SC and OFDM hybrid coherent optical transmission scheme based on 1-bit bandpass delta-sigma modulation.

High-order quadrature amplitude modulation (QAM) can effectively improve the capacity and spectral efficiency of coherent optical transmission systems. However, as the modulation order increases, the signal becomes less tolerant to noise and nonlinear effects during transmission, and the implementation cost also increases. We propose a single carrier (SC) and orthogonal frequency division multiplexing (OFDM) hybrid coherent optical transmission scheme based on a 1-bit bandpass (BP) delta-sigma modulation (DSM). The driving I-channel and Q-channel signals for the optical in-phase/quadrature (I/Q) modulator carry SC-modulated and OFDM-modulated transmitter data, respectively. Optical quadrature-phase-shift-keying (QPSK) modulation is realized by the 1-bit DSM quantizer and I/Q modulator, which can effectively suppress quantization noise and reduce the complexity of digital signal processing (DSP) and the performance requirements of optoelectronic devices. In addition, the hybrid transmission of SC and OFDM can balance the advantages of both to meet the variable channel conditions and complex application scenarios. High-fidelity transmission of SC 512QAM and OFDM 512QAM hybrid signals, in the form of a 60 Gbaud optical QPSK signal, over 60 km single-mode fiber-28 (SMF-28) is verified by offline experiments, and the bit error rates (BERs) of both SC 512QAM and OFDM 512QAM are below the hard-decision forward-error correction (HD-FEC) threshold of 3.8e-3.

Simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals enabled by bandpass delta-sigma modulation.

The application of dual vector millimeter-wave (mm-wave) signals in radio-over-fiber (RoF) systems represents a significant opportunity to enhance spectrum efficiency, transmission capacity, and access flexibility. In addition, facing the increasingly intricate application scenarios, the comprehensive exploitation of high-order quadrature-amplitude-modulation (QAM) signals with hybrid single-carrier (SC) and orthogonal-frequency-division-multiplexing (OFDM) modulation is also vital to rich systematic connotation. Based on bandpass delta-sigma modulation (BP-DSM) and heterodyne detection, we propose what we believe to be a novel scheme for the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals. The innovation lies in the modulation-agnostic nature, accommodating both SC-modulated and OFDM-modulated vector radio-frequency (RF) signals. The BP-DSM is utilized to digitize two independent SC-modulated and OFDM-modulated high-order QAM signals into relatively simple sequences at the transmitter side. With the aid of an optical I/Q modulator, we can integrate both signals after BP-DSM to generate the desired optical quadrature-phase-shift keying (QPSK) signal carrying both information of two original high-order QAM signals. Facilitated by heterodyne detection and a single photodetector (PD), our scheme attains prowess in the detection of both SC-modulated and OFDM-modulated high-order signals. Based on our proposed scheme, we experimentally demonstrate the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated 512QAM signals at 30-GHz mm-wave band, demonstrating bit-error-rates (BERs) below the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 after transmission over 10-km single-mode fiber (SMF) link and 1-m wireless link. In addition, we further investigate the performance impact between SC-modulated and OFDM-modulated high-order QAM signals, and experiment results indicate that the impact is virtually negligible. Moreover, the performance of the generated QPSK mm-wave signal is transparent to the QAM modulation formats of both SC-modulated and OFDM-modulated signals in our proposed scheme.

Hybrid transmission of PS-GS4QAM and QPSK data in the form of a PS-16QAM mm-wave signal enabled by optical asymmetrical dual-SSB modulation.

