Research On Underwater Optical Communication Based On LDPC And PPM

Underwater wireless optical communication is important for human to develop and utilize the ocean marine resources and attracts a wide spread attention because of its advantages,for example,the high transmission speed,the wide bandwidth,small volume and high security.Channel coding and modulation technology is one of the key technologies in underwater optical communication and this thesis mainly studies on the the LDPC code and PPM modulation.In this thesis,the MATLAB simulation is used to analyze the performance of underwater optical communication system based on LDPC and PPM and a simplified soft modulation method which is suitable for underwater channer is proposed.First of all,the thesis introduces the basic knowledge of LDPC code,including the definition and Tanner graph of LDPC,the construction of sparse check matrix and the encoding and decoding methods.By the simulation,it can be found that the performance of LDPC code without the circle whose length equals 4 is better than the LDPC code with the circle whose length equals 4.Besides,three kinds of BP decoding algorithms are compared in thesis.Secondly,the thesis introduces the PPM emphatically and compares the performance of OOK modulation,PPM modulation and DPIM modulation in following aspects respectively: the average transmitting power,bandwidth efficiency and slot error rate.The results indicated that the PPM is best for the underwater optical communication.In addition,based on the simulation of OOK moudulation and PPM modulation,it can be found that PPM performs better than OOK and the PPM soft demodulation is superior to hard demodulation.Then,the underwater optical communication system model base on LDPC code and PPM modulation is proposed,and the system performance is analyzed in various cases,such as different code length and degree distribution,different iteration times,different rate and different PPM orders.All of the simulations indicate the path for the system’s improvement.At last,considering the underwater channel is complicated and unpredictable,a simplified soft demodulation method is proposed to reduce the complexity of the algorithm and the amount of calculation which is convenient for hardware implementation.What’s more,it has a wide range of applications because it does not need the detailed features of channel.By the simulation,it turned out that that the simplified system has a good performance in low PPM order,with bigger code gain than RS code.Howerver,when the PPM order is high,the simplification cause a greate loss of code gain,the simplified soft modulation method is not suggested to be used and the method need a further improvement.