Spread Spectrum Communication Based On Ofdm Technology Research And Implementation Of Algorithm

The modern wireless channel multi-path transmission has the features thatspectrum recourses are limited and always subject to the interference effects. In order tosolve these problems, on one hand, we need OFDM (Orthogonal Frequency DivisionMultiplexing, OFDM) technology, which has higher spectrum efficiency and anti-fadingability. On the other hand, we need spread spectrum technology, which has stronganti-interference ability and advantages of the secrecy. Thus, the combination of OFDMtechnology and spread spectrum technology is a very good option. It not only improvesanti-fading capacity of the system, but also improves anti-interference ability of thesystem. Based on the OFDM spread spectrum communication technology, this paperstudied the key algorithm of this technology, and used the SDRDP (Soft Defined RadioDevelopment Platform) which is a model-based development platform, to actualize thecommunication system on hardware level.This thesis can be divided into the following sections:The first part of this thesis analyzed several schemes of the combination ofOFDM technology and spread spectrum technology: OFDM-based frequency-domain spread spectrum technology, OFDM-based time-domain spread spectrum technology, and OFDM-based time-frequency two-dimensional spread spectrum technology. We also compared the performance of the different technology solutions.Forthe reason that OFDM-based frequency-domain spread spectrum technology is capable of resisting multipath fading and interference and low complexity, we choose this method as the subject of the research in the thesis.The second part presented the system model of OFDM-based frequency-domainspread spectrum technology. And we also investigated the key algorithm ofOFDM-based frequency-domain spread spectrum technology, and focus on the analysisof channel estimation technology. In this section, different channel estimation algorithm(such as LS algorithm, transform domain algorithm and MMSE algorithm) wereanalyzed in terms of theory, and discussed with the respect of performance andcomplexity. Although MMSE is an excellent one in the respect of performance, it cannot to be researched deeply for its huge complexity. And the performance of LS and transform domain channel estimation were simulated and analyzed. When transformdomain channel estimation resets noise to zero outside the point concentrated withsignal energy, it can greatly reduce the system noise and increase the estimatedperformance. Therefore, its performance is better than the LS channel estimation. Thispart simulated and analyzed the system performance using different interpolationalgorithms. According to the performance and complexity, the linear interpolationalgorithm is chose as systems hardware algorithm. At the same time, the performancewas simulated and analyzed in the jamming condition.The third part described the hardware implementation of the OFDM-basedfrequency-domain spectrum systems on the SDR platform. The key module oftransceiver link was given, and the most important module was tested. Meanwhile weanalyzed its property and result of the test. We concluded timing series is a major task inanalyzing system path of chain. Finally, the major difficulties in practice were discussedand the solutions are presented.Finally, hardware system performances were tested. The BER performance of thesystem with the IF and RF noise is tested. The results showed that the performance ofhardware is closed to the theoretical performance and the hardware system can achievebetter performance.