The independent optical dual-single-sideband (dual-SSB) signal generation and detection can be achieved by an optical in-phase/quadrature (I/Q) modulator and one single photodiode (PD). The dual-SSB signal is able to carry two different information. After PD detection, the optical dual-SSB signal can be converted into an electrical millimeter-wave (mm-wave) signal. Therefore, the optical dual-SSB signal generation and detection technique can be employed in the radio-over-fiber (RoF) system to achieve higher system spectral efficiency and reduce system architecture complexity. However, the I/Q modulator's nonideal property results in the amplitude imbalance of the optical dual-SSB signal, and then the crosstalk can occur. Moreover, after PD detection, the generated mm-wave signal based on the optical dual-SSB modulation has a relatively low signal-to-noise ratio (SNR), which restricts the system performance. In this paper, we propose an optical asymmetrical dual-SSB signal generation and detection scheme based on the probabilistic shaping (PS) technology, to decrease the influence of the optical dual-SSB signal's amplitude imbalance and to enhance the system performance in the scenario of the limited SNR. The dual-SSB in our scheme is composed of the left sideband (LSB) in probabilistic-shaping geometric-shaping 4-ary quadrature amplitude modulation (PS-GS4QAM) format and the right sideband (RSB) in quadrature phase-shift keying (QPSK) format. The transmitter digital signal processing (DSP) generates a dual-SSB signal to drive the optical I/Q modulator. The I/Q modulator implements an electrical-to-optical conversion and generates an optical dual-SSB signal. After PD detection, the optical dual-SSB signal is converted into a PS-16QAM mm-wave signal. In our simulation, compared with the normal 16QAM scenario, the PS-16QAM scenario exhibits a ∼1.2 dB receiver sensitivity improvement at the hard-decision forward error correction (HD-FEC) threshold of 3.8×10-3. Therefore, in our experiment, based on the PS technology, we design a dual-SSB signal including a 5 Gbaud LSB-PS-GS4QAM at -15 GHz and a 5 Gbaud RSB-QPSK at 20 GHz. After 5 km standard single-mode fiber (SSMF) transmission and PD detection, the dual-SSB signal is converted into a 5 Gbaud PS-16QAM mm-wave signal at 35 GHz. Then, the generated PS-16QAM signal is sent into a 1.2 m single-input-single-output (SISO) wireless link. In the DSP at the receiver end, the dual-SSB signal can be recovered from the mm-wave signal, and the PS-GS4QAM and QPSK data carried by the dual-SSB signal can be separated. The bit error rates (BERs) of the LSB-PS-GS4QAM and the RSB-QPSK in our experiment can be below the HD-FEC threshold of 3.8×10-3. The results demonstrate that our scheme can tolerate the I/Q modulator's nonideal property and performs well in the scenario of a relatively low SNR.

End-to-end learning strategy based on a frequency domain feature decoupling network emulator with joint probabilistic shaping and equalization for a 300-Gbit/s OAM mode division multiplexing transmission.

Mode coupling and device nonlinear impairment appear to be a long-standing challenge in the orbital angular momentum (OAM) mode division multiplexing (MDM) of intensity modulation direct detection (IM/DD) transmission systems. In this paper, we propose an end-to-end (E2E) learning strategy based on a frequency domain feature decoupling network (FDFDnet) emulator with joint probabilistic shaping (PS) and equalization for an OAM-MDM IM/DD transmission with three modes. Our FDFDnet emulator can accurately build a complex nonlinear model of an OAM-MDM system by separating the signal into features from different frequency domains. Furthermore, a FDFDnet-based E2E strategy for joint PS and equalization is presented with the aim of compensating the signal impairment for the OAM-MDM IM/DD system. An experiment is carried out on a 300 Gbit/s carrierless amplitude phase-32 (CAP-32) signal with three OAM modes over a 10 km ring-core fiber transmission, and the results show that the proposed FDFDnet emulator outperforms the traditional CGAN emulator, with improvements in the modelling accuracy of 30.8%, 26.3% and 31% for the three OAM modes. Moreover, the receiver sensitivity of the proposed E2E learning strategy is higher than for the CGAN emulator by 3, 2.5, 2.2 dBm and the real channel by 5.5, 5.1, and 5.3 dBm for the three OAM modes, respectively. Our experimental results demonstrate that the proposed FDFDnet emulator-based E2E learning strategy is a promising contender for achieving ultra-high-capacity interconnectivity between data centers.

Dual high-order QAM-modulated mm-wave signal transmission in the Q-band enabled by simple IM/DD architecture and bandpass delta-sigma modulation.

