Voice over LTE (VoLTE) is a well-established technology, introduced alongside LTE to support voice services. The technology has six key attributes that make it attractive for use in deployments that require some element of voice communication. These make VoLTE an adaptable, high quality yet easy to utilize technology. The six attributes are:

1. VoLTE offers greater spectrum efficiency than both GSM and WCDMA, meaning that spectrum utilization can be optimized by network operators so more traffic can be carried. This, in turn, can ensure greater capacity is available for customer projects and costs can be lower.
2. VoLTE offers faster call set-up time.
3. VoLTE is optimized for low battery consumption, making it ideal for deployments that are constrained in terms of battery size or access to charging.
4. VoLTE avoids interruption and degradation of data service when introduced in fall-back situations.
5. VoLTE offers better quality of service compared to circuit-switched voice. This is important for deployments which require voice communication to be clearly heard.
6. VoLTE includes inherent support for HD Voice in VoLTE, making it suitable for mission-critical communications and ensuring high quality user experiences.

These attributes have seen growing adoption of VoLTE but there are a set of baseline requirements that must be enabled in order for a deployment to support VoLTE calls. These require the mobile network operator to collaborate with the user equipment. The mobile network operator’s network should support VoLTE and offer IP Multimedia Subsystem (IMS) capability, eNodeB configuration and the required quality of service in the radio and transport network.

From the user equipment perspective, the equipment must be VoLTE-capable. It should also support feature group indicators such as radio link control (RLC) unacknowledged mode (UM), transmission time interval (TTI) bundling, semi-persistent scheduling and continuous discontinuous reception (cDRX). User equipment vendors should deliver VoLTE-capable firmware for the particular model and the specific country in which it will be used. The SIM card also must be provisioned for VoLTE calls and this is the mobile network operator’s responsibility.

Optimization techniques

The are several techniques utilized to optimize VoLTE performance and these include robust header compression (ROHC). This is an algorithm used to compress IP headers of primarily radio bearers carrying VoLTE calls. IPv4 and IPv6 protocol headers are relative to the voice payload and ROHC typically is expected to compress IP/UDP/RTP headers to about 4-6 bytes, increasing coverage and capacity for VoIP with lower block error rate (BLER).

TTI bundling is another optimization technique adopted where the user equipment power is limited and in situations where there are bad radio conditions, such as on the cell edge. These factors can cause packet retransmissions which increases delay and overheads. TTI bundling is a transmission mode designed to improve uplink coverage in LTE, specifically for VoLTE services.

In normal transmission, an uplink scheduling grant allocates uplink resources for one TTI but with TTI bundling an uplink scheduling grant allocates uplink resources for four consecutive TTIs. This allows the user equipment to concentrate its power over a smaller number of sub-carriers for the same transport block size, thereby obtaining better coverage. TTI bundling can improve coverage by around 2-3db.

Discontinuous reception (DRX) is another popular optimization technique. There are two DRX states, active or inactive – when the device is awake or asleep. VoLTE DRX timers encourage more frequent awake periods but these are of shorter duration with the benefit that the end user battery power is saved by allowing the equipment to sleep between voice packets.

Into the 5G era

To enable calls on 5G smartphones and other 5G device, the network infrastructure used for VoLTE will also be used to enable 5G voice calls. In 5G New Radio (NR) deployments, the default voice codec in 5G smartphones enables HD Voice and the 3GPP standardized enhanced voice services (EVS), which can already be used in 4G networks but are not mandatory.

With non-standalone (NSA) 5G, the existing 4G IMS/VoLTE network is used to provide voice services with very minor or no changes. For standalone (SA) 5G, the 5G core is introduced to the network and offers two voice solutions: Voice over New Radio (VoNR) and EPS fallback. VoNR uses IMS-based voice functionality, allowing simultaneous voice and high-speed 5G data experiences. EPS fallback enables a mobile phone to fall back from 5G to 4G LTE during a voice call set-up and data and voice traffic is carried over LTE while the call is active. This solution will be utilized in initial deployments.

These and other methods to take advantage of VoLTE capabilities and ensure smooth operation of VoLTE services were detailed in a recent Quectel Masterclass in which Quectel field application engineers Zoran Romic and Igor Micic introduced Quectel’s VoLTE capabilities, explained the benefits of VoLTE and shared important troubleshooting techniques.

The Masterclass, titled: VoLTE & Quectel modules Activation, testing and troubleshooting, can be listened to here