【作者】 高洋；

【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， 2011， 博士

【摘要】 多输入多输出(MIMO)技术能够充分利用空间资源来对抗无线信道的衰落,从而在不增加系统带宽和发送功率的情况下,可以提高了通信系统的可靠性和频谱利用率。然而在一些实际的应用场景,比如蜂窝系统的上行链路,无线自组织网络和无线传感器网络,移动终端或者网络节点受到自身体积,复杂度和功耗的限制,很难直接应用MIMO技术。于是,一种新的空间分集技术—协作分集技术应运而生。它的基本思想是参与协作通信的多个终端或者节点通过组成一种虚拟的MIMO系统来彼此共享链路资源从而获得空间分集增益。协作分集技术突破了传统MIMO技术对移动终端设备的限制,为其走向实用化提供了新思路。本论文在研究了现有分集技术和MIMO技术的基础上,提出了下述几种协作传输方案:1.差分协作传输方案首先,提出了一种采用正交幅度调制(QAM)的差分空时协作传输(DSTCT)方案。虽然在差分解调过程中需要估计信道功率(CP)和功率归一化系数(NPF),但与传统采用PSK调制的DSTCT方案相比,所提方案具有更高的信噪比(SNR)增益。仿真结果显示,在时变衰落信道下当调制阶数大于8时,DSTCT-QAM方案可以获得比DSTCT-PSK方案更优的误符号率(SER)性能。尤其是当调制阶数为64时,DSTCT- QAM方案可以获得5.35dB的SNR增益。虽然随着多普勒频率的增加,DSTCT-QAM方案的SNR增益随之下降,但只要CP的估计范围选择得合适,仍然可以获得更好的性能。其次,为了克服协作传输中的载波频率偏移,提出了一种采用QAM调制的双差分协作传输(DDCT)方案。推导了闭合的SER近似表达式, SER上界以及高SNR下的渐进近似。基于SER的渐进近似,推导了最佳功率分配(OPA)策略在高SNR下的近似解。仿真结果显示,该OPA策略在实际中等的SNR环境下仍然有效。同时比较了DDCT方案分别采用QAM调制和PSK调制的SER。仿真结果显示,当调制阶数大于8时,DDCT-QAM方案可以获得比DDCT-PSK方案更优的SER性能。尤其是当调制阶数为64时,DDCT-QAM可以获得4.5dB的SNR增益。2.差分中继选择方案研究了一种联合差分调制和中继选择(DM-RS)的多中继协作传输方案。该方案包含一个源节点, N_R个采用检测转发(DetF)协议的中继节点和一个目的节点。首先讨论了采用PSK调制的DM-RS方案,简称为DPSK-RS-DetF方案。利用中继链路的SNR近似,推导了DPSK-RS-DetF方案中断概率的闭合表达式以及高SNR下的渐进近似。该近似表达式证明DPSK-RS-DetF方案可获得的分集增益阶数为N_R+ 1。同时推导了所提方案闭合的SER近似表达式以及表达形式更简单的SER上界。最后,仿真结果验证了推导的理论结果。其次,将QAM调制应用到DM-RS方案中,提出了DQAM-RS-DetF方案。由于在差分QAM解调过程中需要估计CP和NPF,因此在不使用训练符号的前提下,提出了一种CP的估计方法和两种NPF的估计方法。同时分析了CP和NPF估计方法的均方误差(MSE),并在时变衰落信道下比较了这两种NPF估计方法的性能。另外,在假设目的端已知CP的前提下,推导了所提方案闭合的SER近似表达式。同时比较了DQAM-RS-DetF方案和DPSK-RS-DetF方案的SER性能。仿真结果显示,在时变衰落信道下当调制阶数大于8时,DQAM-RS-DetF方案可以获得比DPSK-RS-DetF方案更优的SER性能。3.准正交空时协作传输方案首先,提出了一种在MIMO中继系统中采用准正交空时分组码(QOSTBC)的协作传输方案。与传统采用正交空时分组码(OSTBC)的方案相比,所提方案可以在相同的译码复杂度下获得更高的频谱效率。其次,提出了一种基于1比特信道状态信息(CSI)反馈的自适应译码转发(ADF)协议。根据反馈的CSI,ADF协议可实现在译码转发(DF)协作传输和直接传输之间自适应地选择。再次,分析了所提方案的中断概率性能,推导了中断概率上界的闭合表达式。理论分析结果证明,所提方案可以获得全分集增益且比DF协作传输和直接传输的分集增益都高。最后,仿真结果验证了推导的理论结果。4. MIMO中继系统的发送天线选择(TAS)方案首先研究了MIMO放大转发(AF)中继系统中三种TAS方案在独立同分布(IID) Nakagami-m衰落信道下的中断概率性能,推导了中断概率闭合的渐近下界,比较了这三种方案的分集增益;其次研究了MIMO DF中继系统中三种TAS方案在IID Nakagami-m衰落信道下的中断概率性能,推导了中断概率闭合的渐近近似,比较了这三种方案的分集增益阶数。上述推导的理论结果可以应用于具有任意发送和接收天线数目的MIMO中继系统以及具有任意衰落参数( m≥0.5)的IID Nakagami-m衰落信道。更多还原

