Security enhancement for adaptive optics aided longitudinal orbital angular momentum multiplexed underwater wireless communications.

The frozen-wave-based longitudinal orbital angular momentum multiplexing (LOAMM) system developed in [IEEE Photonics J.10, 7900416 (2018)10.1109/JPHOT.2017.2778238] has the potential to overcome the crosstalk effects induced by turbulence. In this paper, we propose a defocus measurement aided adaptive optics (DMA-AO) technique for turbulence compensation in a LOAMM underwater wireless optical communication (UWOC) system to investigate the enhancement of physical layer security. Relying on a phase retrieval algorithm and probe beam, three amplitude-only measurements obtained from different back focus planes can realize phase reconstruction of distorted OAM beams. Moreover, the so-called mixture generalized gamma-Johnson SB (GJSB) distribution is proposed to characterize the probability density function (PDF) of reference-channel irradiance of OAM. The GJSB allows for obtaining closed-form and analytically tractable expression for the probability of strictly positive secrecy capacity (SPSC) in a single input single output (SISO) system. Furthermore, the average secrecy capacity (ASC) and probability of SPSC for a multiple input multiple output (MIMO) system are investigated. Compared to the traditional OAM multiplexing system based on Laguerre-Gaussian (LG) beams, the LOAMM system with a probe beam assisted DMA-AO technique has potential advantages for improving the security performance in UWOC.

Chaotic physical layer encryption scheme based on phase ambiguity for a DMT system.

Chaotic encryption is a promising scheme for physical layer security. By solving the multi-dimensional chaotic equations and transforming the obtained results, both bit-level and symbol-level encryption can be realized. One of the mainstream symbol-level encryption solutions is the constellation shifting (CS) scheme, which treats the chaotic sequence as artificial noise and adds it to the QAM signal sequence to achieve encryption. However, this scheme has several technical flaws in practical application, in terms of computational complexity and coexistence with blind equalization algorithm and the probabilistic shaping (PS) technique. In this paper, we propose a novel symbol-level encryption scheme based on phase ambiguity (PA), which converts the two sequences originally used to generate artificial noise into a set of phase rotation keys and complex conjugate keys, so that the encrypted symbols are still on the ideal constellation point coordinates. Simulation verification is carried out in a discrete multi-tone (DMT) system with 64QAM modulation. Results show that the proposed scheme can fully retain the shaping gain brought by the PS technique and avoid the error convergence of the blind equalizer. Moreover, the time required to solve the chaotic equations is only 38% of the CS scheme. Experimental verification is carried out, and the obtained results once again prove the superiority of the proposed encryption algorithm, which is a practical alternative for future physical layer secure optical communications.

Optical non-orthogonal multiple access based on amino acids and extended zigzag.

We propose a novel security-enhanced power division multiplexing (SPDM) optical non-orthogonal multiple access scheme in conjunction with seven-core optical fiber in this paper. This scheme could improve the security of data transmission at the physical layer and the split ratio of the access network, ensuring more users can be served at the same time. Additionally, multiple signals can be superimposed in the digital domain, leading to a significant improvement in spectral efficiency. We have further experimentally demonstrated the transmission of 47.25 Gb/s SPDM orthogonal frequency division multiplexing (OFDM) signals in a 2 km seven-core fiber system. The experimental results confirm that our scheme can increase the number of access users by 14 times without influencing the privacy of different users. It is worth mentioning the signal encryption method based on amino acids combine with extended zigzag is proposed for the first time as we know. Meanwhile, the key space reaches 10182, indicating that the data transmission process can be effectively protected from the attack of stealers. The proposed security-enhanced power division multiplexing space division multiplexing passive optical network (SPDM-SDM-PON) support multi-threading and multi-functions, showing a great potential to be applied in the future telecommunication systems.

Bi-directional gated recurrent unit neural network based nonlinear equalizer for coherent optical communication system.

We propose a bi-directional gated recurrent unit neural network based nonlinear equalizer (bi-GRU NLE) for coherent optical communication systems. The performance of bi-GRU NLE has been experimentally demonstrated in a 120 Gb/s 64-quadrature amplitude modulation (64-QAM) coherent optical communication system with a transmission distance of 375 km. Experimental results show that the proposed bi-GRU NLE can significantly mitigate nonlinear distortions. The Q-factors can exceed the hard-decision forward error correction (HD-FEC) limit of 8.52 dB with the aid of bi-GRU NLE, when the launched optical power is in the range of -3 dBm to 3 dBm. In addition, when the launched optical power is in the range of 0 dBm to 2 dBm, the Q-factor performances of the bi-GRU NLE and bi-directional long short-term memory neural network based nonlinear equalizer (bi-LSTM NLE) are similar, while the number of parameters of bi-GRU NLE is about 20.2% less than that of bi-LSTM NLE, the average training time of bi-GRU NLE is shorter than that of bi-LSTM NLE, the number of multiplications required for the bi-GRU NLE to equalize per symbol is about 24.5% less than that for bi-LSTM NLE.

