The main theme of this dissertation is quality of experience (QoE) enhancement of voice over IP (VoIP) on mobile networks. We focus on the development of Mechanisms to cope with the temporal service disconnection due to the movement of subscriber, and the packet loss (PL) due to the temporal signal quality degradation that frequently occurs in mobile cellular networks. In this dissertation, we propose three mechanisms for VoIP: an estimation-based cell search mechanism, an abstracted R-factor based handover (HO) criteria and an opportunistic retransmission mechanism.
A solution exploiting a mobile relay (MR) has been interested in the latest cellular network, which overcomes the QoE degradation caused by frequent HOs and high penetration loss. In this scenario, the MR should support the mobility management features such as the detection of the neighbor base stations (BSs), the signal quality measurement and the group HO. Both the BS detection and the measurement of the signal quality of the detected BSs are the cell search process. Cell search process of the MR makes the MSs experience the service disruption. It also causes to deteriorate the network efficiency. An estimation-based cell search mechanism estimates the signal quality between the MR and the detected neighbor BSs exploiting the cell search results of the MSs. Thus, the MR can obtain the signal quality information of the neighbor BS without the cell search process. By the simulation results, the proposed mechanism reduces the network overhead and increases the delay performance of the MSs. In addition, the proposed mechanism acquires the reasonable estimation accuracy.
To efficiently improve QoE of VoIP services, we should consider the QoE-related mechanisms in the application layer when designing the lower layer mechanism. In this dissertation, we optimize the HO criteria using the R-factor designed to objectively measure the QoE of voice services, as the objective function. The proposed HO criteria are characterized by the linearization coefficients of the R-factor, the minimum required signal quality, the HO interruption time and the variance of the signal quality. These values can be easily obtained in the network. In addition, the derived HO criterion is expressed as the unit of signal quality. Thus, it reduces the system complexity as compared with that of the conventional HO mechanisms considering the R-factor. Consequently, it is a practical solution to improve the QoE of the VoIP services.
The demand for high-quality voice services has been increasing for recently several years. In addition, the performance of the retransmission mechanism has been improved. The interest of the retransmission mechanism for the high-quality VoIP services arises from these factors. Traditionally, the PL of voice services are concealed by the PL concealment (PLC) mechanisms on the receiver side. Thus, the retransmission mechanism for VoIP services should be designed considering the interaction with the PLC scheme. In this dissertation, we propose an opportunistic retransmission mechanism. Basic idea of the proposed mechanism is to only retransmit the successively PLs because the single lost voice packet can be concealed by the PLC mechanism. By the numerical results, the proposed mechanism can improve the delay impairment factor while maintaining almost same effective PL robustness factor compared with the conventional retransmission mechanisms.
To achieve the QoE improvement, we should understand the interactions of various QoS mechanisms through all the communication layers and design a QoS mechanism by the cross-layer approach. However, the cross-layer design approach may reduce the scalability, the efficiency and the flexibility due to the interaction and the operating speed difference between the different layers. In this dissertation, we propose three di_x000B_erent QoE mechanisms. To make up for the weak points in the cross-layer approach, these mechanisms are designed by the loosely-coupled cross-layer approach. All the required information can be easily obtained on the network or itself without any system changes. Thus, these mechanism can be implemented in the current and next generation cellular network.