The intensity-modulation (IM)/direct-detection (DD) systems have been proven effective and low-cost due to their simple system architecture. However, the Mach-Zehnder modulator (MZM) of the IM/DD systems only reserves its driving signal intensity. Therefore, the IM/DD systems are generally unable to transmit vector signals and have a restricted spectrum efficiency and channel capacity. Similarly, the radio-over-fiber (RoF) transmission systems based on IM/DD are limited by their simple architecture and generally cannot transmit high-order quadrature amplitude modulation (QAM) signals, which hinders the improvement of their spectrum efficiency. To address the challenges, we propose a novel, to the best of our knowledge, scheme to simultaneously transmit the dual independent high-order QAM-modulated millimeter-wave (mm-wave) signals in the RoF system with a simple IM/DD architecture, enabled by precoding-based optical carrier suppression (OCS) modulation and bandpass delta-sigma modulation (BP-DSM). The dual independent signals can carry different information, which increases channel capacity and improves spectrum efficiency and system flexibility. Based on our proposed scheme, we experimentally demonstrate the dual 512-QAM mm-wave signal transmission in the Q-band (33-50 GHz) under three different scenarios: 1) dual single-carrier (SC) signal transmission, 2) dual orthogonal-frequency-division-multiplexing (OFDM) signal transmission, and 3) hybrid SC and OFDM signal transmission. We achieve high-fidelity transmission of dual 512-QAM vector signals over a 5 km single-mode fiber (SMF) and a 1-m single-input single-output (SISO) wireless link operating in the Q-band, with the bit error rates (BERs) of all three scenarios below the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. To the best of our knowledge, this is the first time dual high-order QAM-modulated mm-wave signal transmission has been achieved in a RoF system with a simple IM/DD architecture.

Dual vector millimeter-wave signal generation based on optical carrier suppression modulation and direct detection with one photodetector.

We propose a novel, to the best of our knowledge, scheme for dual vector millimeter-wave (mm-wave) signal generation and transmission, based on optical carrier suppression (OCS) modulation, precoding, and direct detection by a single-ended photodiode (PD). At the transmitter side, two independent vector radio frequency (RF) signals with precoding, generated via digital signal processing (DSP), are used to drive an in-phase/quadrature (I/Q) modulator operating at the optical OCS modulation mode to simultaneously generate two independent frequency-doubling optical vector mm-wave signals, which can reduce the bandwidth requirement of transmitter's components and enhance spectral efficiency. With the aid of the single-ended PD and subsequent DSP at the receiver side, two independent frequency-doubling vector mm-wave signals can be separated and demodulated without data error. Based on our proposed scheme, we experimentally demonstrate the generation, transmission, and detection of 2-Gbaud 30-GHz quadrature-phase-shift-keying (QPSK) and 2-Gbaud 46-GHz QPSK signals over 10-km single-mode fiber-28 (SMF-28) and 1-m wireless transmission. The results indicate that the bit-error ratio (BER) of the dual vector mm-wave signals can each reach the hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10-3.

OAM mode-division multiplexing IM/DD transmission at 4.32†Tbit/s with a low-complexity adaptive-network-based fuzzy inference system nonlinear equalizer.

Stochastic nonlinear impairment is the primary factor that limits the transmission performance of high-speed orbital angular momentum (OAM) mode-division multiplexing (MDM) optical fiber communication systems. This Letter presents a low-complexity adaptive-network-based fuzzy inference system (LANFIS) nonlinear equalizer for OAM-MDM intensity-modulation direct-detection (IM/DD) transmission with three OAM modes and 15 wavelength division multiplex (WDM) channels. The LANFIS equalizer could adjust the probability distribution functions (PDFs) of the distorted pulse amplitude modulation (PAM) symbols to fit the statistical characteristics of the WDM-OAM-MDM transmission channel. Therefore, although the transmission symbols in the WDM-OAM-MDM system are subjected to a stochastic nonlinear impairment, the proposed LANFIS equalizer can effectively compensate the distorted signals. The proposed equalizer outperforms the Volterra equalizer with improvements in receiver sensitivity of 2, 1.5, and 1.3 dB for three OAM modes at a wavelength of 1550.12 nm, respectively. It also outperforms a CNN equalizer, with improvements in receiver sensitivity of 1, 0.5, and 0.3 dB, respectively. Moreover, complexity reductions of 67%, 74%, and 99.9% are achieved for the LANFIS equalizer compared with the Volterra, CNN, and ANFIS equalizers, respectively. The proposed equalizer has high performance and low complexity, making it a promising candidate for a high-speed WDM-OAM-MDM system.

Synthesis of Functionalized 4-Hydroxy Carbazoles and Carbazole Alkaloids via Ring Expansion of Indole Cyclopentanone.

