High-speed Ds Communication Systems, Modulation And Demodulation Techniques

Under high speed communication circumstance, the communication method should be efficient in spectrum utilization, and have high level of stability and security. Spread Spectrum(SS) communication is the method which can satisfy such requirements. Spread Spectrum communication is a kind of communication method with excellent performance, and has drawn much attention since it is put forward. This dissertation starts from the modulation and demodulation technologies of the digital Direct Sequence(DS) QPSK. The spread spectrum in direct sequence spread transmitter and receiver, data modulation, fast acquisition and tracking of pseudo noise (PN) sequence, carrier synchronization and data demodulation etc. are studied in this dissertation. A de-spread and demodulation scheme of DS-QPSK signal is designed. Furthermore, the theoretical analysis and performance simulation are given. The scheme is finally implemented on FPGA, and the corresponding test results verify the correctness of the scheme.The main contents of the dissertation are as follows1. Design the schemes of all digital DS-QPSK modulation and demodulation of the high speed Direct Sequence Spread Spectrum communication system。2. To satisfy the need of transmitting system, the common methods of spreading spectrum and QPSK modulation are analyzed, based on which the design and implement schemes adopted in our system are determined.3. The carrier synchronization scheme for big frequency offset is put forward, where the frequency range is shrunk step by step. Generally speaking, in the scheme FLL with good dynamic performance is used to track the residual frequency bias at the initial step when the bias is very big. Then PLL is adopted to correct the phase offset and demodulate the correct QPSK data flow when the frequency bias is reduced to some extent.4. In order to satisfy the need of fast acquisition, common synchronization methods of PN sequence are analyzed. And the fast acquisition and tracking scheme in high speed Direct Sequence Spread Spectrum communication system is presented. Considering the acquisition time and complexity of hardware, the parallel searching is adopted in PN acquisition. Then non-coherent delay lock loop aided with carrier tracking, which has good performance of high dynamic range, is chosen to realize the PN tracking.5. Through the performance testing of modulation and demodulation scheme on hardware platform, it is demonstrated that the proposed scheme and its corresponding test results can achieve all design requirements.