摘要
随着无线技术在移动通信、广播电视、公共安全等领域的广泛应用,现代社会对频谱资源的依赖程度越来越高,认知无线电作为一种提高频谱利用率的无线电技术,在未来的无线通信网络中具有非常广阔的应用前景。频谱共享技术,是认知无线电研究的关键课题之一,其目标是在保证主用户正常通信的前提下,通过认知用户与主用户之间、以及认知用户之间共享频谱,实现认知用户高效可靠的通信。
本论文正是依据频谱共享技术的目标,定位于基于协作的频谱共享技术的研究上。针对不同的频谱共享方式(underlay或overlay),结合不同的协作传输协议,深入研究了协作中继技术在各种频谱共享场景中的应用。主要工作和创新如下：
(1)联合考虑干扰温度限制和频谱共享干扰,构建了完整的underlay式认知中继系统模型；并在此基础上,对系统进行了中断概率和误符号率的性能分析。
针对现有系统模型忽略频谱共享干扰的问题,在联合考虑干扰温度限制和频谱共享干扰的情况下,对采用两种不同协作协议的认知用户系统进行了更准确、更实用的性能分析。在单中继解码转发协作协议的情况下,推导出认知用户系统中断概率的闭式解,以及误符号率的渐近紧的上下限；在多中继选择协作协议的情况下,考虑主用户活动对认知用户性能的影响,推导出中断概率的闭式解。
(2)研究基于协作OFDM的overlay式频谱共享机制,针对主用户的邻频干扰问题,提出了交替迭代优化的功率分配算法
针对overlay式的OFDM-CR系统中可能出现的邻频干扰问题,引入协作中继技术,能够在降低认知用户的发射功率、避免邻频干扰的情况下,保证认知用户的通信质量。以实现认知用户协作OFDM传输最优化为目标,在不影响邻频主用户正常通信的约束下,提出了交替迭代优化的功率分配算法,合理分配源与中继的子载波功率。
(3)提出了一种基于感知中继的overlay式频谱共享机制——感知协作机制,能够提高频谱利用率,实现高效协作传输。
该机制利用具有认知能力的感知中继替代普通的中继用户,共享主用户的频谱来完成认知用户系统的协作传输,节省了系统的带宽资源,能够大大提高传输效率。为该机制设计了两种感知中继的选择策略,一种基于瞬时信道信息,一种基于部分信道信息；并且每种策略都按照是否考虑用户之间的公平性,划分为两种情况分析相应的用户中断性能和系统吞吐率性能。
(4)为了结合underlay、overlay两种频谱共享方式的优点,提出了基于感知结果的混合式协作频谱共享机制。
该混合式机制允许认知用户根据频谱感知的结果自适应地选择underlay或者overlay共享方式；当切换至underlay方式时,依靠协作传输技术,弥补由于功率受限造成的性能损失。在该机制下,考虑主用户的活动状态以及认知用户的频谱感知能力,分析了两种用户之间的相互干扰情况,进而基于信干噪比推导出认知用户网络的中断概率和误符号率的闭式表达式。
(5)为了使认知用户在频谱共享中,更主动和灵活地获得频谱使用机会,提出了基于时隙划分的协作补偿式频谱共享机制。
在该机制中,认知用户主动充当主用户的协作中继,协助主用户传输,以换取自身数据的传输机会。本文讨论了该机制是否适用的判定过程,给出了具体的判定依据；设计出基于时隙分配的传输协议,来协调调度主用户和认知用户的信息传输；评估了补偿式共享后,主用户和认知用户各自的中断性能和误符号率性能。
As the wireless technology rapidly develops in the fields of mobile communications, broadcast TV and social security, the spectrum resource has played a more and more important role in modern society. As a new technology which can carry out a more efficient and flexible program for the usage of spectrum, cognitive radio (CR) has broad application prospects in the future wireless communication networks. The spectrum sharing technology, one of the key research topics of CR, aims to achieve efficient and reliable communications of secondary users (SUs) while guaranteeing the quality of service (QoS) of primary users (PUs).
To achieve the aim of the spectrum sharing technology, this dissertation focuses on spectrum sharing technologies based on cooperation. We design and analyze several cooperative spectrum sharing schemes which are applicable to different spectrum sharing scenarios. The outline of this dissertation is described as follows:
1) A complete system model of the underlay cognitive radio relay system, which takes into account the interference temperature constraint and spectrum sharing interference, is constructed. Based on this model, we investigate the outage performance and the symbol error probability performance.
The existing system model ignores spectrum sharing interference, so we consider the interference temperature constraint and spectrum sharing interference and present more accurate and practical performance analyses for the secondary system with two different cooperative protocols. For the single-relay decode-and-forward cooperation, we derive the closed-form expression of the outage probability, as well as the asymptotically tight upper and lower bounds of the symbol error probability. And for the multi-relays selection cooperation, the closed-form expression of the outage probability is derived with consideration of the activities of the PUs.
2) An overlay spectrum sharing scheme based on OFDM relay transmission is proposed, and an alternate and iterative optimization algorithm is presented to solve the problem of power allocation between the source and the relay.
To solve the problem of adjacent-frequency interference in OFDM-CR systems, we introduce the cooperative relaying technique to avoid the interference by controlling transmit powers of the SUs while guaranteeing the QoS of the secondary transmission. An alternate and iterative optimization algorithm of power allocation is presented to maximize the total rate for the secondary system while keeping the instantaneous interference introduced into the PU below a certain threshold.
3) An overlay spectrum sharing scheme based on cognitive relays, named the cognitive cooperative scheme, is proposed to improve the spectrum efficiency and achieve the efficient cooperative transmission.
This scheme substitutes cognitive relays for conventional ones to utilize the detected spectrum holes to implement cooperative transmission, which can save the dedicated relay resources for the cooperative system and improve the transmission efficiency. We also propose two cognitive relay selection strategies, including the optimal instantaneous channel state information based strategy and the partial CSI based one, and we present the outage and throughput performance analysis.
4) A hybrid cooperative spectrum sharing scheme based on the spectrum sensing result is proposed to combine the advantages of underlay and overlay approaches.
The hand-off between the underlay approach and the overlay approach is based on the spectrum sensing result,ⅰ.e., the SU can adapt its transmit strategy according to the spectrum sensing result. When switching to the underlay approach, the performance loss can be compensated by applying the cooperative relaying technique. Under this scheme, we consider the mutual interference between the PUs and the SUs, and then derive the closed-form expressions of the outage probability and the symbol error probability for the secondary network.
5) In order to make the secondary users obtain spectrum opportunities more flexibly, we propose a compensative cooperative spectrum sharing scheme based on slot allocation.
In such a scheme, the SU acts as the cooperative relay of the PU and helps to transmit the information of the PU in exchange for the spectrum opportunities. Firstly, we analyze the feasibility of this scheme and present the necessary preconditions. Secondly, we design a transmit protocol based on slot allocation to solve the joint scheduling problem of the primary and secondary transmissions. Finally, we evaluate the performance of the primary and secondary systems under the compensative spectrum sharing scheme, including the outage performance and the symbol error probability performance.
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