Research On Security Of Optical Access Networks And Its Improvement Techniques

Internet is one of the most important infrastructures in the present information society. New emerging technologies, such as large data, Internet of Things (IoT) and virtual reality, are promoting the vigorous development of Internet industry. However,with the popularity and widespread use of Internet, Internet security incidents continue to increase, which have brought huge losses to the users. Optical access network is the main infrastructure for carrying the Internet in the last mile, but its broadcasting nature in the downlink makes it vulnerable to various security risks such as eavesdropping and jamming attacks from illegal users. Therefore, the study of security enhancement techniques for last-mile optical access networks is of great significance to ensure the safety of Internet. So far, there are a variety of techniques available to enhance the security of optical access networks, such as chaotic laser secure communications,optical quantum key distribution and physical-layer security technology. Owing to the merits such as flexible implementation, low complexity and high information confidentiality, physical-layer security is one of the most promising techniques that can be applied in next generation high-speed optical access networks. Since orthogonal frequency division multiplexing-based passive optical network (OFDM-PON) has the advantages such as strong tolerance to fiber dispersion, flexible resource allocation and low cost, it is one of the preferred options for optical access networks. This thesis is based on the National Natural Science Foundation of China. The aim of this thesis is to improve the security of last-mile optical access networks. Therefore, this thesis focuses on novel security enhancement techniques in OFDM-PON systems. The key contributions of this thesis are summarized as follows:(1) This thesis proposes three constellation transformation based physical-layer security enhancement techniques, including chaotic IQ encryption, chaotic constellation transformation and Brownian motion based symbol substituting. Compared with the existing security enhancement techniques reported in the literature, the proposed constellation transformation based physical layer security enhancement techniques can enhance the physical-layer security of OFDM-PON systems without degrading the transmission performance of the encrypted signal. The results show that the proposed three constellation transformation based physical-layer security enhancement techniques have different constellation scrambling degrees. The symbol substitution technique has the best scrambling performance and the highest disturbing entropy, while the constellation scrambling information entropy of the chaotic IQ encryption technique is the lowest. To simultaneously improve the security and reduce the PAPR of the transmitted OFDM signals, chaotic IQ encryption-selective mapping (IQ-SLM) and chaotic IQ encryption-optimal frame transmission (IQ-OFT) techniques are further proposed.(2) This thesis proposes three frame transformation based physical-layer security enhancement techniques, including hybrid chaotic confusion and diffusion, hybrid frequency and time domain interleaving, and Brownian motion based frame interleaving.Compared with the widely studied subcarrier scrambling techniques, the hybrid chaotic confusion and diffusion technique can simultaneously realize the confusions of symbol-to-subcarrier mappings and the diffusions of symbols. Since the hybrid frequency and time domain interleaving technique does not need to produce chaotic permutation sequences, it can avoid the permutation generation process which has relatively high complexity. The frame interleaving technique has low complexity, which does not need to produce chaotic permutation sequence. Therefore, its implementation complexity is further reduced. The results show that the proposed frame transformation based physical-layer security enhancement techniques can effectively prevent eavesdropping on transmitted data form the unauthorized users.(3) This thesis studies the key management in the physical layer, which is almost neglected in the current security enhancement technology researches. The static key management technology is suitable for key management in the physical layer of the optical access networks. The process of key management in the physical layer can be divided into three steps including key pre-sharing, key update and encryption synchronization. This thesis proposes a pilot-assisted security key agreement technique,which utilizes the pilot symbols in the channel estimation to ensure that both ends of the security negotiation agree on the same security key. Experimental results verify the feasibility of the pilot-assisted security key agreement technology, which has high security level and acceptable implementation complexity.(4) This thesis evaluates the physical-layer security techniques from three aspects:security, complexity and transmission performance. The comparison results show that the key spaces of the symbol substitution technique and frame interleaving technique are the largest, reaching 2.7 × 1091. The security enhancement technique with a larger number of the initial values and iteration parameters in the chaotic map has a larger key space. All of the proposed physical-layer security enhancement techniques can effectively resist brute force attacks from adversaries. Chaotic constellation transformation technology has the largest scrambling degree, reaching 98.50%. The time complexity of the hybrid frequency and time domain interleaving technique is the highest, reaching O(N2), while the symbol substitution technique and the frame interleaving technology have the highest key complexity. All of the security enhancement techniques proposed in this thesis can keep a relatively low peak-to-average power ratio (PAPR) of the transmitted OFDM signal. Moreover, IQ-SLM and IQ-OFT can reduce the PAPR of the transmitted OFDM signal and hence improve the transmission performance.