| Passive optical network(PON)has made its way as a prominent solution for access network applications owing to the fact that it provides high capacity with low cost.Orthogonal frequency division multiplexing(OFDM)technology in PON has become a hot spot since it has the characteristics of flexible subcarrier allocation,high spectral efficiency,and high resistance to the fiber dispersion.The inherent architecture of PON works as broadcast during downstream transmission.Therefore,considering user data during downstream transmission in the PON,it can be easily eavesdropped by an illegal optical network unit(ONU).Therefore,in order to improve the transmission security of user data,it is necessary to encrypt the transmission data.At present,the data encryption method is mainly in the medium access control layer(MAC)or above.It only encrypts the transmitted data itself,and the other information such as the data header does not adopt encryption measures,which may easily lead to leakage of user data.This unencrypted information may be stored by the illegal party in order to decrypt the user’s data.Therefore,this thesis specifically studies how to implement efficient data encryption schemes in physical-layer for optical OFDM-PON during optical fiber transmission,thereby to effectively enhance the level of security of data transmission and avoid leakage of user information.The thesis proposes some secure technologies based on dynamic quadrature amplitude modulation(QAM)mapping using digital chaos,and studies three physical-layer security schemes for enhancing optical fiber data transmission in OFDM-PON.We proposed new dynamic QAM mapping ideas of completely flexible and dynamic mapping for user data.At the same time,due to the introduction of digital chaos,the system has a huge key space,which greatly improves the security of transmitted data.Compared with the existing physical-layer data encryption schemes in OFDM-PON,digital chaos has been often used for the scrambling or permutation of OFDM symbols.The dynamic mapping schemes of the thesis are novel secure schemes which realizes the effective random diffusion and scrambling of the constellation points,based on digital chaos.The proposed dynamic mapping satisfies the basic requirements of cryptography for data scrambling and further improve the level of security in OFDM-PON.The main research achievements are listed as follows:1.Dynamic mapping for constellation shiftingFor the conventional QAM mapping method,points of all OFDM data symbols on the constellation are fixed at predetermined positions.According to this fixed mapping rule,encrypted data can only occupy specific position on the constellation,and thus cannot provide a sufficiently strong security defense.In order to overcome this shortcoming in the traditional QAM encryption scheme,we propose a new dynamic QAM mapping encryption scheme based on constellation shifting.In this scheme,the original QAM symbol can be dynamically displaced to any new location in the complex plane of the constellation.The displacement parameters are determined by a three-dimensional(3D)digital chaos.Since the dynamic displacements of the individual QAM symbols are independent of each other,the shift of these constellations actually appears as an increase in the noise of the constellation relative to the conventional QAM.For an illegal ONU,even after channel estimation,only a noisy constellation can be obtained.The displacement mapping of QAM objectively increases the dimensions of scrambled constellation points.As the dimension increases,the illegal ONU may confuse it with the higher-order QAM mapping,so the original M-QAM modulation method is safely hidden.Using 20-km long standard single-mode fiber(SSMF),this paper successfully demonstrates the encrypted transmission experiment of 9.4 Gb/s 16-QAM optical OFDM signal data.After the QAM constellation is scrambled by digital chaos,the power penalty is ~2 d B as compared to conventional QAM.Moreover,the peak to average power ratio(PAPR)analysis is also shown as a part of result to verify the power penalty.With the proposed scheme the security of the system has been greatly improved,and its key space is ~10163.2.Dimension-constrained dynamic QAM mappingBased on the results of the first solution,we propose another improved dynamic mapping scheme for QAM.The objective is to effectively control the range and region of QAM dynamic mapping,and reduce the power penalty while implementing dynamic QAM mapping,so that the data transmission performance can be improved.In this encryption scheme,the mapping of QAM symbols is done by a four-dimensional(4D)chaos,in which chaotic parameters are used to select a certain chaotic point in the traditional QAM dimension,and then the original QAM symbol is mapped to its midpoint with the selected chaotic point position.The QAM mapping rule subverts the traditional QAM mapping method and produces a noise-filled QAM constellation.Experimental analysis has shown that only legitimate ONUs with the correct key can correctly decrypt the transmitted data from the received noise constellation.For the illegal ONU,the constellation is a noisy constellation which cannot be recovered without the correct keys.Therefore,the security of the transmitted data is effectively enhanced.Finally,a successful transmission experiment of 9.4 Gb/s encrypted 16-QAM OFDM signals is demonstrated over 25-km SSMF using the above defined encryption scheme.Improved PAPR performance and transmission performance of ~1 d B when compared with our previous work is achieved and the key space provided by the system is ~10310.3.Dynamic mapping for circular QAMCircular QAM mapping,its two independent dimensions are radial and phase.For circular QAM,we also propose a corresponding dynamic circular QAM mapping technique.In this scheme,digital chaos is used to randomly and dynamically change the radius and phase values of points in the constellation.The parameters of the dynamic mapping are controlled by a 4D chaos.Compared with all previously proposed data encryption technologies,the circular QAM mapping scheme proposed here achieves the best transmission performance.The reason is that the circular QAM mapping offsets the phase of the QAM symbol,and in this scheme,the phase offset is controlled by digital chaos and has randomness,so each QAM symbol will have different Dynamic,random phase shift.These random phase shifts can improve the transmission performance of the OFDM signal.In addition,for each QAM symbol,chaotic dynamics are also applied to its radius,and the changed radius will increase or decrease the distance between QAM symbols.As the distance increases,the bit error rate(BER)performance will increase;however,the reduction in distance will also result in a decrease in BER performance.In order to balance the overall performance of the BER,we have predefined the minimum and maximum radius dimensions,so that the dynamic mapping only allows adjustment between the minimum radius and the maximum radius.Under the combination of phase shift and radius mapping,the receiver receives a noise-filled(dynamically mapped)constellation.In the OFDM-PON experiment,a 22-km long fiber was used to successfully demonstrate the encrypted transmission of 9.4 Gb/s,16-QAM optical OFDM signal data.In addition,due to the dynamic circular QAM mapping,the power penalty is only ~0.25 d B.Compared with our previous two related encryption schemes,the system transmission performance and PAPR performance has been improved.In this scheme,the multifold data encryption generates a key space of ~10340,which further enhances the security performance of the physical-layer in data transmission.In summary,this thesis proposes and verifies the secure schemes of dynamic QAM mapping in the physical-layer in OFDM-PON,to effectively improve the transmission security of optical network signals.All of the three proposed secure schemes implement a dynamic mapping of QAM constellations.In addition,we have compared the superiority and transmission performance of various encryption schemes on the basis of ensuring the security performance of the physical-layer.The secure schemes of dynamic QAM mapping based on digital chaos is expected to be applied in future OFDM-PONs to realize enhanced secure transmission of large capacity of user data. |
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