Research On Two-dimensional Modulation Technology Of Space Optical Communication Based On PPM+QPSK

With the rapid development of society,the demands of people for large-capacity communication is increasing.The higher bandwidth utilization is the direction of future communication technology,which gives rise to the space optical communication.It uses laser to carry information,which propagates through the atmosphere.It combines the advantages of optical fiber communication and microwave communication,so that it has large communication capacity,fast communication rate and better anti-interference ability.Therefore,space optical communication has great potential.At present,intensity modulation is widely used in space optical communication systems,which includes OOK intensity modulation and PPM intensity modulation.It is found that PPM intensity modulation has better power utilization than OOK intensity modulation.The thesis analyzes the BER of PPM intensity modulation and OOK intensity modulation in turbulent channels,and it is found that the PPM intensity modulation has lower bit error rate.Therefore,PPM intensity modulation has better prospect for longrange space optical communication.In order to improve the band utilization of PPM intensity modulation,the phase modulation is added on the basis of PPM intensity modulation,and a two-dimensional modulation scheme based on intensity modulation and phase modulation is designed.The thesis combines the 4-PPM intensity modulation and the QPSK phase modulation,designing the two-dimensional modulation scheme of4-PPM-QPSK.Firstly,the thesis analyzes the modulation principles of 4-PPM-QPSK,and then derives the modulation constellation diagram.The power spectrum density and band utilization of 4-PPM-QPSK is theoretically researched.It is found that the band utilization of 4-PPM-QPSK is better than that of 4-PPM.The thesis also analyzes the demodulation principles of 4-PPM-QPSK,including principles of the direct detection systems and the coherent detection systems.Then,the thesis researches the bit error rate of 4-PPM-QPSK in turbulent channels.It is found that the BER of 4-PPM-QPSK is lower than that of 4-PPM in turbulent channel with the same SNR condition.Secondly,from the perspective of engineering implementation,the thesis designs the specific schemes of modulation and demodulation of 4-PPM-QPSK.Then,the thesis verifies the feasibility of the schemes by MATLAB simulation.Finally,the thesis implements the modulation and demodulation schemes of 4-PPMQPSK by FPGA and analyzes the board-level tests.The tests show that the information from the demodulation module is the same as the source.It is proved that the 4-PPMQPSK modulation and demodulation schemes designed in the thesis are effective.