Design and Performance Analysis of Transformed Filter in SMSK Spread Spectrum Modulation

The Serial Minimum Shift Keying (SMSK) modulation is easy to realize, and can meet the hardware demands of the high-data-rate Direct Sequence Spread Spectrum (DSSS) system. Thus, this method has received considerable attention. For the realization of this modulation, building an appropriate Transformed Filter (TF) becomes the crux of modulator. Based on the character of TF, three types of methods are, firstly, proposed in this paper. Butterworth filter, Chebyshev II filter, and Least Squares FIR filter are further designed using Filter Design & Analysis tools. The result of simulation indicates that Butterworth filter and Chebyshev II filter present good performances in restraining side lobes while the main lobe produced by Least-Squares FIR Filter is most close to ideal condition. Then, considering the complexity of implementing band pass filter, the low pass equivalents for TF is discussed, with presenting transverse structure method. The transmit function analysis plot of low pass filter and the spectrum of MSK indicates that this method could essentially realize the conversion of MSK signal, and the characteristics of main lobe could meet the system requirements.