Weak Ldpc Codes Applied Research In Atmospheric Laser Communication System In Turbulent Circumstances

The atmospheric laser communication with the laser beam transmitting in the space as its carrier has the respect advantages of optical communication and wireless communication. It has a huge information capability, narrow transmitting beam, excellent orientation, small aerial size and vigorous capability of anti-electromagnetism interference. Atmospheric laser communication is free of application for permit of frequency resource, has the features of low expenses in setup and maintenance of the network and quick setting up of the loop. It is free of loop lease expenses. Atmospheric laser communication shows a great expectation. All of the main countries in the world are researching expressly on it, hoping to climb to the summit of atmospheric laser communication technology.The channel of atmospheric laser communication is the free space. In theory, can improve the decline of transmission power to overcome, but in practice high-power, high-speed rate of the laser to create a very difficult and not economic. Therefore, channel coding technology used in the study of atmospheric laser communication has been given due attention. Especially , the research, development of high anti-interference performance, high-speed coding and modulation rate of atmospheric laser communication technology as an important topic facing. Some discrepancy in the lower SNR range however coincides with the exact PEP for high SNR.In this paper, LDPC (Low-Density Parity-Check Codes, LDPC) code to improve the performance of atmospheric laser communication link. LDPC codes are linear block codes, so the principle of LDPC codes encoding is the same with linear block codes, the ordinary encoding method of LDPC codes is developed under the principle. But the ordinary encoding method has high encoding complexity and time-delay problems(especially the length of the codes is very large). A novel encoding method--approximate downer triangular (ADT) form encoding method can solve these problems, many simulation results conducted in the additive white Gaussian noise (AWGN) channel show that the performance is almost the same as the ordinary encoding method. LDPC code decoding means algorithm currently has the classic soft-decision belief propagation algorithm (Belief Propagation, BP) and so on, the algorithm uses the transmission of information to carry out iterative decoding, simulation results show that, LDPC codes decoding using BP algorithm has a good time performance, but the lack of rigorous theoretical basis. For the above problem, this paper make cooperative optimization algorithm applied to the LDPC decoding. Simulation results show that, it can achieve good coding gains over the classical belief propagation algorithm; the decoding complexity of the proposed algorithm is low, furthermore, the former don't appear error floor.Different kinds of LDPC codes are studied by using OOK modulation under weak turbulence and the former's coding/decoding algorithm. The simulation results disclose that LDPC codes have a good capability of correcting bursty errors and can greatly improve the reliability of the communication system. Using LDPC codes can guarantee its realization.