In this paper, we propose a key space enhanced chaos-based encryption scheme with no requirement of redundant sideband information in an orthogonal frequency division multiplexing passive optical network (OFDM-PON). For the first time, a simple 1-D chaotic logistic map is employed to scramble the chaotic subcarrier allocation both in time/ frequency domain and logical operation, which provides a huge key space to enhance the physical layer confidentiality.
Due to the dynamic chaotic permutations, the largest key space is achieved in comparison with the reported encryption schemes in OFDM-PON. In addition, a ∼10-Gb/s secure transmission with encrypted OFDM data is experimentally demonstrated over 25-km standard single-mode fiber. The proposed encryption scheme demonstrates the excellent security of data transmission in OFDM-PON and the robustness against exhaustive attacks.
The schematic diagram of the proposed encrypted scheme is plotted in Fig. 1, which is used among optical line terminal (OLT) and ONUs in OFDM-PON. At the transmitter, the pseudo-random binary sequence (PRBS) is injected into the logic model for realizing XOR operation with the chaotic sequence hence obtaining the first-fold encryption. Then, after the serial-to-parallel conversation (S/P), the encrypted data is mapped onto the QAM subcarriers, and is sent for multi-fold encryption permutations time/frequency domain.
The specific sorting process for Sort(X(i)) can be furthermore explained as in Fig. 3, which is based on quick sorting algorithm. The basic idea following as: For each row, pick up an element as pivot, and then execute partition arrangement to realize that all elements smaller than the pivot would be moved to the front of pivot, while all elements bigger than the pivot would be moved to the behind of pivot. After the partition arrangement, the pivot is put in the final exchanged position.
Following the configurations of Fig. 5, we conduct the encrypted OFDM-PON experiment, in which two ONUs are used to emulate the illegal and legal users respectively. At the OLT, all the encrypted OFDM signals are generated offline by MATLAB programs. The IFFT points of the OFDM symbols is set to 256, of which 64 sub-carriers are used to load 16-QAM mapped data and another 64 complex conjugate data of 16-QAM are chosen to be mapped into other subcarriers.
RESULTS AND FURTHER DISCUSSION
Firstly, to qualitatively evaluate our proposed scheme, classical image used in image processing system is encrypted for testing the secrecy of IMDD-based OFDM-PON. As shown in Fig. 6, we give the image, as well its histograms and Welch’s power spectral density for the original encrypted image data. From Fig. 6(a), it is easily got that, due to the roughly histogram distribution feature of original data, the fluctuant power spectral density is obviously observed. And, in the middle of Fig. 6(a), the uneven lines in graph represent the irregular gray-level value information.
Moreover, we also measure the bit error ratio (BER) performance for two ONUs with back-to-back (BtB) and over 25-km SMF transmission in OFDM-PON. The corresponding results are plotted in Fig. 8. Since no BER difference for each ONU within the encrypted and non-encryption cases, we only select the sensitivity under the worst case to analyze. As for the legitimate ONUs, the original data can be correctly decrypted with only ∼0.7-dB power penalty (BER@10−3) for BtB and 25-km SMF transmission.
We demonstrate a physical layer security improved scheme in OFDM-PON. For the first time, by the completed chaotic subcarrier allocation scrambling in both time and frequency domain and chaotic logical operation, the optical OFDM data can realize the encryption. In the encrypted process, the key space can achieve significantly enlarge, and it would increase with the increase of OFDM subcarriers (M) and symbols (N). As for the case of N = M = 64, a large key space of ∼10,287 is created, which is verified that it is higher than current encryption schemes in OFDM-PON, leading to the increasing security for system.
In this scheme, only simple 1-D logistic map is employed to generate the chaotic code and permutation matrix. Moreover, a ∼10-Gb/s secure transmission with encrypted OFDM data is experimentally demonstrated over 25-km SMF, which verifies the feasibility of our scheme. A trail time of 10 266 years and BER of 0.5 are obtained for the illegal ONUs with scrambling matrix of 64 × 64, which provides a promising solution of encryption transmission in future OFDM-PONs.
Source: Hangzhou Dianzi University
Authors: Meihua Bi | Xiaosong Fu | Xuefang Zhou | Lu Zhang | Guowei Yang | XueLin Yang | Shilin Xiao | Weisheng Hu