Reduced-Dimension Decision Feedback Equalizers for High-Speed Wireless Digital Communications

本文主要研究动态多径传播对支持实时视频和多媒体应用的高速无线数字通信的有害影响。 数字电视（DTV）与第四代蜂窝通信系统一起沿着成为目标应用。 建筑物和其他结构的反射在重建的视频流中引起重影和其他伪像。 这些多径效应会对接收机的所有功能造成严重破坏，导致同步丢失和载波/定时恢复丢失以及符号间干扰。离散重影通常与大型平坦表面（如高大钢结构建筑物）的多径条件相关。然而，多径条件可能发生在丘陵和密集的树叶区域，信号可以通过许多路径传播到接收器。当信号从运动物体上反射回来时，会产生复杂的重影，这种重影具有时变特性，均衡器很难克服。为了适应高达40微秒的延迟扩展，当前一代的数字电视接收机芯片采用了具有100多个前馈抽头和400多个反馈抽头的判决反馈均衡器（DFE）。 这构成了大量的抽头来适应，这意味着缓慢的收敛和高计算复杂度。 未来的高速无线多媒体应用将对均衡器的性能提出更高的要求。研究人员将在降维决策反馈滤波器（DFE）领域取得根本性进展，其中包括基于一种强大的降秩自适应滤波算法（称为多级嵌套维纳滤波器（MSNWF））的降维方法。 MSNWF提供比递归最小二乘（RLS）算法更好的收敛速度，但由于MSNWF将总权重向量约束在数据自适应的低维子空间中的事实，因此计算复杂度大大降低。基于CDMA技术和欧洲GSM EDGE系统的第三代蜂窝通信中的线性均衡器。 研究人员亦会研究其他降维方法，包括利用正交频分复用系统中的信道缩短概念来减少循环前缀的长度，以及在接收端通过简单的线性滤波来压缩符号波形。此外，研究人员亦会研究在频率选择性多径条件下，当采用分组空时编码时，如何使用降维DFE来进行均衡。

ABSTRACTThe primary problem addressed is the deleterious effect of dynamicmultipath propagation on high-speed wireless digital communications supporting real-time video and multimedia applications. Digital TV (DTV) is a target application along with Fourth Generation Cellular Communication Systems. Reflections off buildings and other structures cause ghosts and other artifacts in the reconstructed video stream. These multipath effects can wreak havoc on all functions of the receiver causingloss of synchronization and loss of carrier/timing recovery as wellas inter-symbol interference. Discrete ghosts are often associated with multipath conditions of large, flat surfaces, such as tall steel buildings. However, multipath conditions can occur in areas with hills and dense foliagewhere a signal can travel through many paths to the receiver. When the signalis bounced back from a moving object, complex ghosts can arise with a time-varying nature that is very challenging for the equalizer to overcome.To accommodate delay spreads of up to 40 microseconds, current generation Digital TV receiver chips employ a Decision Feedback Equalizer (DFE) with more than 100 Feed-Forward taps and more than 400 Feed-Back taps. This constitutes a large number of taps to adapt implying slow convergence and high computational complexity. Future high-speed wireless multimedia applications will place even more stressful demands on equalizer performance. The investigator will develop fundamental advances in the area ofreduced-dimension Decision Feedback Equalizers (DFE's).These include methods for dimensionality reduction based on apowerful reduced-rank adaptive filtering algorithm referred to as the Multi-Stage Nested Wiener Filter (MSNWF). The MSNWF providesconvergence speeds better than the Recursive Least Squares (RLS) algorithm,but at a much reduced computational complexity due to the fact that that the MSNWF constrains the overall weight vector to lie in a data-adaptive, low-dimensional subspace.The MSNWF has been successfully employed to effect a reduced-rank, linear equalizer for applications in Third Generation Cellular Communicationsbased on both CDMA Technology and the European GSM EDGE System. Theinvestigator will also develop other means for dimensionalityreduction including the use of the channel shortening concept employed in OFDM systems to reduce the length of the cyclic prefix, and symbol waveform compression achieved through simple linear filtering at the receiver.In addition, the investigator will study the use of reduced-dimension DFE'sfor equalization when block space-time coding is employed under frequency selective multipath conditions.