The global telecommunications market is witnessing a paradigm shift in demand as mobile data revenues continue to surpass voice-based revenues in most western countries, accelerating the transition to technologies such as LTE. Mobile networks are witnessing fast-paced development as operators go the extra mile to cater to the changing communication and entertainment needs of their subscribers. New LTE networks will utilize 3G technologies as the underlying infrastructure where no LTE service is yet provided, so testing handovers between different radio access technologies is becoming ever more important. The consequence of inadequate response times can be slow (or no) handover, and poor user experience such as dropped connections.The article expands on this industry background, including the introduction of voice over LTE, and defines and describes the various scenarios for Inter-RAT (Radio Access Technology) handover, including the proposed alternative fall-back scenarios for voice.
What’s the need for LTE anyway?
The global telecommunications market is witnessing a paradigm shift in demand as mobile data revenues surpass voice-based revenues in most western countries. Wireless network operators are focusing on expanding revenues in broadband services, where wireless technologies are perceived by users in the light of broadband wired access. The rapid growth of mobile broadband is driven by demand for the latest devices, applications and services, which enable users to access any type of content on the move. Mobile broadband will also facilitate economic benefits, especially in countries lacking fixed-line broadband infrastructure.
This is driving up data usage on mobile networks at a tremendous rate, and operators need to respond with bandwidth availability, which in turn provides the driving force behind the development of evolved 3G and 4G systems such as HSPA and LTE.Long Term Evolution (LTE) is the project name given by 3GPP to the evolution of the UMTS 3G radio standards. The work on enhancing the original UMTS Terrestrial Radio Access (UTRA) continues in Release 8 of the 3GPP standards with enhancements to High Speed Packet Access (HSPA), but in addition Release 8 includes LTE, or to give it its formal name, Evolved UMTS Terrestrial Radio Access (E-UTRA). Offering higher data rates and lower latency for the user, a simplified all-IP network for the operator and improved spectral efficiency, E-UTRA – or LTE – promises to provide many benefits.
LTE as part of the cellular infrastructureLTE is an all-IP system, designed primarily to provide high-speed data services. Therefore, during network build-out, and until operators choose to implement IP-based voice services, LTE networks will utilize 2G and 3G as an underlying infrastructure for voice calls, and for data services where no LTE service is yet provided. In normal operation, the mobile device (user equipment, or UE) is required to scan for neighbor cells and make measurements which are used as a basis for cell selection and handover decisions. Such processes are very
demanding for today’s UEs, which must also multi-task a large number of other applications, making heavy demands on processor power. The consequence of inadequate UE response times can be slow (or no) handover, and poor user experience such as dropped connections and frozen applications.
Industry research predicts that LTE is likely to experience its most rapid growth from 2012, when the majority of operators launch their networks and a unified approach to delivering voice communications and rich services such as video telephony over LTE become available.Because LTE coverage will not be pervasive, testing handover capability between different radio access
technologies (RAT) is critically important in the verification of UE performance. For a positive end-user experience, UEs need to transition smoothly between these RATs, leading operators to increase their focus on testing the real-world performance of each device before deployment on their networks. Such performance testing goes well beyond the more traditional conformance tests defined by the industry’s standards bodies.
Consider two aspects of testing through the lifecycle:
* Conformance – necessary but not sufficient for deployment
* Performance – reflects real use cases. (e.g., measuring maximum data throughput, battery drain under different conditions)
Figure 1. Simplified handover process
Conformance test might be taken as an industry requirement – ensuring the UE supports a level of functionality and does not cause a problem on the system or to other users – where performance test gives the UE manufacturer the opportunity to differentiate their device based on better user experience: application speed, battery life and generally how the UE fulfills expectations.
Inter-RAT handovers are part of both, and assume different importance depending on what the UE is currently doing. If it’s idle (not using network resources), conformance issues are the main concern. If, however, the user has a data-hungry application active, performance issues become much more important. In idle mode, network selection decisions are made mainly by the UE, and transmitted to the network. Where the UE has an active data connection, the network will decide the transmission channel, based on its own measurements and neighbor cell measurement data returned from the UE. See Figure 1.
A second criterion for Inter-RAT handover is the need for a voice service. As previously mentioned, LTE is a packet-only service, with no provision for the circuit-switched voice connection that is normal in earlier systems. Until operators make the additional network equipment investment required to support voice in LTE, making
or receiving a voice call will not be part of an LTE service. Meantime, many operators are investing in LTE alongside existing voice networks which offer more extensive coverage. In this scenario it makes sense to use the LTE connection for data and the existing network for voice.
Geography plays a part
There’s a separation of inter-RAT requirements depending on the operator’s 2/3G network technology and
UE functionality. If the UE is a wireless card (“dongle”) and supports only data services, things are a little easier. If it is a phone it’s more complicated.
For operators in much of the world, where current networks are 3GPP GSM/W-CDMA/HSPA, there’s a natural evolution to LTE, and standards-based support for full backward and forward compatibility, both in the radio access network and the core network that lies behind it. The final goal is the inclusion of Voice over Internet Protocol (VoIP) as part of LTE, making it easier for operators to run IP-only networks, and full E-UTRA handover to minimize data connection disruption.