The exploration of a ring expansion reaction from indole cyclopentanone to generate a range of diversely functionalized 4-hydroxyl carbazole frameworks, representing the core structure of numerous carbazole alkaloids, has been conducted under mild reaction conditions. This approach exhibits broad functional group tolerance and moderate to good yields. The practical applicability of this strategy has been demonstrated through the concise syntheses of carbazomycins A, D, and G.

Bayesian generative adversarial network emulator based end-to-end learning strategy of the probabilistic shaping for OAM mode division multiplexing IM/DD transmission.

Orbital angular momentum (OAM) mode division multiplexing (MDM) has emerged as a new multiplexing technology that can significantly increase transmission capacity. In addition, probabilistic shaping (PS) is a well-established technique that can increase the transmission capacity of an optical fiber to close to the Shannon limit. However, both the mode coupling and the nonlinear impairment lead to a considerable gap between the OAM-MDM channel and the conventional additive white Gaussian noise (AWGN) channel, meaning that existing PS technology is not suitable for an OAM-MDM intensity-modulation direct-detection (IM-DD) system. In this paper, we propose a Bayesian generative adversarial network (BGAN) emulator based on an end-to-end (E2E) learning strategy with probabilistic shaping (PS) for an OAM-MDM IM/DD transmission with two modes. The weights and biases of the BGAN emulator are treated as a probability distribution, which can be accurately matched to the stochastic nonlinear model of OAM-MDM. Furthermore, a BGAN emulator based on an E2E learning strategy is proposed to find the optimal probability distribution of PS for an OAM-MDM IM/DD system. An experiment was conducted on a 200 Gbit/s two OAM modes carrierless amplitude phase-32(CAP-32) signal over a 5 km ring-core fiber transmission, and the results showed that the proposed BGAN emulator outperformed a conventional CGAN emulator, with improvements in modelling accuracy of 29.3% and 26.3% for the two OAM modes, respectively. Moreover, the generalized mutual information (GMI) of the proposed E2E learning strategy outperformed the conventional MB distribution and the CGAN emulator by 0.31 and 0.33 bits/symbol and 0.16 and 0.2 bits/symbol for the two OAM modes, respectively. Our experimental results demonstrate that the proposed E2E learning strategy with the BGAN emulator is a promising candidate for OAM-MDM IM/DD optic fiber communication.

Secure key real-time update and dynamic DNA encryption for CO-OFDM-PON based on a hybrid 5-D chaos.

A double key (DK) real-time update and hybrid five-dimensional (5-D) hyperchaotic deoxyribonucleic acid (DNA) dynamic encryption scheme is proposed, which can ensure the security in the orthogonal frequency division multiplexing passive optical network (OFDM-PON). Chaotic sequences for DNA dynamic encryption are produced using a four-dimensional (4-D) hyperchaotic Lü system and a one-dimensional (1-D) logistic map. In this scheme, the DK consists of an external key set, which is stored locally, and an internal key, which is associated with the plaintext and external key. In addition, a pilot cluster is used as the carrier of key transmission and key embedding is achieved by converting key to phase information of the pilot. To verify the feasibility of the scheme, a simulation validation is performed on a 46.5Gb/s 16 quadrature amplitude modulation (QAM) coherent OFDM-PON system transmitted over an 80 km transmission distance. The results show that the proposed scheme can improve the security performance of OFDM-PON at a low OSNR cost of 0.3 dB and the key space is expanded to (8.514 × 10102)S. When the correlation redundancy (CR) G⩾7, the 0 bit error rate (BER) of key can be achieved and the key can be updated and distributed in real-time without occupying additional secure channels.

400 Gbit/s 4 mode transmission for IM/DD OAM mode division multiplexing optical fiber communication with a few-shot learning-based AffinityNet nonlinear equalizer.

