In this article we have enhanced the security of an orthogonal frequency division multiplexed passive optical network (OFDM-PON) based on four dimensional (4D) encryption, including constellation, subcarrier, symbol and time, which is proposed for the first time in this paper. 4D-hyperchaotic mapping is used to generate four masking factors to achieve ultra-high security encryption in four different dimensions. During the encryption, dimension coordination optimization is adopted, which effectively reduces the time cost of the system and improves the encryption efficiency by 3 times. At the same time, probabilistic shaping (PS) technology is used to further optimize the system that has effectively improved the bit error performance by about 1 dB. The proposed encryption technique for OFDM-PON has been demonstrated successfully with the help of experiments. The generated OFDM signal is modulated by the quadrature amplitude modulation (QAM) technique, which transmitted 16 Gb/s data rate across a 25 km fiber span of standard single-mode fiber. The values of bit error rate (BER) and peak-to-average-power ratio (PAPR) are analyzed during the experiments, and the obtained results show that the proposed security-enhanced OFDM-PON has high sensitivity and security and can be well compatible with PS and OFDM technologies. The proposed scheme has very reliable security performance and also has excellent benefit improvement, which is very promising in the future PS-OFDM-PON.

This paper proposes a high-security chaotic encrypted power sparse coding division (CE-PSCD) scheme for 7-core fiber based on non-orthogonal multiple access (NOMA) technology. The method utilizes power multiplexing to realize parallel transmission of two signals. Joint encryption of the four-dimensional region is realized using constellation mapping encryption, carrier frequency encryption, symbol scrambling, and sparse code scrambling. What we believe to be a new dimension for encryption of autonomously designed sparse codes is proposed. Meanwhile, we hide the chaotic key in training sequence (TS) to realize the co-transmission of the key and the message. A 70 Gb/s CE-PSCD signal transmission over 2 km of 7-core fiber is demonstrated experimentally. At the limit of forward error correction (FEC) ∼3.8 × 10⁻³, the difference in the encrypted sensitivity among different users at the equal power level is 0.36 dB, which means that the fairness of users will not be destroyed. The key space can reach 10¹³⁴, with a bit error rate (BER) of about 0.5 for brute-force cracking at illegal receivers. As long as the key bits in the hidden TS are wrong by one bit, the BER stays around 0.5. The results show no significant attenuation of the signal before and after encryption at either high or low power, verifying the high-security performance of our proposed scheme.

In this paper, we propose a high-security transmission scheme of power division 3-dimensional carrier-less amplitude and phase (PD-3D-CAP) based on polling-permutation encryption and noise masking key distribution. By implementing the 6D Lorenz digital chaotic system, we scramble the 3D-CAP signal, effectively enhancing the security performance of the system. In our scheme, the proposed noise-masking key distribution can be considered noise-like and transmitted along with the chaotic encrypted signal in the channel. It is important to note that our mechanism also enables uninterrupted authentication and decryption at the receiver to handle continuous key updates. We experimentally demonstrate the transmission of a 12.67Gb/s 3D-CAP signal over a 2km 7-core fiber using the proposed scheme. The scheme allows for simultaneous key transmission without a significant increase in the bit error rate. Additionally, we compare the performance of the PD-3D-CAP scheme to that of the PD-2D-CAP scheme, and observe a sensitivity gain of 0.4 dB with the proposed PD-3D-CAP scheme.

With the expansion of Information and Communication Technology, it is important to develop a communication network that can provide high-capacity ubiquitous connectivity. This work proposes an energy-efficient passive optical network (PON) using orthogonal frequency division multiple access (OFDMA) and wavelength division multiplexing (WDM) to facilitate the dense deployment of radio units (RUs) in a beyond 5G (B5G) communication network. High-speed connectivity is ensured by employing a hybrid PON architecture that includes a combination of free space optics (FSO) links and optical fiber (OF) media to carry OFDM and WDM multiplexed traffic. Furthermore, an optical frequency comb generator (OFCG) is utilized at the transmitter module to generate and leverage the spectrum for transmitting information from baseband units (BBUs) to the RUs situated near the end users. The proposed system is analyzed through (i) simulation analysis using Optisystem for transmission capacity computations and (ii) mathematical analysis to determine the total savings in energy. The simulation analysis shows that the given architecture can carry data across 3 km of FSO medium using 512 subcarriers per BBU transmitting at 10 Gbps of data with QPSK-modulated bit sequence. Additionally, energy efficiency shows that the use of an OFCG cuts the total energy usage by 22% at the transmitter module without negatively impacting the system’s high cardinality and transmission capacity.

In this paper, a new technique for generation of ultra-wide and flattened optical frequency comb (OFC) based on serial cascading of a phase modulator and dual-driven Lithium-Niobate Mach Zehnder modulator (DD-LiNbO3-MZM) is proposed. Over 60 carriers were generated by carefully adjusting the RF switching voltage (RFSV) of the DD-LiNbO3-MZM and the signal frequency of the sinusoidal wave (RF) source. A low power amplified RF source with a signal driving power of ~16.9dBm is applied, and the power of CW laser is kept at 3dBm. The frequency spacing (FS) is kept at 20GHz for generating the maximum number of carriers. Nonetheless, the scheme is also tested for the FSs of 64GHz, and 32GHz. Each scenario is examined in simulation environment and the main outcomes are highlighted. The proposed scheme is comparatively simple and the FS varies with the applied RF source on the modulators. The achieved OFC lines have a tone-to-noise ratio (TNR) of over ~45dB with an undesired side mode suppression ratio of approximately 20dB. The comb lines are almost flat with a varying power deviation of around 0dB-6dB which is optimized to nearly ~0.12dB. Furthermore, the impact of the RFSV on the generated number of carriers is studied in detail. The scheme is analyzed in terms of cost efficiency, power deviations, TNR, optical signal to noise ratio, and number of achieved comb lines.

In this paper, we propose a novel optical non-orthogonal multiple access (NOMA) scheme based on orthogonal chirp division multiplexing (OCDM) in a low-cost intensity modulation and direct detection system for passive optical networks (PON). The proposed OCDM-NOMA scheme can effectively improve spectral efficiency and serve more users by multiplexing spectrally overlaid OCDM signals in power domain. Under the advantages of the chirp spread spectrum, the proposed OCDM-NOMA scheme can also enhance the bit error rate (BER) performance compared to NOMA scheme based on orthogonal frequency division multiplexing (OFDM). A 17.8 Gb/s OCDM-NOMA PON transmission over 25 km standard single-mode fiber is experimentally demonstrated. The results show that, when compared to OFDM-NOMA and precoding OFDM-NOMA, OCDM-NOMA can achieve receiver sensitivity gains of 1.3 dB and 0.5 dB for the high-power branch signal, and 1.4 dB and 0.6 dB for the low-power branch signal at a BER of 3.8×10 ⁻³ . Furthermore, we investigate the feasibility of using higher-order modulation in NOMA technology for the first time to further improve data throughput and spectral efficiency. In a 22.2 Gb/s transmission, the optimal power division ratio is determined, and the transmission performance is investigated. According to the experimental results, OCDM-NOMA with higher-order modulation can be a promising candidate for next-generation PONs.