| Deep-space optical communication technology can significantly increase the return data capacity from deep space to Earth, it will modulate information transmited on the pulse and transmit information in the THz spectral range at high data transfer rate.To achieve high power, ultra-long-distance deep space optical communication, which requires ultra-high speed modulation, So the high rate modulation technique is one of the key to achieve deep space optical communications.This need to select the appropriate way of modulation and demodulation, The modulation mode selection is determined by the channel characteristics of the system, PPM (Pulse Position Modulation) modulation has been widely used in optical communication systems because of its high power efficiencyćhigh transmission efficiency and anti-jamming ability, etc.In this paper, Optical communication modulation techniques are analyzed and compared with the deep space optical communications link characteristics, focusing on analysis of the basic principles of PPM, the signal power spectral density (PSD), the channel capacity and the losses caused by the use of PPM.On the basis of the PPM channel characteristics, the paper studied the deep space optical PPM modulation coding and analyzed the Reed-Solomon (RS) code, serial cascade PPM (SCPPM) codes and low-density parity check (LDPC) codes, Also the performance of the PPM channel has been compared. Whereby the overall PPM modulation and demodulation system architecture are designed.The analysis focused on the working principle of the PPM modulation system and the implementation process of PWM waveform convert to PPM pulse, Also through the application of Matlab Simulink software to achieve the PPM modulation simulation, PPM modulation has a good anti-jamming capability. Finally, the circuit design of the PPM modulation hardware will achieved.This paper selects the TMS320F2812to design the modulator, which can achieve high speed modulation. |
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