Nonlinear impairment in a high-speed orbital angular momentum (OAM) mode-division multiplexing (MDM) optical fiber communication system presents high complexity and strong stochasticity due to the massive optoelectronic devices. In this paper, we propose an Affinity Network (AffinityNet) nonlinear equalizer for an OAM-MDM intensity-modulation direct-detection (IM/DD) transmission with four OAM modes. The labeled training and testing signals from the OAM-MDM system can be regarded as "small sample" and "large target", respectively. AffinityNet can be used to build an accurate nonlinear model using "small sample" based on few-shot learning and can predict the stochastic characteristic nonlinearity of OAM-MDM with a high level of generalization. As a result, the AffinityNet nonlinear equalizer can effectively compensate the stochastic nonlinearity in the OAM-MDM system, despite the large difference between the training and testing signals due to the stochastic nonlinear impairment. An experiment was conducted on a 400 Gbit/s transmission with four OAM modes using a pulse amplitude modulation-8 (PAM-8) signal over a 2 km ring-core fiber (RCF). Our experimental results show that the proposed nonlinear equalizer outperformed the conventional Volterra equalizer with improvements in receiver sensitivity of 1.7, 1.8, 3, and 3.3 dB for the four OAM modes at the 15% forward error correction (FEC) threshold, respectively. In addition, the proposed equalizer outperformed a convolutional neural network (CNN) equalizer with improvements in receiver sensitivity of 0.8, 0.5, 0.9, and 1.4 dB for the four OAM modes at the 15% FEC threshold. In the experiment, a complexity reduction of 37% and 83% of the AffinityNet equalizer is taken compared to the conventional Volterra equalizer and CNN equalizer, respectively. The proposed equalizer is a promising candidate for a high-speed OAM-MDM optical fiber communication system.

Hidden conditional random field-based equalizer for the 3D-CAP-64 transmission of OAM mode-division multiplexed ring-core fiber communication.

As a key technique for achieving ultra-high capacity optical fiber communications, orbital angular momentum (OAM) mode-division multiplexing (MDM) is affected by severe nonlinear impairments, including modulation related nonlinearities, square-law nonlinearity and mode-coupling-induced nonlinearity. In this paper, an equalizer based on a hidden conditional random field (HCRF) is proposed for the nonlinear mitigation of OAM-MDM optical fiber communication systems with 20 GBaud three-dimensional carrierless amplitude and phase modulation-64 (3D-CAP-64) signals. The HCRF equalizer extracts the stochastic nonlinear feature of the OAM-MDM 3D-CAP-64 signals by estimating the conditional probabilities of the hidden variables, thereby enabling the signals to be classified into subclasses of constellation points. The nonlinear impairment can then be mitigated based on the statistical probability distribution of the hidden variables of the OAM-MDM transmission channel in the HCRF equalizer. Our experimental results show that compared with a convolutional neural network (CNN)-based equalizer, the proposed HCRF equalizer improves the receiver sensitivity by 2 dB and 1 dB for the two OAM modes used here, with l = + 2 and l = + 3, respectively, at the 7% forward error correction (FEC) threshold. When compared with a Volterra nonlinear equalizer (VNE) and CNN-based equalizer, the computational complexity of the proposed HCRF equalizer was found to be reduced by 30% and 41%, respectively. The bit error ratio (BER) performance and reduction in computational complexity indicate that the proposed HCRF equalizer has great potential to mitigate nonlinear distortions in high-speed OAM-MDM fiber communication systems.

Photonic-aided W-band dual-vector RF signal generation and detection enabled by bandpass delta-sigma modulation and heterodyne detection.

We propose a photonic-aided dual-vector radio-frequency (RF) signal generation and detection scheme enabled by bandpass delta-sigma modulation and heterodyne detection. With the aid of the bandpass delta-sigma modulation, our proposed scheme is transparent to the modulation format of the dual-vector RF signals and can support the generation, wireless transmission, and detection of both single-carrier (SC) and orthogonal-frequency-division-multiplexing (OFDM) vector RF signals with high-level quadrature-amplitude-modulation (QAM) modulation. With the aid of the heterodyne detection, our proposed scheme can support up to W-band (75-110 GHz) dual-vector RF signal generation and detection. For the validation of our proposed scheme, we experimentally demonstrate the simultaneous generation of a SC-64QAM signal at 94.5 GHz and a SC-128QAM signal at 93.5 GHz and their error-free high-fidelity transmission over a 20-km single-mode fiber 28 (SMF-28) and a 1-m single-input single-output (SISO) wireless link at the W-band. To the best of our knowledge, this is the first time that delta-sigma modulation has been introduced into a W-band photonic-aided fiber-wireless integration system to achieve flexible and high-fidelity dual-vector RF signal generation and detection.

