| The dissertation discusses the scheme of multirate M-ary orthogonal Spread Spectrum (SS), which attracted wide attention and interest of researchers. A new scheme of the variable-processing-gain dual M-ary orthogonal SS is proposed, and the analyses as well as simulations are presented.The main contents of the dissertation are as follows:The performance of dual M-ary orthogonal SS in the proposed system is analyzed, as compared with that of the conventional M-ary orthogonal SS. It is shown that the former is able to outperform the latter at the same transmitted data rate.Considering two main schemes for multirate now, we compare the multicode orthogonal SS scheme with variable-processing-gain one, and choose the latter as the multirate scheme in our system. According to its basic idea, PN codes of different length are adopted to map different processing gains using the control information of data rate in the transmitter, while the blind searching scheme is employed in the receiver to implement the variable processing gain.For the excellent autocorrelation and crosscorrelation characteristics, the phase-shifting sequences of m sequence are chosen as the orthogonal code, which have the constant phase each other in favor of the multipath acquisition and tracking.Based on lots of existing theoretical and experimental works published for the study of the adaptive threshold, it is improved: we present an adaptive threshold at the acquisition stage, which estimate the background power level, and another threshold adjusted in a narrow range loop at the tracking stage. In view of the idea about the multi-integral detection, protection policies of the false synchronization and nonsynchronization are adopted in the proposed system to reduce the false alarm probability greatly.The automatic carrier frequency tracking scheme is accomplished by two steps in our system. One is the frequency searching of time-frequency domain. The other is fine tuning control by frequency counter. |
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