Probabilistic shaping and forward error correction scheme employing uneven segmentation mapping for data center optical communication.

The combination of probabilistic shaping (PS) technology and forward error correction (FEC) technology can significantly boost the performance of a transmission system. In this paper, we propose a probabilistic shaping distribution matching algorithm employing uneven segmentation for data center optical networks, while keeping extremely low computational complexity for both encoding and decoding. Based on the proposed probabilistic shaping distribution matching algorithm, we develop a novel integrated scheme of PS and FEC coding that lifts the restrictions on the use of FEC technology and increases the use of interleaver. An experiment used to evaluate the probabilistically shaped data transmission is successfully conducted over a 25 km standard single-mode fiber (SSMF) with 16 quadrature amplitude modulation (16-QAM). Simultaneously, we use a simulation software to analyze the bit error rate performance at higher resolution. The results show that the joint coding scheme can achieve a 0.4dB performance improvement compared with the single FEC system.

Mitigation of crosstalk between RSB and LSB signals generated by one I/Q modulator.

Optical independent sideband (ISB) signals can be generated by exploiting one external In-phase/Quadrature (I/Q) modulator. Our theoretical analysis shows crosstalk between the two ISB (right and left side) signals can attribute to two main imperfections: amplitude difference and phase unmatched in I/Q data. To reduce the impact of crosstalk between the two ISB signals, we propose three schemes. The first is precise phase match of the I and Q data. The second has been made possible by setting different frequencies for the left sideband (LSB) and the right sideband (RSB) signals, and the last is achieved by adding Multiple-Input Multiple-Output (MIMO) equalization digital signal processing (DSP) at the receiver side. Our experimental results have shown that these schemes can improve the performance of ISB signals. In our experimental system we designed dual ISB system with different modulation formats in two sidebands. Precise phase match can bring a ∼2.2dB improvement at BER of 1×10-2 and a ∼4.3dB improvement at BER of 1×10-3 for 16-ary quadrature-amplitude-modulation (16QAM) and quadrature-phase-shift-keying (QPSK) signals, respectively, in 4Gbaud with carrier frequency of 36GHz system. The BER of 4Gbaud 16QAM ISB signal at 30GHz and 4Gbaud QPSK ISB signal at 38GHz can reach hard-decision forward-error-correction (HD-FEC) when the input power is larger than -5.5 and -7.4dBm respectively in different frequencies system. For 4Gbaud with carrier frequency of 36GHz system, the BER of 16QAM signal and QPSK signal reduce ∼2.1 and ∼2.2dB at HD-FEC after using MIMO. In addition, MIMO can further improve the performance of the matched phase system or the system with different frequencies.

Carrier phase recovery friendly probabilistic shaping scheme based on a quasi-Maxwell-Boltzmann distribution model.

A novel probabilistic shaping (PS) scheme based on the quasi-Maxwell-Boltzmann (quasi-MB) distribution model is proposed in order to solve the incompatibility between PS and carrier phase recovery (CPR) algorithms, such as blind phase search (BPS) and principal component-based phase estimation (PCPE). In the proposed quasi-MB model, the same occurrence probability is assigned to each constellation point on the same square-ring, rather than on the same circle. Signals obeying the quasi-MB model have superior CPR friendliness compared to traditional PS signals. For a PS 64 quadrature amplitude modulation system, the simulation results indicate up to 51% and 21% normalized generalized mutual information (NGMI) improvements for PCPE and BPS, respectively. Experimental verification of the proposed quasi-MB scheme was demonstrated in a 10 Gbaud coherent detection system. The results show that when the optical signal-to-noise ratio (OSNR) is low, the quasi-MB model can help the BPS algorithm to achieve better NGMI performance and, when the OSNR is high, the proposed model can also solve the incompatibility between the PCPE algorithm and PS.

Robust weighted K-means clustering algorithm for a probabilistic-shaped 64QAM coherent optical communication system.

