The physical layer security of radio-over-fiber (RoF) system is a very important problem for future communication. In this paper, a novel probabilistic shaping (PS) based constellation encryption scheme is proposed in which two bit-level encryption operations are firstly performed according to chaotic sequences and hash values. The chaotic sequences are generated by hyperchaotic system and hash values are obtained by SHA-512. Then PS is applied to enhance transmission performance. After PS-16-quadrature amplitude modulation (QAM), constellation encryption is implemented aiming at maintaining overall shaping distribution unchanged and improving security. An encrypted PS-16-QAM orthogonal frequency division multiplexing (OFDM) signal is successfully transmitted over 50 km standard single-mode fiber (SSMF) and 5 m wireless channel in our experiment. The results demonstrate that the key space of 10¹²¹ is achieved to defend malicious attacks. Moreover, the proposed PS-based encryption scheme can obtain approximately 2.4 dB gain at a BER of 10⁻³ compared with traditional OFDM signal. Thus, the proposed scheme has a good application prospect in the future OFDM-RoF system due to the dominant BER and security performance.

This paper proposes a novel hybrid secure method based on improved deoxyribonucleic acid (DNA) encoding encryption and spiral scrambling in chaotic OFDM-PON for enhancing the physical-layer security. In the improved DNA encoding encryption, the odd-even cross-bit DNA encoding, base scrambling and base-level substitution are determined by chaotic sequences. For each binary and base, the selected encoding rules and base scrambling methods are dynamically changing, which enhances the robustness against malicious attacks by attackers. In the spiral scrambling process, the QAM symbol matrix is divided into several blocks, and these blocks are scrambled. In the scrambling of the plural matrix, the position where the spiral starts and the orientation and direction of the traversal are also controlled by the chaotic sequences. By employing DNA encoding encryption and spiral scrambling, a key space of ~10 $^135$ can be achieved in the multi-fold encryption of the proposed scheme, which can improve the physical-layer security. The encrypted 16-QAM OFDM data are successfully transmitted over a 60-km SSMF in OFDM-PON. The simulation results demonstrate that it has better BER performance at the BER of 10 $^3$ than other schemes. The proposed encryption method can effectively protect data from attacks by eavesdroppers or illegal users.

A secure transmission scheme of a W-band millimeter-wave radio-over-fiber (RoF) system based on cellular neural network (CNN) and dynamic resource allocation is proposed, in which 7-D CNN chaotic system is composed to heighten the anti-deciphering ability of a secure system, and the spectrum resources are optimized by dynamic allocation. The analyses show that the key space of our scheme can reach approximately 10 112 . The encrypted W-band millimeter-wave signal is successfully transmitted in the filter-OFDM RoF system of 50-km SSMF and 5 m wireless channel, and the experimental results illustrate that our proposed scheme can enhance the bit error ratio performance by approximately 0.3 dB compared with the traditional one.

Chaotic encryption is a potential scheme for the communication security of radio-over-fiber (RoF) system. In this paper, a one-time pad strategy combined with dynamic key embedding and multi-level chaotic encryption is experimentally demonstrated for RoF system. The keys are divided into dynamic and static key. The static key is randomly selected and encrypt dynamic key which is extracted according to the transmission information. The dynamic key is employed in polar coding encryption (bit- level) and static key is employed in dynamic key embedding, constellation masking (symbol-level) and chaotic synchronization. The receiver selects the required static key according to the synchronization information and recovers the dynamic key from the received data, which realizes the one-time pad strategy and greatly enhances the security of the system. Transmission experiments are successfully conducted over 50 km standard single-mode fiber (SSMF) and 5 m wireless channel. Compared with the traditional OFDM, the experimental results show the proposed scheme adopting successive cancellation list 8 (SCL8) decoder can acquire ∼2.1 dB gain at the BER of 10 ⁻³ . In addition, experiments verify the influence of code length, code rate and different modulation formats, which further explains the feasibility of the scheme. Experiments show that the proposed scheme has great practical potential for future optical wireless secure communication.

The cross-domain image captioning, which is trained on a source domain and generalized to other domains, usually faces the large domain shift problem. Although prior work has attempted to leverage both paired source and unpaired target data to minimize this shift, the performance is still unsatisfactory. One main reason lies in the large discrepancy in language expression between two domains, where diverse language styles are adopted to describe an image from different views, resulting in different semantic descriptions for an image. To tackle this problem, this paper proposes a Style-based Cross-domain Image Captioner (SCIC) which incorporates the discriminative style information into the encoder-decoder framework, and interprets an image as a special sentence according to external style instructions. Technically, we design a novel “Instruction-based LSTM”, which adds the instruct gate to collect a style instruction, and then outputs a specified format according to that instruction. Two objectives are designed to train I-LSTM: 1) generating correct image descriptions and 2) generating correct styles, thus the model is expected to accurately capture the semantic meanings of an image by the special caption as well as understand the syntactic structure of the caption. We use MS-COCO as the source domain, and Oxford-102, CUB-200, Flickr30k as the target domains. Experimental results demonstrate that our model consistently outperforms the previous methods, and the style information incorporating with I-LSTM significantly improves the performance, with 5% CIDEr improvements at least on all datasets.

The physical layer security of millimeter-wave radio-over-fiber (MMW-RoF) system at W-band is a promising topic. This paper experimentally demonstrates a one-time pad scheme based on polar code and orthogonal frequency division multiplexing (OFDM) for MMW-RoF system. In the proposed scheme, the initial key is obtained by looking-up table mapping randomly, and chaotic sequence is generated by using 4-D cellular neural network (CNN). The key for next frame is placed in the partial frozen bits of polar code, which can realize the one-time pad mechanism. The encrypted OFDM signal carried by 100GHz millimeter-wave is successfully transmitted over 50 km standard single-mode fiber (SSMF) and 5 m wireless channel in this experiment. The experimental results show that, compared with the traditional OFDM signal, the proposed scheme in SCL8 decoder can achieve ∼4.1 dB gain at the BER of 10⁻³, which can effectively use the frozen bits of polar code to transmit more information and improve the security and reliability of the whole system.