In the initial network implementations, losing LTE service or making/receiving a voice call will cause an automatic fall-back to a 3G bearer, where both voice and data service can be provided. Voice service is then provided by a circuit-switched fall-back mechanism, and continuing data service managed by radio resource release and re-assignment messages. The current specifications contain fall-back scenarios all the way back to a basic GSM/GPRS connection if that is the only available network infrastructure, and Release 9 includes improvements to circuit-switched fall-back to improve access speed.
In countries where current networks are 3GPP2 cdma2000/1xEV-DO the issues are harder to resolve. While there is support in LTE for the discovery and measurement of neighbor 3GPP2 cells, the core LTE and 3GPP2 networks have major differences. The first LTE implementations will support only “non-optimized” handovers where, when the UE loses LTE service it has to acquire 1xEV-DO service. If it has previously been connected to a local 1xEV-DO cell, this may happen relatively quickly, if not it will take much longer. In idle mode this is not really an issue, but during an active data session will cause some disruption. Later implementations will support “optimized” handovers where the UE will be directed to a new serving cell and have much more information about it.
To ensure the UE connects to LTE service where it is available, LTE is set as the “preferred service” by the network operator. Voice service is supplied by a separate cdma2000 radio in the UE and a separate
network connection with no requirement for interworking. This is known as simultaneous voice and LTE (SVLTE); there is no integration of voice and data services, and battery power consumption may be compromised. Later LTE specification releases and changes to the 3GPP2 core network will address data
and voice integration, with the goal of providing the same final solution as the 3GPP case. However, there is an additional level of complexity as the solution needs to accommodate not just the transition between different physical layer technologies but also interworking between separate core networks. To support this “tunneling” between the 3GPP and 3GPP2 core networks ir required. This allows the UE to perform, for instance, pre-registration on the 3GPP2 network while still connected to LTE, speeding the handover process when a cell change is required.
Making sure it all works
Key to developing LTE UEs for markets worldwide is the ability to test their functionality under controlled, repeatable conditions. Consider these three stages: firstly, the UE must correctly interpret and properly respond to protocol messages from each network technology it supports; secondly the protocol messaging must result in the desired connection between the UE and the network; and thirdly the connection must provide the means to access the required service to the satisfaction of a user. A system such as that shown in Figure 2 contains the basic elements required for development testing: an environment for running the signaling conformance test scripts to validate adherence to system standards, LTE and 2/3G system emulation to provide the controlled RF environment for both UE characterization and handover testing, the ability to log and analyze protocol messaging amongst the test sets and UE, and an application server to provide the services needed for performance test.
More comprehensive systems can be used to provide a more complex RF environment for testing – for example additional test sets to provide multiple neighbor cells and both 2G and 3G cells of different technologies, and adding fading simulators to provide a better representation of a truly mobile environment.
Figure 2. A typical system for UE characterization using Agilent Wireless Communications Test Sets
Signaling conformance test (sometimes called protocol conformance test) uses scripts describing specific test cases and provided by accredited standards organizations, and is the basis for proving interoperability. The device must pass the required cases for each technology that it supports. The TTCN- 3 (Test and Testing Control Notation version 3) Environment shown in the figure allows test cases to be portable from one test platform to another using an interface which has been standardized by ETSI. Manufacturers provide the standard TTCN-3 compiled test cases and an adaptor that connects the standardized interface to the unique characteristics of their test instrumentation. This testing is typically done under good RF conditions, and the goal is to demonstrate that the UE responds correctly during a sequence of protocol messages. Pass/fail is
determined from the messages and their content – no parametric measurements are made.
In the RF domain, a test system which can provide cell emulation for different technologies, including parametric changes and corresponding measurements, can be used to investigate both the outcome and the timing of
inter-RAT handovers. 3GPP standards cover expected Radio Resource Management (RRM) behavior between LTE and earlier releases. In the case of operators moving to LTE from 3GPP2 technologies (cdma2000/1xEV-DO) operators are creating their own requirements and test plans for handover testing, and working with suppliers to ensure new UEs meet the needs of their customers. This type of testing helps UE developers ensure their devices conform to the latest needs of network operators as the various inter-RAT scenarios are fully specified and implemented.
When the ability to change the RF environment is used in conjunction with a live application server, it can provide valuable insight into the behavior of both application and UE during disruptions caused by changes to
the physical connection: for example, RF channel conditions, system handovers, dropped calls, connection speed changes, and interference from other applications. The application server can also be used to ensure the UE handles multiple processes correctly and in a timely way. Examples might include delivering an SMS during video streaming, downloading multiple files and using an interactive gaming application. This capability gives the developer the view of overall device or application performance that helps differentiate their product from its competitors.
Today’s smartphones are full-blown communications and computing platforms that tap into high-speed data networks to access all manner of compelling content. With thousands of downloadable applications covering everything from location-based advertising to streaming internet radio and tv and interactive on-line gaming,
expectations for mobile and network performance are steep; operators need to make revenue from seamless data services and consumers expect the mobile Internet to perform on par with their home and office. LTE service is evolving, and thorough testing – particularly of the integration between LTE and legacy networks – to ensure customers get the performance and continuous connectivity they expect – is, and will remain, critical to the deployment of new-generation networks and services.