Research On Modulation And Demodulation For Laser Communication In Atmosphere

Atmospheric laser communication has been widely used because of its wide bandwidth,high transmission rate,strong anti-jamming ability,good security and economic practicability.And atmospheric laser communication commonly adopts intensity modulation direct detection(IM/DD)system.On-Off Keying(OOK),Pulse Position Modulation(PPM)and Digital Pulse Interval Modulation(DPIM)have been used most as the classical modulation method in IM/DD system.In view of low bandwidth utilization and frame synchronization of PPM and unfixed symbol length of DPIM,many kinds of modulation methods were proposed.Fixed Length Digital Pulse Interval Modulation(FDPIM)is based on the advantages of PPM and DPIM,which can solve the problem that PPM needs frame synchronization and DPIM symbol length is not fixed,but has the disadvantage of low bandwidth utilization and low channel capacity.Based on the research status of modulation and demodulation technology for laser communication in atmospheric,the main work of this paper are as follows:1.We propose a new modulation method--Fixed Symmetrical Pulse Interval Modulation(FSPIM),which is suitable for atmospheric laser communication and is easy to realize,to solve the problem of the low bandwidth utilization and low channel capacity of FDPIM.The symbol structure and principle of FSPIM modulation are introduced in detail.The main principle is to reduce the transmission bandwidth by using symmetric slot mapping,and to realize the distinction of mapping methods and symbol synchronization by adding an identified pulse.2.The modulation and demodulation system of FSPIM based on FPGA is proposed and is discussed in detail in this paper.We realize the system of FSPIM by VHDL language under Quartus II 9.1.The feasibility and practicability of the proposed FSPIM modulation method in atmospheric laser communication system is further verified.3.The symbol length,average transmit power,bandwidth requirement of FSPIM are elaborated,formula derived and numerical simulated.We compare FSPIM with other six modulations methods to analyze the advantages and disadvantages of FSPIM modulation.The result shows that FSPIM modulation has the advantages of bandwidth efficiency.Meanwhile it has fixed symbol length and frame synchronization function.The defect of FSPIM is the average emission power is higher which is still far better than OOK modulation.