Communications through time-varying subspace channels

“无线革命”和不断增长的移动通信需求不断带来新的工程问题和挑战。这些挑战之一是能够在随机时变信道上传输高数据速率的通信系统的设计。在移动无线通信系统的分析和设计方面存在大量的工作。这些工作几乎总是假设信道变化相对较慢，从而可以认为信道在符号块上近似恒定。因此，现有移动通信系统的基本操作与为时不变信道设计的系统中的操作相同。诸如强大的纠错码、信道均衡器和空间或时间分集的各种方法被用于事后减轻时变信道的影响。这项研究项目正在开发新的通信系统，用于快速变化的信道，以至于传统通信系统的性能严重下降。设计广泛的数字通信系统的一个基本假设是，接收器具有估计无线电或声传播信道的能力，并在检测过程中使用该估计。在具有大的多普勒和时延扩展的随机快时变信道中，可靠的信道估计是不可能的。本课题考虑的是在信道实现未知且不可估测，且信道的统计信息已知(或可以估计)的情况下，通信系统的设计问题。因此，信道实现仅已知位于其维度为信道协方差矩阵的等级的子空间中的某个位置。单位等级的情况将这归结为一类已知的通信问题。然而，在等级大于1的情况下，需要对当前理论进行重大扩展。这一问题的提出导致了新的接收器结构和设计通信系统的新方法，这些通信系统必须在恶劣的时变环境中工作。该项目正在开发对子空间通道通信的最佳和次优解决方案，并对其性能进行分析。正在研究各种类型的通信系统，包括：通过频率选择性信道操作的单输入单输出系统、MIMO系统以及多用户和多接入系统。

The ``wireless revolution" and the ever increasing need to communicate on the move is continually bringing up new engineering problems and challenges. One of these challenges is the design of communication systems capable of delivering high data rates over random time-varying channels. An extensive body of work exists on the analysis and design of mobile wireless communication systems. Almost invariably these works assume that the channel variations are relatively slow so that the channel can be considered to be approximately constant over a block of symbols. Thus, the fundamental operation of existing mobile communication systems is the same as in systems designed for time-invariant channels. Various methods such as powerful error correcting codes, channel equalizers, and spatial or temporal diversity are used post-facto to mitigate the effects of the time-varying channel. This research project is developing novel communication systems for channels which vary so rapidly that the performance of conventional communication systems is severely degraded.A fundamental assumption underlying the design of a wide range of digital communication systems is that the receiver has the ability to estimate the radio or acoustic propagation channel, and to use that estimate in the detection process. In random rapidly time-varying channels with large Doppler and delay spreads, reliable channel estimation is not possible. This project considers the problem of designing a communication system when the channel realization is unknown and inestimable, and only the statistics of the channel are known (or can be estimated). Thus, the channel realization is only known to lie somewhere in a subspace whose dimension is the rank of the channel covariance matrix. The unit-rank case reduces this to a known class of communication problems. However, the case where the rank is greater than unity requires a significant extension of current theory. This problem formulation leads to new receiver structures and novel ways of designing communication systems which must operate in harsh time-varying environments. This project is developing optimal and sub-optimal solutions for communications through subspace channels and an analysis of their performance. Various types of communication systems are being studied including: single-input single-output systems operating through frequency selective channels, MIMO systems, and multi-user and multi-access systems.