摘要
随着移动通信技术的发展，下一代移动通信系统将面临用户数量急剧增加，移动业务逐步走向多元化，用户对服务质量的要求不断提高。人们希望下一代移动通信系统不仅具有更大的容量，还要支持除传统话音业务以外的视频、数据等移动多媒体业务。不同业务有不同的QoS要求(QoS：Quality of Service)，如何在恶劣的无线传播环境里以及用户运动且相互干扰的情况下为各类业务提供QoS保证，同时充分利用宝贵的无线频谱资源，是基于CDMA技术的下一代蜂窝通信系统以及未来无线个人通信系统都必须致力解决的问题，因此无线资源管理(RRM：Radio Resource Management)成为近年来研究的热点。呼叫接纳控制(CAC：Call Admission Control)是无线资源管理的重要功能实体，它通过接纳或者拒绝一个用户的服务请求，来保持系统的正常运行，是平衡用户服务满意度与系统资源最大化利用矛盾的主要手段，对于维护系统稳定性，增加系统容量并防止系统过载具有非常重要的意义。
本论文在充分总结以往研究成果的基础上，从保证用户QoS的角度出发，重点研究了下一代移动通信系统中有QoS保证的呼叫接纳控制策略。论文第一章介绍了论文的研究背景、研究内容与组织结构，第二章主要介绍了无线多媒体通信网络中的无线资源管理和CAC技术，并重点说明本文所采用的一种针对未来无线通信网络的特点，以3GPP定义为基础，更细致、更加符合实际情况的多媒体业务模型和建模方法。
在本文第三章中，以WCDMA网络为依托，在第二章中所介绍的业务模型的基础上，提出了一种新的CAC算法，该算法将系统上行链路负载因子作为主要接纳准则，既考虑到了不同业务之间的优先级，也考虑了同一业务切换呼叫和新呼叫之间的优先级，并且是一种多门限的动态接入控制策略。仿真结果表明，本算法在优先保证切换呼叫和话音业务新呼叫成功接入的同时，还能将优先级较低的非实时数据业务的新呼叫阻塞率保持在一个比较低的水平，有效改善了系统性能，较好的保证了各业务的QoS要求。
未来蜂窝网络发展的趋势是小区范围越来越小，切换变得更加频繁，用户切换到资源不足的小区时就会引起服务质量下降甚至服务中断。频繁中断正在进行的通信比发起通信时受到阻塞更令人难以忍受。为保证通信的连续性，常见的呼叫接纳控制算法都会赋予切换呼叫更高的优先级，并预留一部分资源专供切换呼叫使用。但在很多情况下，这部分预留资源可能没有被切换呼叫使用，从而造成系统资源的浪费。为了改善这种状况，提高资源预留算法的准确性，本论文第四章中提出了一种基于预测移动台切换基站并动态预留资源的呼叫接纳控制算法。仿真结果表明，本文提出的算法能够较好的适应网络业务量和用户移动路径的实时变化，在保证切换用户QoS要求的前提下能获得更高的带宽利用率和更低的新呼叫阻塞率，使系统获得更好的性能。
最后对全文进行了总结，并提出未来的工作方向。
Rapid progress of mobile communication and Internet networks in recent years has seen a dramatically growing demand for wireless multimedia services which require reliable and high-speed data transmission over a wireless channel. One of the most challenges for the next generation wireless communication systems is to support a large number of subscribers and to ensure the fulfillment of quality-of-service (QoS) requirements. How to support the different QoS requirements of heterogeneous services and utilize the scarce wireless resource efficiently in the situation of error-prone wireless channel get more attention in recent years. Call admission control (CAC) is one of the most important parts of resource management in wireless cellular networks, and an efficient CAC scheme is essential to provide QoS guarantee of different services to as many users as possible. It keeps the system work stable through accepting or rejecting users' service requests. CAC solves the conflict between the service satisfaction of user and system resource utilization.
In chapter 1, we give the research background and the signification of the study. Then, the radio resource management for CDMA system especially the CAC technology in wireless networks is introduced in chapter 2, and a particular presentation of the system evaluation model for wireless multimedia networks which is used in this thesis is also delivered.
Based on the traffic model we introduced in chapter 2, a novel call admission control algorithm for the WCDMA network is proposed, which uses the load factor of the up-link as the main admission criteria. It doesn't only take into account the different priorities of different services, but also the different priorities between a new call and a handoff call. The threshold of the algorithm is also dynamic. The simulation results indicate that the algorithm can minimize the dropping rates of all handoff calls and blocking rate of the new voice calls, meanwhile, the blocking rate of the non-real-time data service can also be kept on the relative low level.
In mobile cellular systems, forced termination of a call in progress in the target cell is less desirable than blocking a new call attempt. The usual CAC algorithms often give handoff calls higher priority and assign a part of resources particular for these calls .The mechanism of the reservation of resources only for handoff calls sometimes may results in the poor utilization of the resource in system. In chapter 4 of this thesis, the location of an existed call is estimated based on the measured pilot signal strengths by this mobile station (MS) from a number of nearby base stations (BSs), so the target BS of this call can be predicted to intentionally and efficiently reserve resource, the amount of reserved resource will also be dynamic. The simulation results show that this policy can guarantee the QoS of handoff calls efficiently, and also improve the resource utility significantly.
Finally, the thesis is summarized and the future work is brought forth.
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