallback (CSFB) as well as single radio voice call continuity (SRVCC) and enhanced single radio voice call continuity (eSRVCC) solutions. With CSFB, an LTE network only provides data service, and wireless devices fall back to the CS domain to send or receive voice calls. To implement CSFB, an operator only needs to upgrade their existing mobile switching centers (MSCs) without deploying an IP multimedia system (IMS). SRVCC and eSRVCC require the operator to deploy an IMS for IP-based voice service in the LTE network. When a user moves outside the LTE coverage area, voice calls are seamlessly handed over to the CS domain.

ZTE’s voice over LTE (VoLTE) provides excellent user experience and guarantees service quality. The VoLTE solution helps operators win in the 4G era.

Faster Connection for HD Voice and Video Calls

SRVCC and eSRVCC enable smooth evolution from 2G and 3G to 4G and guarantee seamless voice handover. Compared with legacy CS voice, VoLTE with SRVCC or eSRVCC shortens call set-up time, ensures good handover to 2G and 3G, and provides better user experience. HD voice relies on the adaptive multirate wideband (AMR-WB) speech compression standard to capture voice between 50 Hz and 7000 Hz (the ordinary voice spectrum is between 300 Hz and 3400 Hz). HD voice also has a mean opinion score (MOS) of 4–4.5 (the MOS for ordinary voice is around 3.5), which indicates a deeper clarity that is almost the same as speaking directly into a person’s ear. AMR-WB adds significant depth and nuance to the transmitted voice signal and supports seamless voice-video switching with LTE.

End-to-End QoS Guarantee

VoLTE voice and video calls are sensitive to QoS metrics. To achieve high QoS, IMS requires low latency and low bit error during transmission. IMS voice uses dedicated bearer QoS class identifier (QCI), as defined by 3GPP, for end-to-end QoS guarantee and uses the policy and charging rule function (PCRF) for QoS policy control and resource allocation. ZTE’s intelligent QoS perception system enables an operator to monitor QoS metrics in real time and receive alarms in case of failure. In wireless IP environments, ZTE uses enhanced wireless technologies, including transmission time interval (TTI) bundling, robust header compression (RoHC) and semi-persistent scheduling (SPS), to guarantee high-quality voice calls.

Consistent User Experience

Users need not change their phone numbers after migrating to IMS. With one number, a user can enjoy rich VoLTE and multimedia services while continuing to subscribe to 2G and 3G.

In a legacy network, CS and IMS user profiles are stored separately and may have differences. To guarantee consistent service, ZTE provides an intelligent user data convergence (UDC) architecture that ensures consistency between stored data and phone-operation data in both the CS and IMS domains. For SMS interworking between LTE and 2G/3G networks, ZTE uses an IP short message gateway (IP_SM_GW) to guarantee a consistent user experience. ZTE also uses an IP multimedia service switching function (IM-SSF) to inherit 2G/3G services and maintain a consistent user experience.

IMS enables a mobile operator to integrate voice services into enriched applications while keeping legacy voice and SMS services. In this way, a variety of services can be provided.

Innovative and Differentiated Solutions

The coexistence of CS, PS and IMS domains means that user data is distributed. Selecting a terminated domain in VoLTE call flow is a key issue. An improper target domain selection may cause slow call connection and repeated service triggers. This results in bad service experience. To address this issue, ZTE provides the 3GPP-defined terminating access domain selection (T-ADS) solution as well as the dual-trigger solution. A proper domain selection solution helps an operator accurately select a domain and reduce call set-up time in order to provide the best voice service.

With mature VoLTE solutions and rich experience in IMS deployment, ZTE is the vendor of choice for operators wanting to deploy a VoLTE network. ZTE has been recognized by operators worldwide for its VoLTE performance at multiple trial sites.

ZTE’s VoLTE Development Milestones

●    April 2014: ZTE made the world’s first IPv6 VoLTE call.

●    February 2014: ZTE demonstrated NFV and voice & video service over LTE on vIMS and vEPC at MWC 2014.

●    December 2013: ZTE made the world’s first VoTD-LTE call with eSRVCC.

●    June 2013: ZTE made the world’s first end-to-end VoTD-LTE call.

●    February 2013: ZTE made the world’s first HD VoLTE call with eSRVCC.

●    January 2013: ZTE deployed the world’s first multi-tenancy and VoLTE solution based on the IMS and CS convergence.

●    November 2012: ZTE trialed the world’s first commercial VoLTE call with eSRVCC.

●    September 2011: ZTE completed the world’s first VoLTE IOT.

●    November 2010: ZTE made the world’s first VoLTE call.