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 infrastructure
LTE 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)
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.
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