Chaotic Signal Scrambling For Secured Optical OFDM Transmission

Nowadays,fiber-optic communication is playing a leading role in modern high-speed data communication networks for its low loss,large capacity and robustness against interference.Among other optical communication techniques,optical orthogonal frequency-division multiplexing(OFDM)is characterized by mutually orthogonal sub-carriers.The modulated frequency spectrum of each sub-channel can overlap to decrease guard intervals and thus improve spectrum utilization in OFDM modulation format.Since discrete Fourier transform(DFT)was used to achieve multi-carrier modulation,the complexity of the system was reduced.From 2005,OFDM has been introduced into optical communication and optical OFDM has become one of the most promising technologies for next generation high-speed optical fiber communications.Meanwhile,with the increasing amount of data in communication,information security has become more and more important.Digital chaos has great practical value in secure communication due to its pseudo-randomness,autocorrelation and high sensitivity to the initial values.Optical OFDM has flexible sub-carriers allocations both in time and frequency domains,and it can be easily generated and processed by Digital Signal Processing(DSP),which make it easy to be incorporated with digital chaos to realize high secure communication.This thesis will focus on the application of digital chaos-based signal scrambling techniques in optical OFDM.Transmission experiments based on chaotic data encryption have been also demonstrated.The research work can be summarized as follows:(1)Chaos-based OFDM symbol synchronization algorithm has been studied for intensity modulation and direct detection(IM-DD)optical OFDM passive optical network(PON).Synchronization training sequence is composed by digital chaotic sequence because its pseudo-randomness and perfect autocorrelation.Using chaos-based synchronization algorithm,the computational complexity of synchronization can be reduced and the security of system can also be enhanced.Experiment was carried out to prove the feasibility of the chaotic synchronization.(2)Chaos-based data encryption technique has been studied incorporated with signal scrambling techniques in optical OFDM-PON.By employing digital chaos into the conventional selected mapping(SLM)and partial transmit sequence(PTS)algorithms,high-secured data encryption is achieved with a huge key space.Meanwhile,the peak to average power ratio(PAPR)is also reduced due to the signal scrambling,which enhances the transmission performance of OFDM signals.Besides,digital chaos provides scrambling information in SLM and PTS algorithms,which leads to less requirement of the sideband information.Successful transmission experiments have been demonstrated the feasibility of the proposed data encryption algorithms.(3)A novel data encryption algorithm against chosen-plaintext attack has been proposed and demonstrated for the first time.The algorithm is based on two-dimension time and frequency permutation in sub-carrier allocation and phase rotation of OFDM signals,which makes the ciphertext to be associated with the previous input plaintexts,therefore it can effectively resist chosen-plaintext attacks.The algorithm has also been demonstrated by experiments.Besides,if some specific sub-carriers has low signal to noise ratio,for instance under attack,the transmission performance of these sub-carriers can also be improved by using the proposed algorithm.