FPGA-based MSK DS-SS modulator for digital satellite communications

Minimum shift keying (MSK) modulation fits in satellite communications links due to its superior performance in providing low sidelobe spectral energy and reduced sidelobe regrowth. This paper investigates design, implementation and testing of MSK modulator on FPGA. Direct sequence spread spectrum (DS-SS) modulation is also considered because it provides low power spectral density and allows accurate ranging of the satellite. Direct digital synthesis (DDS) is employed to generate the modulated signal. A novel technique for digital implementation of type II MSK modulator is shown. The MSK modulator is designed and simulated using VHDL language and then implemented on Xilinx XtremeDSP Development Kit Pro which contains Xilinx FPGA XC2VP30-4FF1152 and Analog Devices AD9772A digital to analog converters (DAC). Evaluation of the implemented modulator is done by comparing the resultant MSK modulation spectrum and the modulated waveform to the theoretical ones, which showed good performance.