摘要
无线电频谱（以下简称频谱）是无线通信中一种稀缺而又宝贵的自然资源，已开发出来的频谱绝大部分已分配。随着无线通信业务的快速发展，频谱需求迅速增长，致使频谱资源紧缺问题更加突出，成为制约无线通信发展的新瓶颈。认知无线电（CR）技术因其能够有效地解决当前无线频谱资源紧缺问题而成为近年来无线通信领域中重要的研究热点。CR技术的基本思想非常简单，但其设计和实现却面临很多挑战。为使认知无线电技术较好地实用化，仍需要开展大量的研究工作，尤其在面向服务质量保障的核心技术方面。
本文重点研究了面向服务质量保障的认知无线电部分相关核心技术，主要涉及面向服务质量保障的模型框架、频谱预测、频谱接入、频谱分配、分组调度等。
论文工作主要体现在以下几个方面：
（1）在面向服务质量保障的模型框架方面，分析了认知无线电环境下影响服务质量保障的主要因素，探讨了服务质量保证策略，设计了一种保障服务质量的动态频谱使用模型。此模型可适应不同环境的需求，灵活性强，也可以充分利用现有的成熟的研究成果。本文研究内容为基于此模型下的一系列关键技术，包括频谱预测、接入控制、频谱分配及调度技术等，这些研究都以服务质量保障为基本出发点。
（2）在频谱预测方面，通过对频谱历史信息的分析，研究了多变量灰色模型MGM和自适应的多变量灰色模型AMGM，提出了两种适合于频谱可用性预测的改进多变量自适应灰色模型eeAMGM和eeeAMGM。仿真结果表明，所提出的两种模型较原模型在预测性能方面都有较大的改进，各项误差指标、精度等级、可靠性等都很好，能够较好地用于频谱可用性的预测。研究了信道状态剩余时长序列的产生，并利用混沌与神经网络理论进行信道状态剩余时长的预测，设计了基于混沌神经网络的的剩余时长预测模型，给出了其预测流程和预测模块结构图，并对其进行了性能评估，仿真结果验证了预测模型的有效性。
（3）在频谱接入方面，采用强制优先排队和马尔可夫理论对认知无线电网络中的动态频谱接入过程进行模拟建模和分析，得出了网络适合次用户（或称为认知用户）接入的条件；在此基础上，提出了一种针对次用户的接纳控制方案，此方案能够根据当前频谱资源情况和次用户服务质量需求，自适应地对拟接入的次用户予以接纳或者拒绝，通过这种接纳控制可以更好地保障次用户的QoS要求和网络的整体性能。
（4）在信道分配方面，提出了一种基于最小化分配风险的信道分配模型。基于此模型进一步设计了基于最小化切换风险的信道分配方案MHR。MHR统筹考虑了多个分配需求和包括繁忙及空闲在内的所有信道，仿真结果表明，此方案能够大大减小系统中因主用户出现而引起的次用户发生切换的次数，减少了切换开销，也能够很好地改善其他性能指标；通过对MHR中切换风险的重新定义，设计了两种支持两种等级的服务质量区分方案：MHR-TCS-NP和MHR-TCS-PM，其中MHR-TCS-PM中引入了抢占，MHR-TCS-NP没有抢占，仿真结果表明，这两种方案均能有效提高高优先级次用户的成功通信率，降低高优先级次用户的平均等待时延。其中，MHR-TCS-PM还可以更有效地降低Service-I的平均等待时延及切换率等各项指标，提高其通信的成功率。
（5）在认知无线电分组调度算法方面，从理论上分析了一种典型的分组调度算法M-LWDF的公平性，由分析得知，在队首分组等待时间服从指数分布的情况下，M-LWDF的业务队列具有相同的调度机会，而与业务类型无关；其公平性在用户业务QoS需求一定的情况下，仅与两队列用户数之比有关。提出了改进的分组调度算法EM-LWDF，EM-LWDF严格依据QoS需求公平准则进行设计，在队首分组等待时间服从指数分布的情况下，其公平性不受用户数之比的影响，理论上其公平指数为1，仿真结果表明，EM-LWDF的公平指数的变化曲线与理论分析结果基本一致，比较接近于1，且不受两队列用户数之比的影响，而且业务的丢包率能够被有效控制在丢包率上限之内。
Radio spectrum (hereinafter referred to as spectrum) is a type of scarce andprecious natural resource in wireless communications, and most of the exploitedspectrum have already been assigned. With the fast development of wirelesscommunications services, there has been a rapid growth of demand on spectrum inrecent years, which causes the problem of spectrum shortage to be acute. So, spectrumhas been a new bottleneck of the development of wireless communications. Recently,cognitive radio (CR) technology has become an important research hot topic in the fieldof wireless communications since it can effectively deal with the current problem ofwireless spectrum scarcity. The design and implementation of CR technology are facinga lot of challenges although its basic idea seems to be very simple. To promote itspractical realization, there has been much research works to be carried out, especially inthe respect of QoS (quality of service) guarantee.
