Phenomena in 4G Networks

After introducing the main concepts related to mobility in 4G networks and the relevant methods and models linked to the evaluation of QoE, the potential effect that mobility across heterogeneous networks may have on the QoE of mobile users in 4G networks. There are three main sources for potential degradations in overall call quality: (1) changes in the underlying network conditions, (2) network handover, and (3) codec adaptation to network changes. These phenomenona will be briefly described next, as they are the core conditions studied in this work.

Changes in Network Conditions

A main characteristic of 4G Networks is heterogeneity. This translates into potentially severe changes in network conditions when seamlessly roaming across wireless technologies. The variations in packet loss, delays and bandwidth to which mobile applications are confronted with in 4G networks may lead to changes in the user's quality experience. Variations in network conditions are common, but the magnitude of the changes will increase in 4G networks, and new methods to deal with this will be needed in order to maintain an acceptable QoE. A first step towards building these new methods is to understand the impact of these variations on user experience. This chapter discusses this impact for an ongoing VoIP call when a user moves between independent wireless access networks.

Network Handovers

In Next Generation Mobile Networks (NGMN), the convergence of a multitude of different network technologies will provide the user with transparent and ubiquitous access to data and media services. The independence of network and service layers also introduces a new level of mobility. Equipped with appropriate terminals, users may move through geographical areas covered by different wireless network technologies while service access is preserved. In order to guarantee the provision of seamless connectivity especially for time-critical services like media streaming or Voice-over-IP (VoIP), sophisticated mobility-enabling protocols like Mobile IP are needed which ascertain a fast and robust roaming between heterogeneous wireless networks. This roaming between disparate wireless technologies (vertical handovers) while keeping ongoing sessions forces a new level of application level adaptation, causing an effect on user perception. This effect has not been thoroughly quantified as it is not present on existing communication systems.

Codec Adaptation

Current voice over IP codecs can adapt to variations in the underlying networks, but may not cope with the kind of changes caused by vertical handovers, as these could result in the need to change codec families (e.g., narrowband to wideband). In this case, for an ongoing VoIP call the network handover can occur in conjunction with the appropriate application layer adaptation in order to account for the newly encountered conditions. Most important, a codec re-negotiation might be enforced to meet sudden changes in network conditions. This should ideally be done in such a manner that neither the call nor the audio stream is interrupted during the handover. Thus, the continuity of the speech is maintained, resulting in an optimal speech quality.

Special attention has to be paid to these aspects as they produce new auditory artifacts that can invalidate speech quality models in the context of forthcoming communications systems, causing mismatches in the design of networks and mobile devices and applications.