ITR/SI-COLLABORATIVE RESEARCH: Investigations on CDMA Systems using Multiple Level Sequences and Partitioned Reduced Complexity Multiuser Detection Receivers

自1994年第一个stis -95系统在西雅图部署以来，CDMA系统在无线通信领域取得了巨大的商业成功。事实上，未来大多数无线通信系统将采用CDMA技术。然而，为了实现CDMA在更高的系统容量、更低的误码率和更低的丢包率方面的巨大潜力，在存在多址干扰的情况下的解调问题仍然是所有CDMA系统的重要瓶颈。为了解决这种多址干扰问题，研究人员和工程师从20世纪80年代初就开始研究所谓的多用户探测器（MUD）。不幸的是，到目前为止，由于这些结构过于复杂，没有一个多用户检测器在真正的CDMA系统中实现。造成这种情况的主要因素有两个。首先，由于一些实际原因，CDMA系统必须使用长扩频序列，而多用户检测器必须使用短扩频序列。其次，即使是最简单的多用户检测器的复杂性仍然太大，无法在最快的电子设备中实现，除了最小的数据速率。在这项工作中，我们建议以合作的方式解决这两个问题。此次合作的组织方式是，密歇根州罗彻斯特市奥克兰大学的刘庆冲博士将首先专注于第一个目标，而俄亥俄州雅典市俄亥俄大学的David W. Matolak博士将首先（同时）专注于第二个目标。结果将经常交换，会议将每季度举行一次。在一些早期的进展之后，我们的合作将变得更加紧密，因为我们结合了工作目标和考虑系统（发送器和接收器）的性能。我们建议首先推广最近关于由短序列构造长扩展序列的一些结果。这可以称为系统目标。这些新序列既适用于传统的CDMA系统，也适用于多用户检测器。这种方法是由我们中的一个人发明的，并已在休斯电子公司的Spaceway系统中以400mbps的速度运行，这可能是最快的无线网络。它大大降低了接收机的复杂性和成本，并提供了本质上最佳的性能。通过在多个层面上测量序列相关性，该方法为序列设计提供了新的见解，并极大地降低了宽带无线通信信号设计和检测的复杂性。这种方法有潜力帮助弥合目前CDMA系统和多用户检测器之间的差距。我们还建议从降低复杂性网格搜索技术的角度，结合最有前途的MUD接收器结构之一，以分区的方式研究降低复杂性的多用户接收器。这可以称为接收目标。我们感兴趣的MUD接收器是最小均方误差（MMSE）接收器，它具有良好的性能和适度的复杂性。分区方法旨在在mmse前端和降低复杂度的网格处理器之间共享检测任务。这些接收器将使用指定用于未来第三代和第四代CDMA无线通信系统标准的新闻阅读序列，以及系统目标中开发的多级序列。降低复杂度的网格搜索技术将探索使用类似的技术，但不能完全应用于CDMA多用户检测问题。这两个目标的目标是创建一个基本的桥梁，将现有的和计划中的CDMA系统与理论的多用户检测器连接起来，以便克服inCDMA系统的多址干扰问题的瓶颈。这项拟议工作的两个目标都将采用分析，然后是计算机模拟。这两个目标也需要研究生和本科生的协助。建模工作旨在协调理论与实际实施，从而提供最佳意义上的工程教育：将原则与实践联系起来。培养学生在CDMA系统设计和接收机设计方面的能力自然对无线行业也很有价值。此外，研究将为两所大学的几门新的研究生课程和本科生设计项目提供材料。

CDMA systems have achieved great commercial success in wireless communications since the firstIS-95 system was deployed in Seattle in 1994. In fact, most of the future wireless communicationsystems will be using CDMA technology. Yet, as far as reaching the great potential of CDMA in termsof higher system capacity, lower bit error rate, and lower packet loss rate, the problem of demodulation inthe presence of multiple access interference remains as a significant bottleneck for all CDMA systems.To solve this multiple access interference problem, researchers and engineers have worked since the early1980's on so-called multiuser detectors (MUD). Unfortunately, thus far none of the multiuser detectorshas been implemented in a real CDMA system, because of the prohibitive complexity of these structures. There are two major factors that contribute to this situation. First, CDMA systems must use longspreading sequences for several practical reasons, whereas multiuser detectors must use short spreadingsequences. Second, the complexity of even the simplest multiuser detector is still too great to beimplemented in the fastest electronics, for all but the smallest of data rates. In this work, we propose toaddress both of these problems in a collaborative effort. The collaboration will be organized so that Dr.Qingchong Liu of Oakland University in Rochester, Michigan, will initially focus on the first objective,and Dr. David W. Matolak of Ohio University in Athens, Ohio, will initially (and concurrently) focus onthe second objective. Results will be exchanged frequently, and meetings will be held quarterly. Aftersome early progress, our collaboration will grow closer as we combine the work objectives and considersystem (transmitter and receiver) performance. We propose first to extend some recent results obtained for the construction of long spreadingsequences from short sequences. This can be termed the system objective. These new sequences willbe appropriate for both conventional CDMA systems and for multiuser detectors. This method wasinvented by one of us and has been implemented in possibly the fastest wireless network running at400Mbps in the Spaceway system by Hughes Electronics. It has significantly reduced receivercomplexity and cost, and gives essentially optimum performance. By measuring sequence correlations atmultiple levels, the method provides new insights on sequence design and tremendously reducescomplexity in signal design and detection for broadband wireless communications. This method has thepotential to help bridge a gap between current CDMA systems and multiuser detectors. We also propose to study reduced complexity multiuser receivers from the perspective of reducedcomplexity trellis search techniques, combined with one of the most promising MUD receiver structures,in a partitioned manner. This can be termed the receiver objective. The MUD receiver of interest isthe minimum mean-squared error (MMSE) receiver, which is attractive for its good performance andmodest complexity. The partitioning approach will aim to share the detection tasks between the MMSEfront-end and the reduced-complexity trellis processor. These receivers will make use of the newspreading sequences specified for use in future 3 rd and 4 th -generation CDMA wireless communicationsystem standards, and the multiple level sequences developed in the system objective. The reducedcomplexity trellis search techniques will explore use of the analogous techniques researched forequalization, but not fully applied to the problem of CDMA multiuser detection. Our goal for these twoobjectives is to create a fundamental bridge to connect the existing and planned CDMA systems with thetheoretical multiuser detectors so that the bottleneck of the multiple access interference problem inCDMA systems can be surmounted. Both objectives of this proposed work will employ analysis, followed by computer simulations.Both objectives will also require the assistance of graduate and undergraduate students. The modelingwork will aim to reconcile theory with practical implementation and thus provide engineering educationin the best sense: connecting principles and practice. Training students in both system design andreceiver design for CDMA will also naturally be valuable to the wireless industry. In addition, theresearch will provide material for inclusion in several new graduate courses, and in undergraduate designprojects, at both universities.