A novel weighted K-means scheme for a probabilistic-shaped (PS) 64 quadrature amplitude modulation (QAM) signal is proposed in order to locate the decision points more accurately and enhance the robustness of clustering algorithm. By using a weighting factor following the reciprocal of Maxwell-Boltzmann distribution, the proposed algorithm can combine the advantages of PS and K-means robustly while reducing the overall computational complexity of the clustering process. Experimental verification of the proposed clustering technique was demonstrated in a 120-Gb/s probabilistic-shaped 64QAM coherent optical communication system. The results show that the proposed algorithm has outperformed K-means with respect to bit error rate (BER), clustering robustness and iteration times in both back-to-back and 375km transmission scenarios. For the back-to-back situation, the proposed algorithm is capable of achieving about 0.6dB and 1.8dB OSNR gain over K-means clustered signals and unclustered signals. For the case of transmission, the proposed clustering procedure can robustly locate the optimal decision points with launched signal power ranging from -5dBm to 5dBm, while the working range for K-means procedure is only -4dBm to 2dBm. In addition, the proposed weighted algorithm takes less iteration times than K-means to converge, especially when the signal impairments caused by fiber Kerr nonlinearity is severe.

Independent dual single-sideband vector millimeter-wave signal generation by one single I/Q modulator.

We propose a new scheme to realize the independent dual single-sideband (SSB) vector millimeter-wave (mm-wave) signal generation based on one single in-phase/quadrature (I/Q) modulator. The two SSB vector mm-wave signals can have independent carrier frequencies and modulation formats. We experimentally demonstrate the simultaneous generation and transmission of 38 GHz 16-ary quadrature-amplitude-modulation (16QAM) and 40 GHz quadrature-phase-shift-keying (QPSK) mm-wave signals based on this scheme. The penalty of the 16QAM and QPSK mm-wave signals after transmission over 10-km standard single-mode fiber and 0.5-m wireless link can be neglected.

Probabilistic shaped trellis coded modulation with generalized frequency division multiplexing for data center optical networks.

With the requirement for large capacity communication systems, the technology to close the gap between the achievable information rate and the Shannon capacity limit attracts more and more attention. In this paper, we present a novel scheme of trellis coded modulation combined with probabilistic shaping (PS-TCM) in the intensity-modulation/direct detection (IM/DD) system, using generalized frequency division multiplexing (GFDM). The principle of PS-TCM is analyzed with mutual information. We experimentally demonstrate the proposed scheme in the optical GFDM system. The results show that 4 Gb/s PS-TCM-32QAM signal achieves ∼2dB gain over the regular 16QAM signal in the back to back case. After 20 km transmission, the scheme of PS-TCM-32QAM provides 1.8 dB performance gain over that of the regular 16QAM signal. The bandwidth effect of optical filter on the performance of PS-TCM-32QAM signal is analyzed. The proposed scheme has significant gain, flexible spectrum efficiency, and approached Shannon limit, which brings a better trade-off between effectiveness and reliability performance for the multi-carrier optical system.

Photonic vector mm-wave signal generation by optical dual-SSB modulation and a single push-pull MZM.

We propose and experimentally demonstrate single-sideband (SSB) photonic vector millimeter-wave (mm-wave) signal generation enabled by optical dual-SSB modulation and a single push-pull Mach-Zehnder modulator (MZM). We use software-based digital signal processing to generate the dual-SSB driving signal for the push-pull MZM. The dual-SSB driving signal has a vector-modulated sideband and an unmodulated sideband, which are spaced by our desired mm-wave carrier frequency. After linear modulation of the push-pull MZM, optical filtering, and single-ended photodiode detection, we can get an electrical mm-wave signal displaying the same vector modulation as the aforementioned vector-modulated sideband. Based on our proposed scheme, we experimentally demonstrate the generation and transmission of a 40 GHz mm-wave signal carrying 1 Gbaud 16-ary quadrature-amplitude-modulation or 4 Gbaud quadrature-phase-shift-keying transmitter data over a 10 km single-mode fiber-28 and 0.5 m air-space link, with a bit-error-ratio less than the hard-decision forward-error-correction threshold.

Mixture-of-Gaussian clustering-based decision technique for a coherent optical communication system.

A decision technique using mixture-of-Gaussian (MoG) clustering algorithms is proposed in the context of a coherent optical communication system. For an 80-Gb/s single-carrier polarization-division multiplexed 16 quadrature amplitude modulation (QAM) transmission system at 975 km, an improvement in $Q$Q factors up to 0.41 dB is observed for the entire range of the considered optical signal-to-noise SNR. Compared with the traditional minimum Euclidean distance-based decision, the MoG clustering-based decision achieves a transmission distance increase of 175 km at a $Q$Q factor of 9.96 dB. Experiment results show that the proposed decision technique is insensitive to the system's nonlinear impairments and can effectively improve nonlinear tolerance of the system. We also propose a majorization method for decision-directed least mean square (LMS) using the MoG clustering-based decision algorithm, called MoG-LMS. The performance improvement of the MoG-LMS algorithm is verified by experiments.