[Research progress on traditional Chinese medicine in treatment of neurodegenerative diseases by delaying neurovascular unit aging].

Neurodegenerative diseases are a collective term for a large group of diseases caused by degenerative changes in nerve cells. Aging is the main risk factor for neurodegenerative diseases. The neurovascular unit(NVU) is the smallest functional unit of the brain, which regulates brain blood flow and maintains brain homeostasis. Accelerated aging of NVU cells directly impairs NVU function and leads to the occurrence of various neurodegenerative diseases. The intrinsic mechanisms of NVU cell aging are complex and involve oxidative stress damage, loss of protein homeostasis, DNA damage, mitochondrial dysfunction, immune inflammatory response, and impaired cellular autophagy. In recent years, studies have found that traditional Chinese medicine(TCM) can inhibit NVU aging through multiple pathways and targets, exerting a brain-protective effect. Therefore, this article aimed to provide a theoretical basis for further research on TCM inhibition of NVU cell aging and references for new drug development and clinical applications by reviewing its mechanisms of anti-aging, such as regulating relevant proteins, improving mitochondrial dysfunction, reducing DNA damage, lowering inflammatory response, antioxidant stress, and modulating cellular autophagy.

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.

Prediction of Stable Ground-State Binary Sodium-Potassium Interalkalis under High Pressures.

The complex structures and electronic properties of alkali metals and their alloys provide a natural laboratory for studying the interelectronic interactions of metals under compression. A recent theoretical study (J. Phys. Chem. Lett. 2019, 10, 3006) predicted an interesting pressure-induced decomposition-recombination behavior of the Na2K compound over a pressure range of 10-500 GPa. However, a subsequent experiment (Phys. Rev. B 2020, 101, 224108) reported the formation of NaK rather than Na2K at pressures above 5.9 GPa. To address this discordance, we study the chemical stability of different stoichiometries of NaxK (x = 1/4, 1/3, 1/2, 2/3, 3/4, 4/3, 3/2, and 1-4) by an effective structure searching method combined with first-principles calculations. Na2K is calculated to be unstable at 5-35 GPa due to the decomposition reaction Na2K → NaK + Na, coinciding well with the experiment. NaK undergoes a combination-decomposition-recombination process accompanied by an opposite charge-transfer behavior between Na and K with pressure. Besides NaK, two hitherto unknown compounds NaK3 and Na3K2 are uncovered. NaK3 is a typical metallic alloy, while Na3K2 is an electride with strong interstitial electron localization.

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.

Receding Dynamics of Droplet Deposition on a Smooth Surface from a Central Jet to Secondary Droplet Emission.

A droplet impacting a solid surface undergoes deposition, splashing, or rebounding, depending on the properties of the droplet and impacted surface. The deposition pattern attracts lots of attention for the reasons of its predictable behavior on wetting and dewetting. The phenomenon of the central jet, even breaking up into a secondary droplet, is observed during gentle deposition on partially wettable surfaces. The mechanism behind the central jet is addressed by the dual role of the inertia force. In this paper, the dynamic characteristics of droplet deposition are analyzed with a focus on the receding process from jetting to secondary droplet emission. The jet is pinched off into a small secondary droplet when the impacting Weber (We) number is within the range of approximately 26-54. The secondary droplet is then ejected because of the collision with the upward jet. The convergence of capillary waves on the liquid film is employed to explain the two-stage upward jet. The jet tip is pinched off because of Rayleigh-Plateau instability, leading to the generation of the secondary droplet. A geometric model of a two-floor cylinder is further proposed to describe the equivalent recession of the capillary wave. The equivalent radius of the receding wave is linear with time, and the jet height exhibits a scaling law of Djet ∼ (t - 0.8)1/2 with normalized time. Additionally, the dynamic characteristics are investigated from time and size views. With various Weber numbers, the normalized receding, jetting, and tip times are found to remain almost constant. A piecewise relationship with the Weber number is revealed for the normalized receding wave and the tip height. The wave recedes at the characteristic velocity of 0.23 m/s. Moreover, the normalized jet height and tip diameter, as well as secondary droplet diameter, are independent of the inertia force. The emission velocity and kinetic energy of the secondary droplet are found to reach the maximum at We ∼ 41.6.

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.