【作者】 李靖；

【导师】 葛建华；

【作者基本信息】 西安电子科技大学 ， 通信与信息系统， 2009， 博士

【摘要】 多输入多输出(MIMO)技术能抵抗无线信道中多径衰落的影响,在提高未来无线通信系统的容量和可靠性等方面具有巨大的潜力。然而,由于功率、体积或硬件复杂度等限制,在无线终端实现多天线较为困难。近年来,一种基于用户间协作和中继协作等方式的协作分集技术应运而生,受到了广泛的关注。其基本原理是:在多用户环境中,各单天线终端通过分布式传输和处理,共享彼此的天线和其它资源构成多天线发射机,形成虚拟的MIMO系统来获得空间分集增益。本论文针对协作分集领域中的协作方案设计和资源优化分配策略展开研究,提出了特定场景下的解决方案,并进行了详细的理论分析和仿真验证。论文的主要工作概括如下:1.在协作方案设计方面针对传统两用户协作方案要求用户在正交子信道上进行中继传输,导致系统频谱利用率较低的问题,分别提出了单天线和多天线目的端场景下结合分布式空时编码(DSTC)和分布式预编码(DP)空时预处理技术的增强型协作方案。在单天线目的端场景下,1)提出了一种基于放大转发的分布式空时码(AF–DSTC)协作方案。该方案利用空时分组码的正交特性,使协作用户在共享的信道资源上进行中继,从而提高了传统AF协作方案的频谱效率。仿真结果表明所提方案在高信噪比条件下可获得完全分集增益。2)提出了一种基于译码转发的分布式空时码(DF–DSTC)协作方案,该方案的思想与AF–DSTC相同。推导了系统中断概率性能的上下界以及采用二进制相移键控(BPSK)星座调制时系统误比特率性能的闭式解,从理论上证明了所提方案在高信噪比条件下能获得二阶分集增益。仿真结果验证了理论推导的正确性。在多天线目的端场景下,1)对单天线目的端场景下的DF-DSTC协作方案进行了研究扩展。假定存在一个协作传输发起者,新的DF-DSTC协作方案规定:当两用户均译码失败时,协作发起者可以重传自己的信息,从而保证其性能不劣于直接传输。当用户采用多进制相移键控(MPSK)星座调制且目的端配置m(m≥1)副天线时,推导并分析了系统误比特率性能的上界和下界。最后,通过仿真验证了理论分析结果的正确性。2)提出了一种基于译码转发的分布式预编码(DF–DP)协作方案。该方案利用目的端的多天线特性,将中继信号进行预编码处理后在共享的信道资源上发送,从而同时提高了传统DF协作方案的频谱效率和可靠性。将虚拟的两输入多输出信道在矢量空间分解为两个正交子信道,提出了一种系统误码率最小准则下的预编码方法。仿真结果表明,所提方案在低反馈条件下能显著提高系统误码率性能。2.在资源优化分配方面主要针对协作伙伴选择和功率分配展开研究,提出了在特定场景下“何时协作”、“与谁协作”和“如何分配功率”的解决方案。1)提出了固定式中继和选择式中继正交调制协作方案中的最优功率分配和伙伴选择方法。根据用户间信道和上行信道的统计特性及可调功率分配因子,分别推导了两种协作方案下系统误码率性能的闭式解,并以误码率最小作为准则,提出了功率优化分配(OPA)方法来对本地和中继信号的发送功率进行自适应调整。将伙伴选择与OPA方法联合考虑时,确定了不同协作增益下的伙伴位置,并提出一种简单的次优伙伴选择方法,即最优伙伴位置应从源侧距离源和目的端的中点最近。仿真结果表明,与未优化的方法相比,所提资源优化方法能显著降低误码率,提高协作增益,且可用来简化实际的伙伴选择过程。2)提出了旋转编码协作方案中的一种功率优化分配方法。该方法首先利用Chernoff界推导了端到端系统误符号率(SER)的上界,然后在总功率一定的条件下,以最小化SER上界为目标,根据平均信道增益来计算源和中继间的功率分配因子。最后将功率分配信息通过反馈信道发送给协作用户,从而实现了功率的自适应分配,提高了系统的资源利用率。仿真结果验证了所提方案的有效性。此外,还比较了不同伙伴选择方法对旋转编码方案性能的影响。更多还原

