Modeling and Design for the Lower Layers of 4th Generation Indoor/Outdoor Wireless Networks

虽然当前的无线通信系统为语音和诸如电子邮件和寻呼服务的简单数字数据的传输提供了可接受的性能，但是它们不能充分地支持例如图形丰富的多媒体应用或视频会议所要求的更高的数据速率。必须克服这一吞吐量限制，以满足人们日益增长的对即时信息可访问性的需求，而不管一个人在世界各地的位置，有时也被称为“全球无缝漫游”。直到最近，人们还认为无线系统中较高数据速率的主要障碍是多径--由于发送器和接收器之间的物体，信号沿许多不同路径的散射。然而，引人注目的新研究表明恰恰相反；特别是，已经证明，如果在多径丰富的通信链路的两端使用多个天线，并使用适当的数字“空时”码，则可以实现吞吐量的显著增加。直观地说，这是因为每条传播路径都提供了一个单独的“独立”通道，数据可以在该通道上传输。为了利用这些在空间和时间上不同的信道，链路的接收端和发送端都需要天线阵列。这显然导致了系统复杂性的增加，并提出了当使用多个Tx和Rx(MTRx)天线时必须使用什么类型的通信协议的问题。调查人员正在对MTRx无线通信系统的设计、实施、测试和性能分析进行彻底的评估。具体任务包括：(1)室内和室外无线信道的实验空时信道特性和建模--设计和实施灵活、低成本的MIMO数据采集系统，该系统能够有效地测量不同环境下的空间和时间信道特性以及不同的天线方向性和极化特性；开发统计模型，基于与观测数据的拟合提供对MIMO信道时空特性的准确描述；(2)分析基于理想化假设的空时算法在实际多径环境中的性能如何--调查不精确已知的信道如何降级性能，为了获得可接受的无差错的数据传输必须以多高的精度获知信道；研究频率选择性信道如何降级假定平坦衰落的算法的性能；确定诸如多普勒等信道非平稳性对空时算法性能的影响以及更新信道以获得合理性能的频率；分析同信道干扰的影响和考虑共信道干扰的模型。(3)发展新的、实用的空时处理策略，该策略对现实信道的影响是稳健的，并且面向多用户接入--研究真实信道的空时差错控制编码策略，重点是三维交织器设计、空时ARQ和混合ARQ以及空时编码组合；利用MTRx无线系统实现各种多址协议，包括对空间、时间、频率和码分多址的研究，以及基于分组的方法；开发适当的MIMO信道估计器以与需要可靠的信道状态信息的空时处理算法一起使用。

While current wireless communications systems offer acceptable performance for transmission of voice and simple digital data such as e-mail and paging services, they cannot adequately support the higher data rates required for example by graphics-rich multimedia applications or video conferencing. This throughput limitation must be overcome in order to meet the growing desire for instant information accessibility regardless of one's location around the world, sometimes referred to as "global seamless roaming". Until recently, it was believed that the primary obstacle to higher data rates in wireless systems was due to multipath -- the scattering of a signal along many different paths due to objects between the transmitter and receiver. However, remarkable new research suggests just the opposite; in particular, it has been demonstrated that if multiple antennas are used on both ends of a multipath-rich communications link, and appropriate digital "space-time" codes are employed, then dramatic increases in throughput can be achieved. Intuitively, this is because each propagation path offers a separate ``independent'' channel over which data can be transmitted. To take advantage of these spatially and temporally diverse channels, antenna arrays are required on both the receive and transmit ends of the link. This obviously leads to an increase in system complexity, and raises questions concerning what type of communication protocols must be used when multiple Tx and Rx (MTRx) antennas are employed. These are issues addressed by this research project.The investigators are conducting a thorough assessment of the design, implementation, testing, and performance analysis of MTRx wireless communications systems. Specific tasks include the following:(1)Experimental space-time channel characterization and modeling for indoor and outdoor wireless channels -- Design and implementation of a flexible, low-cost MIMO data acquisition system capable of efficiently measuring spatial and temporal channel characteristics in various settings as well as for different antenna directivity and polarization properties; Development of statistical models that provide an accurate description of the MIMO channel space-time characteristics based upon fits to the observed data; Utilization of the model in assessing the validity of the assumptions used in the development of MTRx algorithms, and the degradation in performance of the algorithms when model mismatch occurs.(2)Analysis of how space-time algorithms based on idealized assumptions fare in real multipath environments -- An investigation of how imprecisely known channels degrade performance, how accurately the channel must be known in order to obtain acceptably error-free data transfer; A study of how frequency selective channels degrade the performance of algorithms that assume flat fading; A determination of the effects of channel non-stationarities such as Doppler on space-time algorithm performance and on how often the channel be updated for reasonable performance; Analysis of the effect of co-channel interference and models for taking it into account.(3)The development of new, practical strategies for space-time processing that are robust to realistic channel effects and geared to multiple user access -- Investigation of space-time error control coding strategies for real channels, with a focus on 3-dimensional interleaver design, space-time ARQ and hybrid-ARQ, and space-time code-combining; Implementation of various multiple access protocols using MTRx wireless systems, including a study of space, time, frequency, and code division multiple access, as well as packet-based approaches; Development of appropriate MIMO channel estimators to be used with the space-time processing algorithms requiring reliable channel state information.