ITR/SI Collaborative Research: Investigations on CDMA Systems using Multiple Level Sequences and Partitioned Reduced Complexity Multiuser Detection Receivers

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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.