【Abstract】 Multiple input multiple output (MIMO) technique can overcome the limitations of multi-path fading in radio channels, and therefore holds the potential to drastically improve the capacity and reliability in future wireless communication system. However, due to size, cost or hardware complexity limitations, it may be difficult for a wireless terminal to support multiple antennas. Recently, cooperative diversity technique, which is based on user cooperation and relay cooperation etc, has emerged as a promising technique and received considerable interests. The essence of this technique lies in that single-antenna terminals in a multi-user scenario are allowed to share their antennas and other resources through distributed transmission and processing so that a virtual multi-antenna transmitter is achieved, and hence reap the spatial diversity benefits of the virtual MIMO system. Focusing on the design of cooperative schemes and resource optimization allocation strategies in the cooperative diversity fields, the solutions for particular scenarios are introduced and further analyzed from theoretical and simulation aspects. The main contribution of this work can be summarized as follows:1. The design issues of cooperative schemesTo solve the resource inefficiency of the traditional two-user cooperative schemes which require the users to relay on orthogonal sub-channels, several improved cooperative schemes combined with distributed space-time code (DSTC) and distributed precode (DP) techniques are proposed when the destination is configured with single and multiple antennas, respectively.For the single-antenna destination scenario, 1) An amplify-and-forward based DSTC (AF-DSTC) cooperative scheme is proposed, which utilizes the orthogonal characteristic of space-time block code (STBC) to make cooperative users relay on the shared channel resources so that the spectral efficiency of the classical AF cooperative scheme can be improved. Simulation results show that the proposed scheme can achieve full diversity order in the high signal-to-noise ratio (SNR) regimes. 2) A decode-and-forward based DSTC (DF-DSTC) cooperative scheme is proposed, which adopts the same idea of AF-DSTC scheme. The upper and lower bounds of outage performance and the closed-form bit error rate (BER) expression of binary phase shift keying (BPSK) signal are derived. The theoretical results demonstrate that the full diversity order does exist in the high SNR regimes, and further validated by simulation results.For the multi-antenna destination scenario, 1) The DF-DSTC cooperative scheme under the single-antenna destination scenario is extended. Assume there exists a user that originates the cooperation, the new DF-DSTC scheme indicates that when both users fail to decode the other, the originator should retransmit its own information so that its performance is no worse than direct transmission. When M-ary phase shift keying (MPSK) constellation modulation is employed at the user and m(m≥1) antennas are configured at the base station, the upper and lower bounds of system BER performance are derived and verified by simulations. 2) A decode-and-forward based DP (DF-DP) cooperative scheme is proposed, which utilizes the multi-antenna characteristic at the destination to make the preprocessed relay signals transmitted on the shared channel resources, so that the spectral efficiency and reliability of the traditional DF cooperative scheme can be improved simultaneously. With the virtual two-input multiple-output channel decomposed into two orthogonal sub-channels in the vector space, a precoding scheme is proposed under the minimum BER criterion. Simulation results show that the proposed scheme is effective in improving the BER performance and has a low feedback overhead.2. The optimization issues of resource allocationFocusing on the cooperative partner selection and power allocation issues, the solutions to“When to cooperate”,“Whom to cooperate with”and“How to allocate the power”are presented for particular scenarios of interests.1) The optimal power allocation and partner selection algorithms for quadrature modulation cooperative schemes with fixed and selective relays are investigated. The closed-form expressions of the BER performance are derived for both cooperative schemes, in terms of the inter-user channel and uplink channel statistics and a scalable power factor. Then OPA algorithms are devised to optimize the power allocation between the local and relayed signals under the minimum BER criterion. By joint consideration of the partner selection and OPA algorithm, the partner locations specified by various cooperation gains are determined, and then a simple sub-optimal partner selection algorithm is proposed, i.e, the best partner should be located nearest to the midway between the source and the destination from the source side. Simulation results show that the proposed resource optimization algorithms are superior to the unoptimized algorithms by significantly reducing the BER and improve the cooperative gain, which is also useful to simplify the practical partner choice process.2) An optimal power allocation (EPA) algorithm is proposed for the cooperative scheme based on rotation code. The upper bound of the end-to-end system symbol error rate (SER) is first derived using the Chernoff bound, and then under the total power constraint, the power factor is obtained to minimize the SER bound according to the average channel gains. The power allocation information is transmitted to cooperative users through feedback channels, so that the system source utilization can be improved. The effectiveness of the proposed OPA algorithm is verified by simulations. Furthermore, the effect of various partner selection algorithms on the system performance are also investigated.更多还原