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.

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In this Letter, an effective integrated waveform generation method where an orthogonal frequency division multiplexing (OFDM)-based non-orthogonal multiple access (NOMA) communication signal combined with a linear frequency modulation (LFM) radar signal is proposed and experimentally demonstrated in a W-band communication and radar detection system. The proposed method can generate communication and radar signals simultaneously. In addition, multiple user communication and multi-target radar detection can be realized. Furthermore, for the OFDM-NOMA signal, discrete Fourier transform (DFT) precoding is used to keep the optimal power ratio consistent on different subcarriers, thus, the communication performance can be effectively improved. The experimental results show that the distance estimation error is 1.2 cm for radar signals. Meanwhile, after transmission over 0.8-m free space, the total aggregate rate for two users with the OFDM-NOMA signal is 3.125 Gb/s under the bit error rate of 8.5 × 10⁻⁴.

Underwater optical wireless communication (UOWC) is of great interest to the academic and the industry community. In this paper, we propose a low-complexity and effective joint transmitter-side digital signal processing (DSP) including geometric shaping, time-domain tone reservation (TR), and clipping. The peak-to-average power ratio reduction performance and implementation complexity of the proposed time-domain TR are extensively analyzed. We then develop a real-time 2.2-Gb/s system, which is optimized for its shaping ratio and clipping ratio. With the help of the efficient DSPs, an 8-dB received optical power enhancement is realized under the bit-error-rate threshold of 3.8x10 $^3$ . We successfully demonstrate a time-multiplexed four 4K video transmission in real-time using the proposed scheme over a 3.6-m underwater and 8-m free-space channel. The implementation details, as well as the analyses of system stability, resource utilization, and latency, are presented. The results validate the feasibility and effectiveness of the proposed scheme and a 2.2-Gbit/s real-time OWC system is demonstrated for a water-air communication link.

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.

Visible light communication (VLC) with light emitting diodes (LEDs) is an emerging technology for 5G wireless communications. Recently, using complementary metal-oxide-semiconductor (CMOS) image sensor as VLC receiver is developed owing to its flexibility and low-cost. However, two illumination levels such as on-off keying (OOK) signal are used. To improve the system throughput and reduce complexity of the hardware design, in this paper, we propose and experimentally demonstrate a multilevel modulation scheme for VLC system utilizing the overlapping of two light sources for the first time, and the two light sources are modulated by an OOK and a Manchester signal respectively. At the receiver, a CMOS camera can demodulate the Manchester and the OOK signal simultaneously. Meanwhile, a low-pass filter (LPF) is used to enhance the system performance. The experimental results demonstrate that the proposed multilevel modulation scheme can achieve a net data rate of 4.32 kbit/s.