摘要
在对无线通信系统的研究当中,多天线技术凭借其较高的频谱效率和多样化的实现方案,近年来引起了人们日益广泛的观注。目前单用户技术发展的已经较为成熟,而通过基站多天线来获取多用户空间复用增益正成为现在开发的热点。本文主要研究了多天线多用户系统各种有效的实现方案,并对其上/下行链路的通信性能进行了详细的分析,同时获得了以下的创新成果:
首先,针对引入多用户空间复用能力的多天线系统下行链路,提出了一类基于不同拥塞线性约束条件的预编码算法,并证明了此类算法同现有的基于迫零块对角化和最大SLR准则的线性预编码方案之间的等价性。然后,在此基础之上我们还推导出了其最优发射权值的闭式表达式,从而有效的避免了实现现有算法所必需的奇异值分解(或广义特征值分解)等运算,显著降低了系统的复杂度。此外,利用上述闭式解,还推导出了不同预编码算法接收信干噪比(信干比)的概率密度函数,进而得到了相应的性能解析式。
其次,由于发射端获得信道状态信息(CSI)的多少决定了多天线系统的通信性能,所以对于需要通过反馈来获得下行CSI的系统,我们研究了基于码书量化反馈的线性预编码方案。这里首先分析了基于用户信道方向信息的最小拥塞预编码算法,然后在此基础之上,提出了一种基于随机矢量量化码书的有限反馈预编码方案。同传统的迫零算法相比,这种基于最小拥塞的预编码算法具有明显的性能优势。不过由于存在量化噪声不匹配效应,导致了这种直接信道量化算法的性能随着信噪比的增加而出现波动,并且逐渐恶化最终趋近于与迫零体制相同。所以为了提高系统的稳定性,该章还推导出了相应的稳健预编码算法。仿真结果和分析表明,这种稳健算法在保证了相对迫零算法的优势的前提下,其性能随着信噪比的增加仍然会稳定上升,从而有效地提高了系统的稳健性。
再次,针对用户数量较多的无线网络,研究了借助用户选择技术来提取多用户分集增益的实用算法。首先分析了一种基于广义最小拥塞预编码算法的多用户选择策略,并将其同现有的迫零体制进行了比较。仿真结果表明,该方案能够获得更优的通信性能,并且在保证一定通信质量的基础上,同时支持更多的用户工作,因而更适合在大用户数量的网络中应用。然后,根据以上策略给出了与其相对应的低复杂度实现方案,以及一种基于最大信拥噪比准则的多用户快速选择算法。这两种快速算法能够在获得与穷举搜索算法相接近的性能的同时,显著的降低所需的计算复杂度,从而便于在实际的通信系统中应用。
最后,研究了采用分集技术时,多天线系统上行链路所能获得的性能。这里为了对抗日益严重的阴影衰落等因素的影响,重点考察了广义分布式天线系统(GDAS)的情况。基于该系统中信道的复合衰落分布形式复杂的特点,我们借用了一种Lognormal变量近似的方法,推导出了使用最大比合并和相干等增益合并时,分集输出信噪比的概率密度函数,并给出了高精度的中断概率和误码率的解析结果。
In the research on wireless communication systems, multiple antenna techniques have attracted extensive attention in recent years, due to their potential for high spectral efficiency and high flexibility. Although the application techniques for single user systems are well developed, it is still an open issue for exploiting the spatial multiplexing gain by using multiple antennas at the basestation (BS). In this dissertation, we investigate various realization strategies for multi-antenna multi-user systems, and analyze the uplink/downlink communication performance specificly. The main contributions of this thesis are as follows:
Firstly, for the downlink of multiple antenna systems with the ability of multiuser spatial multiplexing, a novel precoding algorithm based on a series of linear constraints on jamming is proposed, and a proof on its equivalence to the linearly precoding schemes of block diagonalization (BD) and signal to leakage ratio (SLR) maximization is also given. Besides, closed-form expressions of the optimal precoding weights of the algorithm proposed are derived, therefore resulting in the remarkable reduction on the complexity of the existed schemes, by avoiding the operations of singular value decomposition (or generalized eigenvalue decomposition). Also, using the results above, we derive the exact probability density function (PDF) of the receiving SINR (SIR), and furthermore the analytical expressions of the performance evaluation metrics.
Secondly, it is well known that the communication performance of multiple antenna systems highly depends on the amount of channel state information (CSI) known at the basestation. So, for the systems that the downlink CSI is obtained through feedback channel, we investage a precoding scheme based on the limited feedback quantization codebook. Here a minimum jamming precoding algorithm utilizing channel direction information only is derived, and then we present a limited feedback linear precoding strategy based on random vector quantization (RVQ) codebook. Compared with the conventional BD scheme, the algorithm proposed exhibits much higher performance. However, due to the effect of quantization noise mismatch, this improvement fluctuates with the increasing of the SNR, and eventually converges to the BD scheme. To eliminate the impact above, a robust precoding algorithm is also presented. Simulation results show that, this algorithm can ensure the advantage over the BD scheme, and at the same time, maintain the performance while the SNR increasing, therefore improve the robustness of the system.
Thirdly, we study the multiuser selection strategy for the downlink linearly precoding systems. Here a scheme based on the generalized minimum jamming criterion is considered, and simulation results show that it can obtain better performance compard with the BD algorithm, and more importantly, it can surport much more users at the same time (frequency) slot. Based on the selection scheme above, its fast realization versions are also proposed. The performance of the fast selection algorithms is close to that of the exhaustive search, whereas the reduction in the system complexity is quite large.
Lastly, we analyze the uplink performance of multiple antenna systems using receiver diversity techniques. Especially, we focus on the situation of the generalized distributed antenna system (GDAS), which can significantly reduce the impact of shadowing in the wireless fading channel. Because of the mathematical complexity of the composite fading in GDAS, here an approximation method is used to derive the PDF of the combining output SNR, and the high precise analytical expressions of outage probability and bit error ratio are also presented.
引文
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