Voice over LTE



Voice over Long-Term Evolution (acronym VoLTE) is an LTE high-speed wireless communication standard for voice calls and SMS using mobile phones and data terminals. VoLTE has up to three times more voice and data capacity than older 3G UMTS and up to six times more than 2G GSM. It uses less bandwidth because VoLTE's packet headers are smaller than those of unoptimized VoIP/LTE. VoLTE calls are usually charged at the same rate as other calls.

To be able to make a VoLTE call, the device, its firmware, and the mobile telephone providers on each end, as well as the inter-carrier connectivity must all implement the service in the area, and be able to work together. VoLTE has been marketed as "HD voice" by some carriers, but this is a broader concept. Moreover, HD+ (EVS) is used only in LTE; HD voice was available in 3G too.

Overview
VoLTE is based on the IP Multimedia Subsystem (IMS) architectural framework, with specific profiles for control and media planes of voice service. This facilitates VoLTE on the LTE wireless broadband service defined by GSMA in PRD IR.92. The approach results in the voice service (control and media planes) being delivered as data flows within the LTE data bearer, with no dependency on (or ultimately, requirement for) the circuit-switched voice network to be in the call path.

As of February 2019 there were 253 operators investing in VoLTE in 113 countries globally, including 184 operators with commercially launched VoLTE-HD voice service in 87 countries, up from 137 operators in 65 countries 12 months previously, according to data from the Global Mobile Suppliers Association. By August 2019, these numbers had risen to 262 operators investing in VoLTE in 120 countries and 194 operators with launched VoLTE-HD voice services in 91 countries.

Requirements

 * Phone service provider has to enable the voice service on its LTE cellular network. Called "VoLTE profiles", it contains the necessary information, for each service provider, to enable VoLTE on the customer's equipment.
 * Device manufacturers have to update their devices, for all providers all over the world, with each operator's profile.
 * If the service provider of a mobile is present within the device, the operating system will automatically connect and enable Voice over LTE.

Encountered problems
There are several issues of the VoLTE deployment :


 * All subscribers were not automatically eligible to VoLTE, it was firstly enabled for specific-subscribers only
 * Some providers had VoLTE only compatible with equipments sold by their shops
 * Old LTE devices are not updated by their manufacturer, thus are not compatible
 * Emergency services encountered issues as being reachable in VoLTE
 * In USA, lot of M2M devices were not LTE-compatible
 * Lot of second-range devices brands are not supported by cellular operators
 * There is no possibility to "check" which operators in the world, are supported on a specific device

History
Beginning in August 2012, MetroPCS launched the world's first commercial VoLTE services in Dallas, Texas, in the United States, alongside the first VoLTE phone, the LG Connect 4G. In May 2014, Singtel introduced the world's first commercial "full-featured" VoLTE service in Singapore, only in combination with Galaxy Note 3, it was subsequently expanded. In June 2014, KT showcased the world's first cross-border roaming services based on Voice over LTE. The South Korean operator partnered with China Mobile to develop VoLTE roaming services.

In November 2014, Verizon and AT&T announced the companies are enabling VoLTE-to-VoLTE connections between their respective customers. VoLTE interoperability between Verizon and AT&T customers began in 2015. Testing and design were performed between both companies using third party networks such as Alcatel-Lucent. This was stated to have been completed in November 2017.

On July 11, 2015, SEATEL Cambodia announced the world's first commercial 100% VoLTE service without 2G/3G in Cambodia.

As of 2020, almost all new phones for sale have the potential to support VoLTE.

On December 31, 2022, Verizon shut down their CDMA network, therefore requiring devices to support LTE or 5G. Customers with CDMA-only devices and LTE devices without VoLTE support would have been required to switch to a VoLTE-capable device by that date. Verizon stopped activating CDMA-only devices on their network in 2018, and had previously planned to shut down 3G service in 2019, but extended the timeline for shutting down the 3G network twice — first to December 31, 2020, and then to December 31, 2022, which the VP of Network Engineering says "will not be extended again." The company indicated the delays were in an effort to "minimize disruptions" to its customers still utilizing the 3G network. As of March 2021, less than 1% of Verizon's customers were still using 3G, and many of the 3G-connected devices are integrated devices, such as smart utility meters and home burglar alarms.

Additionally, certain Verizon-compatible handsets were blocked from Verizon's CDMA network even before December 31, 2022, even if the device supported CDMA (for instance, unlocked devices with support for Sprint or China Telecom), therefore requiring VoLTE support when used on Verizon. This includes CDMA-capable versions of the Apple iPhone 12 and 13 series, Samsung Galaxy S21, Google Pixel 5a, and OnePlus 6T to 9. Devices lacking CDMA support such as the Pixel 6 and above, US-sold OnePlus 10 Pro and 10T devices, and select iPhones including the iPhone 14 as well as older "GSM" models will only work on Verizon in VoLTE mode.

Voice quality
To ensure compatibility, 3GPP demand at least AMR-NB codec (narrow band), but the recommended speech codec for VoLTE is Adaptive Multi-Rate Wideband (AMR-WB), also known under the trademark HD Voice after GSMA's certification program. This codec is mandated in 3GPP networks that are capable of 16 kHz sampling.

In addition, many carriers and devices can use Enhanced Voice Services (EVS). This is an up to superwideband (20–16,000 Hz) or fullband (20–20,000 Hz) codec backwards-compatible with AMR-WB. This codec is also known under the trademark HD Voice+, after GSMA's certification program. GSMA has proposed to make EVS mandatory just like AMR-WB. (Both the carrier(s), any interconnect and the two calling devices must be capable of using the codec for it to be used.) The AMR-WB+ codec has a wide bit-rate range, from 5.2 to 48 kbit/s. EVS offers a wide range of bit rates from 5.9 kbit/s to 128 kbit/s, allowing service providers to optimize network capacity and call quality as desired for their service, so VoLTE does not ensure high call quality.

Fraunhofer IIS has previously demonstrated an implementation of the AAC-ELD codec in VoLTE that they call "Full-HD Voice". It has not gained any standard status or real-world adoption. They have since reused the term "Full-HD Voice" for EVS in fullband mode (HD+ is also used).