Studying For The Modulation Coding Technology Of The Atmospheric Optical Communication

Because of atmosphere channel’s specificity and randomness in the atmosphericoptical communication system, laser signals which carry digital information are severelyattenuated by the atmospheric molecular absorption and scattering, light fluctuating thatcaused by the atmospheric turbulence occurs wavefront distortion, those factors lead tocommunication system instability, which causes high bit error rate and low reliability of thesystem. In this paper, the modulation and coding technology in the atmospheric opticalcommunication must be the appropriate modulation format encoding which can ensure hightransmission rate and low bit error rate, be better able to reduce atmospheric interferenceand improve the reliability of communication systems and overcome the randomness of thechannel.In this paper, on the basis of the characteristics of atmospheric opticalcommunication system and the feasibility of practical application, the system uses the wayof intensity modulation/direct detection. The performance of several modulation schemesare analyzed, while the communication channel model is set up with additive whiteGaussian noise and multiplicative interference of the atmospheric turbulence. the averagetransmit power, bandwidth utilization, error rate expressions of the various modulationformats are derivation and simulated. It’s indicating that the pulse position modulation(PPM) signal has optimal anti-interference performance with high power efficiency in theatmosphere compared to other modulation formats, but the PPM’s bandwidth is relativelylager. Especially the modulation order is larger, the better the performance of PPM’santi-interference, the corresponding bandwidth is also greater. In the actual communicationapplication system, using the appropriate modulation order combination of transmissionrate can guarantee the performance of PPM modulation signal.Based on theoretical analysis, this paper designed and implemented voice-sourceatmosphere communication system that used binary off keying (OOK) and4-PPM modulation and demodulation. It used field programmable logic array(FPGA) as the coremodulation encoding system and completed the voice signal modulation and demodulationcommunication system experimental hardware platform. At the same time the logic of theFPGA implementation of voice signal4-PPM’s modulation and demodulation, sending andreceiving were detailed designed, and each module’s timing simulation was completed.Experiment used the designed hardware platform and formulated a reasonable program,completed the voice of the optical signal modulation and demodulation of the4-PPMcommunication debugging and measurements to the paper’s communication system designrequirements. Compared the measured waveform of OOK and PPM signal whichtransferred in the atmosphere channel, PPM signal’s anti-interference performance arebetter than OOK. Thus the results of experiments verify the theoretical analysis of the paper,which achieved paper communication system design requirements.