【Abstract】 Multiple input multiple output (MIMO) technology can take full advantage of spatial resources to combat wireless channel fading, therefore it can improve the reliability and frequency spectrum utilization of communication systems without additional system bandwidth and transmit power. However, owing to the limitation of mobile terminals or network nodes with regard to the size, complexity and power consumption, MIMO technology hardly can be applied in some practical situations, such as the uplink of cellular system, wireless ad hoc networks and wireless sensor networks. Hence, a new spatial diversity technology, cooperative diversity technology, has been proposed. The basic idea is that multi terminals or nodes compose a virtual MIMO system and share the link resources with each other to obtain spatial diversity gain. Cooperative diversity technology has broken the restriction of the traditional MIMO technology on mobile terminal devices and provided a new method for its utilization in engineering practice.Motivated by the previous studies on the diversity technology and MIMO technology, we propose the following cooperative transmission schemes in this dissertation.1.Differential cooperative transmission schemeA differential space-time cooperative transmission (DSTCT) scheme using quadrature amplitude modulation (QAM) has been presented. Although it is required to estimate the channel power (CP) and normalized power factor (NPF) in the differential demodulation, the proposed scheme has the advantage of signal-to-noise ratio (SNR) gain compared with the conventional DSTCT scheme using PSK modulation. Simulation results show that when the modulation order is larger than 8 in time-varying fading channels, the DSTCT-QAM scheme has better symbol error rate (SER) performance than that of the DSTCT-PSK scheme. Specially, when the modulation order is 64, the DSTCT-QAM scheme obtains a 5.35 dB SNR gain. Although the SNR gain decreases as the Doppler frequency increases, the DSTCT-QAM scheme still can achieve a better performance than the DSTCT-PSK scheme, provided the CP estimation interval is selected suitably.In addition, a double differential cooperative transmission (DDCT) scheme using QAM modulation has been proposed to overcome carrier frequency offset in cooperative transmission. A closed-form SER approximation formulation, an SER upper bound as well as an asymptotic approximation at high SNR have been derived. Based on the asymptotic approximation, an approximate solution of optimum power allocation (OPA) at high SNR has been derived. The simulation results show that the approximate solution of the OPA also provides a good solution to a realistic moderate SNR scenario. Moreover, the SER of the DDCT scheme with QAM modulation and that with PSK modulation have been compared. Simulation results show that the DDCT- QAM scheme has better SER performance than that of the DDCT-PSK scheme, when the modulation order is larger than 8. Specially, when the modulation order equals 64, the DDCT-QAM scheme obtains a 4.5dB SNR gain.2.Differential modulation and relay selection schemeThe performance of a multi-relay cooperative communication scheme with differential modulation and relay-selection (DM-RS) has been studied. The scheme consists of a source, N_R relays, and a destination, where the best relay is selected to forward the source node’s signal with detect-and-forward (DetF) protocol. The performance of the DM-RS scheme with PSK modulation (which can be called the DPSK-RS-DetF scheme) has been investigated first. In particular, a closed-form outage probability and an approximation at large SNR have been derived using the SNR approximation of the equivalent relay link. The outage probability approximation shows that the scheme can achieve a full diversity order of N_R+ 1. Furthermore, a closed-form SER approximation and an upper bound have been derived. Simulation results confirm the presented analytical results.In addition, a cooperative communication scheme using QAM modulation in the DM-RS scheme has been proposed, which can be named as the DQAM-RS-DetF scheme. Since the CP and NPF are required in the differential demodulation, one CP estimation method and two NPF estimation methods have been developed forgoing training symbols. In addition, the mean square error (MSE) of the estimation methods have been investigated, and the performances of the two NPF estimation methods have been compared in time-varying fading channels. Moreover, assuming the CP knowledge at the destination, a closed form SER approximation of the DQAM-RS-DetF scheme has been derived. Simulations results show that when the modulation order is larger than 8 in time-varying channels, the DQAM-RS-DetF scheme has better SER performance than that of the DPSK-RS-DetF scheme.3.Quasi-orthogonal space-time coded cooperative transmission schemeA cooperative transmission scheme using quasi-orthogonal space-time block codes (QOSTBC) for MIMO relay networks has been proposed. Comparing with the conventional cooperative transmission scheme using orthogonal space-time block codes (OSTBC), the proposed scheme can achieve higher bandwidth efficiency with the same decoding complexity. Moreover, an adaptive decode-and-forward (ADF) relaying protocol has been proposed based on one-bit channel state information (CSI) feedback. According to the CSI feedback, a better transmission mode can be selected adaptively between the direct transmission and decode-and-forward (DF) cooperative transmission. In addition, the outage performance of the proposed scheme has been investigated and a closed-form upper bound on the outage probability has been derived. The analytical results show that the proposed scheme can achieve a full diversity order, which is higher than that of the direct and DF cooperative transmissions. Simulation results verify the obtained analytical results.4.Transmit antenna selection (TAS) schemes in MIMO relaying systemThe outage performances of three TAS schemes in amplify-and-forward (AF) MIMO relaying over independent and identically distributed (IID) Nakagami-m fading channels are investigated. In particular, asymptotic lower bounds on the outage probabilities of the schemes are derived and the obtained diversity orders are compared. Moreover, the outage performances of three TAS schemes in DF MIMO relaying over IID Nakagami-m fading channels are investigated. In particular, the asymptotic outage probabilities of the schemes are derived and the obtained diversity orders are compared. Both of the presented expressions are general for the three TAS schemes with an arbitrary antenna configuration and a fading environment with arbitrary values of severity parameters m≥0.5. Simulation results confirm the presented analytical results.更多还原