Study On Channel Estimation In FSO-OFDM System

OFDM is a special kind of multi-carrier transmission technology,that is concerned in the field of optical communication,in especial have been proposed as the coherent detection and direct detection of long-distance transmission.Advantages of OFDM is to improve the spectrum utilization and against frequency selective fading.Combined with FSO technology can achieve high-speed,high-quality data transmission,and higher spectral efficiency and greater system capacity.Channel estimation is one of the key technologies for FSO-OFDM system,it is very important in the overall system performance and close contact with a lot of problems.In addition,signal loss and sub-carrier frequency offset can be carried out by channel estimation compensation processing.In the paper,the analysis of the working principle and characteristics of FSO-OFDM.LS、 LMMSE、SVD-LMMSE algorithm and LS-liner interpolation and LS-cubic spline interpolation algorithm based on pilot are researched in IF coherent FSO-OFDM system model.Using MATLAB to the analysis and comparison of the performance of them,the performance of LMMSE algorithm is best,LS algorithm is the most convenient and simple.Compared with LMMSE algorithm,the performance of SVD-LMMSE algorithm is worse,however,it achieves a good compromise between the complexity and accuracy.Contrary to the problems of the sub-carrier frequency offset when the optical signal through the atmospheric channel,LS channel estimation method is given based on training sequence,the algorithm is simple and ICI can be well to reduce since the sub-carrier frequency offset generated,the error rate of the system has been greatly reduced.The estimated effects of the channel estimation method based on the pilot and the training sequence have a great relationship with the length of the CP,a reasonable CP length is critical to the system’s overall performance.In order to enhance the tolerate of optical,channel estimation method is given based on the channel coding.The channel estimation algorithms are researched about RS code、Turbo code and LDPC code.The results show that the increase of transmit information’s redundancy because of channel coding by MATLAB simulation,but the performance of the system is greatly improved.RS code is simple and mature,the performance of Turbo code is best,compared with RS code,the performance of system has been greatly improved,but its complexity is higher.Compared with Turbo code,the performance of LDPC code is slightly poor,but the complexity is greatly reduced.