Research On The Key Techniques Of FSO-OFDM

Due to the Free Space Optical(FSO) communication has the advantages of low cost, flexible networking, convenient installation and dispense with frequency permission, it has become a major technology in the present information technologies. But due to light dispersion and speed restriction of electronic devices the transfer rate is often limited and the performance is not satisfactory. In order to sufficient exploit the potentials of FSO to enhance the transfer rate and the bit error rate(BER) performance of the optical communication system, we have to adopt a new transmission technology which has high spectrum efficiency and high ability to combat the atmospheric scattering effect. Hence, the two mature technologies of OFDM and FSO are combined. But the FSO-OFDM system also has many theory and technology problems remain yet to be solved. In this paper, we focus on these problems.Firstly, the OFDM technology is introduced into FSO. The baseband model based on polarization and the frequency model based on coherent detection are proposed in this paper. The signal processing of the two models are demonstrated.Secondly, because of the baseband model can not make full use of hardware resource and the frequency model requests coherent detection, a new FSO-OFDM model based on time frequency code is proposed in order to avoid the disadvantages of the two models. This coding method is proposed by applying the space-time code to OFDM system, it can reduce the system complexity and improve the system BER performance by changing the signal structure, providing redundancy for receiving signals and combining signals at receiver.Thirdly, because of the traditional algorithm's frequency deviation estimation range is limited, the symbol timing and frequency deviation joint estimation algorithm by adopting FH conjugate symmetrical sequence is proposed on the basic of Schmidl& Cox and Minn algorithm. The timing curve of this algorithm will present a spike pulse only at the right time, the resolution is greatly enhanced, so the symbol timing will be more accurate. The estimation of frequency deviation range will be expanded with rational training sequence length. Fourthly, the energy attenuation caused by raindrop, fog drop and dust are calculate by applying the Mie theory. The symbol error rate formula of the FSO-OFDM system under these channel conditions is deduced. Then its correctness is verified through simulation. The results show that the FSO-OFDM system performance in scattering channel primarily depends on the variance of light intensity attenuation caused by scattering, and also depends on light intensity attenuation coefficient caused by scattering.Fifthly,the LMS self-adapting filter channel estimation algorithm based on time domain training sequence is putted forward. As this algorithm utilizing the feedback signals to estimate in frequency selective fading channel, so the pilot signal is unnecessary, and present higher system efficiency and better channel estimated quality.This paper makes a further research on the several key problems about the FSO-OFDM system listing as forward. And bring forward some new algorithms which achieves perfect effect testified both by theory analysis and by simulation experimentation.