| Atmospheric wireless optical communication is realized by using laser beam. It has wide application prospects in national defense and civil applications as well because of its some significant advantages such as good confidentiality, strong anti-electromagnetic interference ability and broadband transmission. Hence, the wireless optical communication has become a significant research topic in the areas of large capacity, information security and wireless communication.However, the laser transmission process is necessarily affected by the atmospheric attenuation and turbulence, background light, and many other factors. Hence, the atmospheric channel has the fading effect, multipath interference, and other phenomena. These effects degrade in different extents the performance of atmospheric optical wireless communication links.The reasons that degrade the quality of the atmospheric wireless optical communications can be roughly categorized into two classifies. One is that the atmosphere fading reduces the loss of the transmission power. The second reason is that the degeneration of transmission beam and the inter-interference of codes degrade the link performance. It has been shown that one cannot solve the second problem only by the way of increasing launch power or receiving sensitivity. Use of anti-interference channel coding scheme is one of better solution schemes.High anti-interference channel coding technology is a hot and key research topic in atmosphere wireless optical communications.In this paper, two popular modulation schemes, OOK and PPM, are analyzed and compared in detail. In particular, their waveforms, bandwidth efficiencies, average power transmission efficiencies and slot error rates are compared point by point. The results show that the PPM is a preferred modulation scheme for high quality atmospheric wireless optical communication due to its higher average transmission power efficiency and lower slot error rate.LDPC is now a popular coding scheme. LDPC combining with efficient BP decoding has the performance of approaching Shannon information bound. Therefore it would be also very useful to combine the PPM with LDPC code for improving the atmospheric wireless optical communication system. This paper adopts this idea and addresses the method in detail.In this paper, three cases are considered and analyzed. They are weak atmospheric turbulence channel model with OOK modulation, weak atmospheric turbulence channel model with PPM modulation, and Poisson channel model with PPM modulation as well. For all the three cases, BP algorithm in probability domain and log domain as well has been researched respectively. And the communication performances in the three cases are simulated and analyzed.For studying the influence of turbulent intensity on the system performance, we simulate the system performances in different turbulence intensity cases. The simulation results show that as the turbulence intensity increases, the performance of the system gets increasingly worse, and the channel capacity becomes smaller.In addition, we have also considered the affect of channel code on the system performance. The system performance with using LDPC code is simulated and analyzed in various cases, such as different code length, different the maximum iteration numbers, different rate and different decoding algorithms. The simulation results show that LDPC code has better error correction capability, longer code and lower code rate. And the simulations also show that the performance of LDPC code cannot be improved any more if the greatest number of iterations becomes larger than a certain value.In this paper, we finally study how the slot number of PPM modulation affects on the system performance. In the cases of different time slots, the system performances are simulated. The simulations result show that with the increasing time slot, the communication system has worse performance, but higher information transmission rate. |
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