Research On Multiuser Detection In Active Anti-interference Receivers

The Fourth-generation (4G) mobile technology trend is driven by the notion of designing mobile terminals which provide diversity by supporting advanced multimedia services and global compatibility by integrating various air interface standards into a single terminal. In order to achieve interconnection, co-operation and seamless communication between different terminals, active anti-interference mobile receivers based on software radio are studied in this thesis. The receivers can receive a variety of signals from different communication systems, such as DS-CDMA, MC-CDMA, OFDM and MIMO-OFDM, etc. DS-CDMA and MC-CDMA as the main researching objects are selected. CDMA is a self-interfered system. Its performance and capacity is reduced by the thermal noise, multipath fading, multiple access interference and Near-far effects. Especially when the number of users increasing, CDMA system performance is restricted by multiple access interference, which is as the main factor. Multi-carrier CDMA is a new mobile broad-band communication system, which is an integrated technology of OFDM and CDMA. It's a new multi-carrier modulation technology which own both advantages and disadvantages. In addition to multiple access interference, MC-CDMA has its own shortcomings:it is extremely sensitive to a frequency offset. So far the multi-access interference is reduced and the Near-far problems are solved by the efficient techniques for interference suppression, such as good spreading sequences, efficient power control, multi-user detection and smart antenna technology. In addition, frequency offset estimation can be used to eliminate the ICI caused by the frequency offset. The research of this thesis are surrounded the "Research and simulation on the active anti-interference mobile receivers" project. In this thesis we study on the techniques for interference suppression in the receivers.Firstly, the mathematics model of active anti-interference mobile receivers was introduced in this thesis. The fundamental principles and system models of DS-CDMA and MC-CDMA were emphasized on, then their characters were summarized, and the main interference and its causes in both systems were analyzed. Secondly, some methods used for resisting interference were introduced and compared. Then the matrix inversion in the decorrelating detector was improved, and a detailed mathematical derivation was given. The performance and computational complexity were compared between the improved algorithm and the decorrelating detection algorithm under the software MATLAB. The simulation results indicated that the performance of the improved algorithm is similar to that of decorrelating detection algorithm, but its computational complexity is much smaller than the decorrelating detection algorithm' s complexity.