Anritsu tackles the challenge of LTE testing
It is easy to be dazzled by the array of applications and services provided by smartphones, tablets and other sophisticated kinds of mobile user terminal, and to forget that traditional voice call and SMS (Short Message Service) functions are still the biggest source of revenue for mobile network operators, writes Lee Roberts, wireless business development manager at Anritsu (UK)
2G and 3G cellular telephone technologies, still used in today’s feature phones, employ circuit-switched techniques to implement standard voice call and text services. In theory, this changes in the leap from 3G to LTE (Long Term Evolution), the next generation of mobile phone technology. As originally conceived, LTE is an all-IP (Internet Protocol), packet domain technology.
In the LTE standards specified by industry body the 3GPP, voice traffic is meant to be handled by Voice Over LTE services using the ‘IMS-VoIP’ (Internet Protocol Multimedia Subsystem – Voice Over IP) architecture network – in other words, voice signals are to be fragmented into packets carried over the internet.
But upgrading the network to build this architecture entails a huge financial investment, so in fact the implementation of voice and SMS using IMS-VoIP will only be completed in stages over a number of years. In the mean time, transitional arrangements for supporting voice and SMS on LTE handsets are required, using the old circuit-switched techniques.
Unfortunately, engineers responsible for testing the functionality of LTE handsets cannot simply replicate testing techniques and routines applied to voice and SMS services in the GSM and W-CDMA environments in the test regimes applied to LTE terminal equipment.
The reason, as this article describes, is because of the way circuit-switched operation is implemented in LTE equipment.
Carrying circuit-switched voice on an IP network
A number of techniques have been defined in the LTE standards for carrying circuit-switched traffic on LTE equipment (see Figure 1). Circuit-Switched Fall Back (CSFB) is a mechanism for enabling an LTE mobile device connected to an IP packet-switched network to alert the handset to an incoming standard voice call or text message from a 2G or 3G circuit-switched network. CSFB enables the LTE device to switch (‘fall back’) to circuit-switched technology, using existing 2G and 3G networks.
This CSFB technique has drawbacks: the LTE packet-switched network cannot be used simultaneously during a voice call connection, and the delay in establishing a voice call can be noticeable.
Fig. 1: techniques for handling circuit-switched functions on an LTE handset
Single Radio Voice Call Continuity (SR-VCC) is a circuit-switched technique used to maintain voice call continuity when no other VoIP packet-switched systems are available for inter-RAT (Radio Access Technology) handovers, which occur when a user terminal moves from one cell to another during a voice call. The call is maintained by transferring the connection using an active handover sequence to a suitable 2G or 3G access network, such as GERAN, UMTS, or CDMA2000.
Correct operation of this function is of crucial importance to network operators, since it ensures continuity of service to subscribers as they move between areas with LTE coverage to areas with only 2G or 3G coverage.
Simultaneous Voice LTE (SV-LTE) is a process which enables LTE packet-switched data services to function simultaneously with and independently from a circuit-switched voice call on a CDMA2000 network. As there are effectively two communication paths, the handset can accept an incoming call while transferring data via the LTE packet connection in parallel.
Test platform requirements
Each of the hybrid LTE/circuit-switched voice functions described above must be tested in the design and manufacture of an LTE handset. Even the most rudimentary test solutions should provide the ability to undertake a routine set of test procedures for CSFB, SR-VCC and SV-LTE. But complexity is the bane of the test operator’s life; one new approach, created by Anritsu, is aimed at making the testing of these functions intuitive and quick.
The test solution developed by Anritsu provides an intuitive graphical user interface which requires little intervention by the user when creating or running test sequences. It also supports the generation and execution of pre-defined scripts, based on program language code. This means the user can customise voice and messaging test-case scenarios when necessary.
Of the three voice functions described above, CSFB has proved to be particularly important to handset manufacturers and network operators during the early-stage testing of LTE devices. CSFB tests can be carried out on the Anritsu MD8475A signalling test platform running the Smart Studio GUI (see Figure 2).
Fig. 2: MD8475A Smart Studio user interface
On the MD8475A, CSFB testing can be quickly set up using the LTE-to-WCDMA simulation model settings defined in the Smart Studio simulation parameters. The appropriate cell parameter information, such as operating band and power level, for the LTE and circuit-switched cells can easily be specified by directly clicking one of the base station icons in the GUI’s main window.
Testing of parameters such as jitter buffer, lost packets, packets with errors, call set-up latency, completion rates and inter-cell handovers allows the user to measure performance when simulating the environment of a typical live call. Verifying that all the procedures are being implemented efficiently and reliably will ensure that the user has the best possible experience in the real-world cell conditions of any given operator’s network.
Furthermore, the test scenarios are flexible enough to allow mobile an d network operators not only to test conformance to 3GPP specifications, but also to provide extra customised test coverage to support additional handset features not defined in the 3GPP’s standards.
Complementary VoLTE test solutions have been developed by Anritsu to target different test requirements, such as the Anritsu Rapid Test Design (RTD) and MD8430A LTE signalling tester, which support a comprehensive test capability for SR-VCC and VoLTE VoIP handsets.
The operation of circuit-switched technology in a handset designed primarily for packet-switched operation is necessarily complex. The implementation of simple and highly automated test routines for CSFB and other LTE voice functions in instruments such as the MD8475A promises to save design and production engineers a vast amount of time and effort.