Optimising Voice over LTE, VoLTE in Public Safety Systems

Juha Laukkanen
Testing LTE / VoLTE for public safety systems
Current public safety networks, such as TETRA, are lacking the high data speeds required by new public safety services. The new public safety networks are based on LTE - a mainstream technology that enables high data speeds and also Voice over LTE (VoLTE) services.

LTE is gaining traction in the public safety and defence markets with authorities looking to take advantage of the economies of scale that mainstream cellular markets offer, where volumes are far greater than in markets with proprietary waveform solutions that are specifically developed for one or two customers.

Voice over LTE as mission critical voice service

FirstNet in the US is one of the first public safety initiatives looking into the use of Voice over LTE (VoLTE). The network will be based on LTE giving first responders the ability to leverage data services and ultimately incorporate VoLTE for voice. The trend is gaining ground in Europe as well, where national authorities are seriously considering LTE as the future network for first responders and defence forces. Operators are gradually migrating more and more customers on to VoLTE to provide them with next-generation telephony services. This model would be applicable in the public safety domain as well. Not only is VoLTE expected to improve voice service quality, operators will also benefit from customers switching to VoLTE services as it will free up capacity in the core network for bandwidth-hungry services like video – which is one of the key use cases for first responders.

Voice calls across LTE networks can be handled using Circuit Switched Fallback (CSFB) and VoLTE. CSFB provides a mechanism to transfer an initiated voice call to legacy circuit-switched networks. VoLTE, on the other hand, is a fully packet switched technology which uses Voice over IP (VoIP) technologies. With the SR-VCC (Single Radio Voice Call Continuity) functionality, voice calls made using VoLTE can be switched to legacy networks when the user moves out of LTE network coverage.

In mainstream cellular, although the first VoLTE networks are already in use with multiple trials on-going in all corners of the world, the setup for the service is far more complex than traditional cellular voice. The implementation of VoLTE services are twofold, comprising implementations to the device and implementations to the network. The radio network IMS (IP Multimedia Subsystem) core has to work with the IMS client on the device. Complexity is caused by the existence of several different vendors, both on the network and the device side. Currently, various IMS vendors have their own specific implementations of the IMS client, and device (chipset) manufacturers have their own IMS clients in the terminals. This is extremely challenging in terms of compatibility.  

In a typical drive test setup, devices supporting the first responders / defence RF bands are connected to a laptop running the drive test software. The drive test solution measures capacity, quality, coverage and performance of wireless LTE networks, giving detailed network-level KPIs, such as cell dominance, overshooting cells and interference, in addition to device-specific parameters, such as serving and neighbour cells. The resulting data is very granular. For example, physical channel information includes BLER (block error rate) information, SNR/SNR (signal to noise ratio) per antenna port, timing advance and TX power information. The data can then be presented in detail as numerical or graphical output, illustrated on maps and graphs (see image below).

Finding the optimal drive test solution

Test and measurement technology providers have introduced full drive test solutions for the global first responders market, giving public safety operators a single solution for their drive test needs; the drive test solution and the device. These products can be enhanced with post-processing solutions for further analysis and advanced visual reporting. To enable this, the drive test solution needs to produce measurement files in open ASCII (American Standard Code for Information Interchange) file format. With so many LTE-based public safety networks around the world now in the trial phase, enabling operators to measure and optimise network and device performance – even in challenging conditions – is key to delivering an optimised network that will meet the stringent requirements of first responders.

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