This paper is mainly focused on the study of some parts of core technologiestoward the guarantee of QoS, namely the framework model toward the guarantee ofQoS, spectrum forecast, spectrum access, channel allocation, and packet scheduling.
The main works of this paper is as follows.
(1) In the respect of the framework model toward the guarantee of QoS, the keyfactors influencing QoS in CR environments are analyzed, the strategies forguaranteeing QoS are explored, and a model for the use of dynamic spectrum isdesigned in term of environment requirements and flexibility, in which some existingmature technologies can be also used. The following works of this paper are based onthis model.
(2) As for spectrum forecast, based on the analysis of spectrum historic information,the spectrum forecast performance of the multi-variable-grey-model (MGM) and theadaptive MGM (AMGM) are evaluated, and two improved AMGM named eeAMGMand eeeAMGM for spectrum availability forecast are presented. The simulation resultsshow that eeAMGM and eeeAMGM outperform AMGM in the performance ofspectrum availability forecast, such as error degree, accuracy, reliability, etc. In addition,a mechanism for forecasting the remained length of the duration of ON state and OFFstate is designed by introducing the theories of chaos and neural networks, and thesimulation results validate its effectiveness.
(3) As far as spectrum access is concerned, a simulation model of the process ofdynamic spectrum access in CR networks is built by the use of queuing theory withcompulsory priority, and the theoretic analysis with Markov theory is given. Accordingto the analysis, the most favorable conditions for the access of secondary users areobtained. Based on the access conditions, an admission control approach is designed,which can adaptively admit or refuse secondary uses in terms of the actual conditions ofspectrum holes and QoS requirements.
(4) So far as channel allocation is concerned, a novel channel allocation model forminimizing allocation risk is proposed, and then a channel allocation approach namedMHR for minimizing handoff risk is designed. MHR overall plans multiple channelallocation requirements and also considers multiple data channels together, includingboth idle and busy channels. In addition, two improved MHR named MHR-TCS-NPwithout preempt and MHR-TCS-PM with preempt for supporting two classes of serviceare designed by redefining handoff risk.
(5) In the case of packet scheduling in CR networks, the theoretic analysis ofM-LWDF fairness is given, which shows that M-LWDF fairness is relevant to channelcondition, packet’s arrival process and the ratio of QoS requirements of different servicequeues. Given QoS requirements and other parameters related to channel model andpacket’s arrival process, it is merely relevant to the ratio of the number of users in theservice queues. Based on the theoretic analysis of M-LWDF fairness, an enhancedM-LWDF algorithm named EM-LWDF is proposed and demonstrated. EM-LWDF isstrictly designed in light of QoS requirement fairness criteria. Its fairness is almost notrelevant to the ratio of the number of users in the service queues and the theoreticalvalue of fairness index is equal to1.
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