7.1 Conclusions
Nowadays, in the era of wireless communication, the services with rich multimedia content have dominated the overall mobile traffic. In respond to the tremendous demand for these services from the customers, the 3GPP standardization body has defined eMBMS as an efficient solution for delivering multimedia data to massive number of users via the cellular network. It is expected to replace other traditional broadcast systems such as DVB, DMB or satellite broadcasting. Realizing the promising future of eMBMS, this thesis aims to give an introduction to the cellular broadcast technology and put emphasis on the performance of eMBMS in the context of mobility. Both theoretical and practical aspects of the broadcast service in LTE network are mentioned in our work. The theoretical part describes the whole picture of the eMBMS service and points out the problems that a user might face to when using eMBMS in a high mobility environment ; whereas the practice part expresses the performance of the eMBMS system in a real-time platform. The major achievements of the thesis are the following :
— Identify and analyze the mobility-related issues of the eMBMS service: In this thesis, we have studied in depth the eMBMS standard and classified the situations that affect the reception of LTE-Broadcast service due to the movement of mobile terminals. The detail analysis has clarified the limitation of mobility support procedures for eMBMS in 3GPP standard and in the literature at the moment. It also allows us to recognize the missing information that causes the service disruption and suggests the idea to solve this problem.
— Propose a complete solution for eMBMS service continuity in LTE/LTE-Advanced network : Based on the drawbacks of the existing mobility support mechanisms, we provide a method that helps the users to continue receiving their interested eMBMS service in all possible mobility scenarios. According to the pro-posal, if all the neighboring cells do not belong to the same MBSFN Area with the current cell, the UE will try to look for a candidate that belongs to other MBSFN
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Areas in the same frequency layer but still supports the required service. If no sui-table candidate was found, the UE continues searching cells in other frequencies to find out the appropriate one. At the last attempt when all neighboring cells in other frequencies do not transmit the needed service, the network will inform the device about the eMBMS service supported by other broadcast technologies. With our solution, the service continuity is guaranteed through different MBSFN Areas and on all frequencies while the current LTE standard only supports for the movement within one MBSFN Area. Moreover, even though the convergence among broadcast technologies is under research, a signaling method is available in our proposal so that the mobile device can change to another radio access technology and receive its ongoing service.
— Present a mechanism to reduce the media stream interruption time du-ring handover period: An important objective of mobile network operators is to provide a seamless service (particularly video-related services) to mobile customers in a high mobility environment. The eMBMS service, with its streaming feature, is thus expected to be as smooth as possible in the transition period. However, dif-ferent from the unicast transmission, there is no data forwarding for an individual user in eMBMS to moderate the impact of the disruption time on the quality of media stream. Additionally, when changing from one MBSFN Area to another one, the UE cannot get the data right after connecting to the target cell, it needs time to gather control informations and identify the position of its required service in the new cell. For these reasons, the eMBMS users will face to a stream interruption when the handover occurs. In order to minimize the influence of the eMBMS signaling retrieving time, we suggest using the Handover Command message to convey the necessary information to the UE. Our mechanism comprises a method of exchan-ging the eMBMS information between the serving and target eNB in both X2-based and S1-based HO procedure. With this suggestion, the terminal is able to quickly access to the real media data without spending time for listening to eMBMS control messages and hence, the stream disruption time is reduced significantly.
— Provide a real implementation of eMBMS system in an open-source plat-form - the OpenAirInterface: Started from the need to evaluate our proposed solution concerned the service continuity, we have implemented the eMBMS service into real-time OAI platform. Our work also responses to the shortage of a simu-lation/emulation tool for LTE-Broadcast in the research community. The imple-mentation of eMBMS in OAI platform covers the access stratum in LTE including RRC level, PDCP/RCL/MAC sublayers and full PHY layer. It not only helps us to evaluate our mobility support solution through experiments with high degree of confidence but also is available as a simulator/emulator for other researchers doing further research in eMBMS.
— Highlight the importance of eMBMS handover on the quality perceived by human users (QoE) in LTE-Broadcast service: Despite the limitation of OAI platform in supporting X2-based HO procedure, we managed to evaluate the QoE of eMBMS service in mobile scenario by combining the results from eMBMS emulation and the experiment in real cellular network. The OAI emulator was used to determine the stream interruption time while in the field-test, we use TEMS Investigation tool and its VSQI algorithm to derive the QoE estimation for the video
stream during handover. The deterioration of VSQI value related to the disruption time in the experiment indirectly reveals the improvement in QoE that we can attain if our method is applied.
In conclusion, this thesis has provided the fundamental knowledge of the multimedia broadcast/multicast service in LTE/4G networks as well as the state of the art for eMBMS mobility support. A solution to ensure the service continuity and improve the service quality is also presented together with the performance evaluation through real experiment.
7.2 Perspectives
Due to the difficulty of implementation in real-time system as well as the time constraint, this thesis is not able to provide a realistic mobility scenario for studying the behavior of eMBMS in full protocol stack (DASH with FLUTE and Raptor Code for FEC) and X2 interface between two base stations. This could be a good direction for extending our work.
Furthermore, the Carrier Aggregation feature is under development in our platform and when it is done, we can investigate the impact of frequency switching procedure on the service quality received by the eMBMS users. The ultimate goal of these studies is to build an optimized criteria system based on QoE metric for the network selection procedure taking into account the eMBMS reception.
Although, LTE-Broadcast is about to be commercialized shortly, many aspects are potential for future research to improve the performance of eMBMS system : in physical layer, a longer cyclic prefix (33,3µs instead of the current 16.7µs) or MIMO technique for eMBMS transmission should be studied ; An uplink to report the quality and optimize the MBSFN transmission or a dedicated component carrier for eMBMS are also the potential research objectives.
Another direction is the convergence of broadcast technologies. Some studies have been done with the aim to merge two terrestrial broadcast systems : eMBMS and DVB-T2. Such a common broadcast system allows increasing the coverage and service availability as well as ensuring the good service quality for the customers. The hybrid satellite/LTE network also becomes a prominent topic in offering a seamless video delivery system. The cooperation of satellite transmission in LTE eMBMS might be seen as the broadcast service in cellular network with an extra component carrier. A significant adjustment in PHY is envisioned for the switching between these two access technologies.
In a different point of view, we can exploit the usage of LTE-Broadcast in other appli-cations rather than just for conventional download or streaming services. eMBMS can be used as an efficient way to transfer messages for the public safety purpose. The network may need just a small portion of radio resources to send the warning via eMBMS bearer to concerned habitants in a specific area about the natural disasters such as tsunami, ear-thquake or volcano eruption, etc. The eMBMS’s application may appear in the Intelligent Transport System (ITS) as well : new generation vehicles whose operating system connects to the smartphone can get the traffic information from cellular network through eMBMS and then inform to the owners.
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