Voice solutions in LTE

The original idea behind LTE is that it would provide only wireless internet services. However, major revenue for cellular operators comes from voice calls and SMS and therefore Voice in LTE has become a hot topic. Recently, I got an opportunity  to work with various voice solutions-the experience which I believe would be useful to share here.

LTE does not have a 'circuit switch core' which means that we cannot have voice calls as it is in 2G and 3G technologies. In the initial LTE deployment cases however, operators are using their legacy networks along with their 4G network for voice services.

So far we have heard of the following available voice solutions which I will discuss briefly.

• Circuit Switched Fall Back (CSFB)
• Simultaneous Voice and LTE (SV-LTE)
• Voice over LTE (VoLTE)
• Voice over LTE via Generic Access (VoLGA)
• Over the top (OTT)

Circuit Switched Fall Back 

An operator who deployed LTE network, already owning a 3G or 2G network can take benefit from the feature called "Circuit switched fall back'. The main idea is that 4G smartphones are going to have a radio capabilities for 3G/2G networks as well. Such handsets can connect at a time either to LTE or 2G/3G . The shortcoming is that someone on voice call will not be able to use LTE network for browsing or chatting etc.

CSFB for operator means very little investment since only few modifications are required in the network. Additional interface between MME and MSC is required (SGs). CSFB solution has also been standardized by 3GPP and has gained large industrial support.

Simultaneous Voice and LTE

SV-LTE is handset specific in which handset is capabile of using two radios (LTE and WCDMA/GSM/CDMA) at one time. So a user can use packet services from LTE while voice call can be made on other networks simultaneously unlike CSFB. The shortfall here is high battery utilization due to dual radio operation. 

For CDMA and  LTE pair, the SV-LTE is the standard solution and being widely adopted. There are already SV-LTE smartphones available in the market. I came across a few available for LGU+ in Korea and Verizon in USA. Both operate LTE networks as an overlay to their old CDMA networks. 

SV-LTE is the cheapest option for operators as no new modification is required to the network. Nevertheless, as mentioned earlier it is at the cost of high battery utilization

Voice over LTE (IMS)

I believe this is going to be the most popular and widely adopted future voice solution for LTE. Instead of using legacy networks, VoLTE utilizes IP Multimedia Subsystem (IMS) and provides voice services  using the application layer on LTE.
IMS is a group of core network entities responsible for providing rich multimedia services over IP network. VoIP call, SMS, MMS, LIVE TV are a few such services. IMS has been in the communication industry for long but with the emergence of 4G networks, it is gaining popularity again.

Voice over LTE via Generic Access

I think, operators will accept VoLGA as a last option for voice capability. This solution uses CS core only from legacy networks and also require new network elements. Therefore LTE handsets do not need 3G/2G radio capabilities since radio part won't be used from legacy networks. Good thing about this solution however is that unlike CSFB, LTE handset will be able to use voice and data simultaneously.

Over the top VoIP application

OTT is actually not LTE specific but a generic solution that we already have been using on 3G/WiFi networks. OTT application is completely transparent to network and also out of operators' control. I am talking about generic VoIP clients like Viber, skype, Tango etc. They do not give the real taste of voice flexibility as in other 3GPP networks and also lack the QoS for voice. Nonetheless,  these will be widely used by the consumers as an alternative, because of the fact that it gives them full flexibility to choose their own service.