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

虽然目前的无线通信系统在传输声音和简单数字数据（如电子邮件和寻呼服务）方面提供了可接受的性能，但它们不能充分支持所需的更高数据速率，例如图形丰富的多媒体应用程序或视频会议。必须克服这种吞吐量限制，以满足对即时信息访问的日益增长的需求，而不管一个人在世界各地的位置，有时被称为“全球无缝漫游”。直到最近，人们还认为无线系统中提高数据速率的主要障碍是多径——由于发射器和接收器之间的物体，信号沿着许多不同的路径散射。然而，一项引人注目的新研究表明，情况恰恰相反；特别是，如果在多路径通信链路的两端使用多个天线，并采用适当的数字“时空”编码，则可以实现吞吐量的显着增加。直观地说，这是因为每个传播路径都提供了一个单独的“独立”通道，可以通过该通道传输数据。为了利用这些空间和时间上不同的信道，在链路的接收端和发送端都需要天线阵列。这显然会导致系统复杂性的增加，并提出了当使用多个Tx和Rx （MTRx）天线时必须使用哪种类型的通信协议的问题。这些都是本研究项目要解决的问题。调查人员正在对MTRx无线通信系统的设计、实现、测试和性能分析进行全面评估。具体任务包括：(1)室内和室外无线信道的实验空时信道表征和建模——设计和实现一个灵活的、低成本的MIMO数据采集系统，能够有效地测量各种设置以及不同天线指向性和极化特性下的时空信道特性；发展统计模型，根据对观测数据的拟合提供对MIMO信道时空特征的准确描述；利用该模型评估MTRx算法开发中使用的假设的有效性，以及当模型不匹配发生时算法性能的下降。(2)分析基于理想化假设的时空算法在真实多径环境中的表现——研究不精确的已知信道如何降低性能，为了获得可接受的无错误数据传输，必须知道信道的精确程度；频率选择信道如何降低假设平坦衰落的算法性能的研究确定信道非平稳性（如多普勒）对时空算法性能的影响，以及为获得合理性能而更新信道的频率；分析了同信道干扰的影响，并建立了相应的模型。(3)开发新的、实用的、对现实信道效果具有鲁棒性的、面向多用户接入的时空处理策略——研究真实信道的时空误差控制编码策略，重点研究三维交织器设计、时空ARQ和混合ARQ以及时空编码组合；使用MTRx无线系统实现各种多址接入协议，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；开发合适的MIMO信道估计器，用于需要可靠信道状态信息的空时处理算法。

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.