Back-to-back Research Articles

Investigation into the effect of dimerization on PKR activation energetics and dynamics with enhanced sampling molecular dynamics

Protein kinase R (PKR) is a serine/threonine kinase that acts as a central component of the innate immune pathway, the cell’s first line of defense against pathogens, by phosphorylating its substrate, eIF2α, and halting translation. PKR is activated upon binding viral dsRNA, leading to autophosphorylation within the activation loop. Crystal structures and mutational scanning suggest that activation of PKR requires homodimerization in a back-to-back (BTB) geometry. To investigate, we use a computational approach to calculate the free energy changes associated with activation in the context of a monomer and a BTB dimer.

DC‐link capacitor current observation based stability enhancement control for back‐to‐back converter

This article presents a stability enhancement control scheme applied in the three-phase back-to-back (BTB) converters with L filter. The stability enhancement control scheme is concocted using the feedback of DC-link capacitor current without additional sensors. Both grid-side converters are equipped with this control scheme, which can also be called the coordinated compensation control scheme. In this article, DC terminal impedance models of two converters before and after optimization and stability criterion are introduced. Furthermore, means of impedance analysis, including Bode plot and Nyquist curve, are discussed to categorize the causes of BTB converter instability caused by the increased power. The coordinated scheme is compared with the conventional scheme for revealing the theoretical basis. The proposed method is capable of optimizing the disturbance suppression capability of DC-link voltage, which would potentially reduce capacitor volume and physical size of the DC-link. The impedance-based stability analysis indicates that the controllability and stability of the BTB converter is enhanced with the proposed DC-link capacitor current coordinated control. Furthermore, the proposed control scheme is implemented in the MATLAB/Simulink environment. Moreover, for further validation, the feasibility of the coordinated control scheme is verified with the experimental results.

Effect of Chirped Dispersion and Modal Partition Noise on Multimode VCSEL Encoded With NRZ-OOK and PAM-4 Formats

Chirp-dispersion and data transmission analyses of the multimode vertical-cavity surface-emitting laser (MM-VCSEL) with a 7.5-μm oxide-confined aperture and a root-mean-square spectral linewidth of 1 nm is performed for intra-data-center application. For directly modulating the non-return-to-zero on-off keying (NRZ-OOK) and the 4-level pulse amplitude modulation (PAM-4) data streams, an optimized bias current of 17 mA and a data amplitude of 800 mV are selected to provide the MM-VCSEL with an output optical power of 6.1 mW and a transverse mode number of 17. Such an optimized operation enables the MM-VCSEL with a differential resistance of 45.3 Ω to approach the impedance matching by driving the circuit such that the -3-dB bandwidth can be achieved to 23.8 GHz. The relative intensity noise and mode-partition-noise of the MM-VCSEL are analyzed to perform the optimized NRZ-OOK/PAM4 data encoding. By utilizing the pre-emphasis technique to compensate for the intensity and phase variation, the encoded NRZ-OOK and PAM-4 can be respectively promoted to 61 Gbit/s and 96 Gbit/s in the back-to-back (BtB) case. After passing through the 100-m OM5 multi-mode-fiber (MMF) link to suffer the severe modal dispersion, the data rate would respectively degrade to 45 Gbit/s (or 4.5 Gbit·km/s) and 70 Gbit/s (or 7 Gbit·km/s) for OOK and PAM-4 transmission.

An Improved Design of Synthetic Loading Method for a Rapid In-Wheel Motor Characterization in Different Operating Points

In-wheel motors (IWMs) as a crucial component of an electric vehicle need to be tested at the end of the production line, at service intervals, and during motor design validation. This article presents an improved synthetic loading (SL) method for the rapid characterization of the IWMs. The proposed method emulates mechanical loading at a specific operating point by current harmonic injection. We introduce a novel algorithm for the current reference setting that is based on the manufacturer’s motor design data and accounts for the motor’s nonlinearities. Also, the method features a trapezoidal reference instead of a sinusoidal, which lowers the instantaneous peak current of the converter and, thereby, increases the attainable torque–speed range. The performance of the proposed SL method is comparable to the classic back-to-back (BB) testing method, which is confirmed by the theoretical analysis, simulations, and experimental results. The difference in motor efficiency measured with the BB and the improved SL method is smaller than 1% in most operating points. The proposed method offers considerable time and cost savings for the manufacturer—eliminating the mechanical coupling/decoupling operation—and can be applied to surface-mounted permanent magnet synchronous machines in general.

Imbalanced Mach-Zehnder Modulator for Fading Suppression in Dispersion-Uncompensated Direct Detection System

In this work, we systematically analyze the impact of three kinds of Mach-Zehnder modulator (MZM) imbalances, including bias deviation, amplitude mismatch, and differential time skew in intensity-modulation direct-detection (IM-DD) links. It is shown that, for power fading limited transmission, the imbalances can be utilized as advantages rather than impairments. Specifically, the bias deviation with single-arm driven mode and amplitude mismatch with differential driven mode can increase the available bandwidth by shifting the frequency of fading notches. Meanwhile, time skew provides another way to avoid fading by shaping the double sideband (DSB) signal into a vestigial sideband (VSB) with an asymmetrical transfer function. In the transmission experiment, 34 Gbaud Nyquist 6/8-ary pulse amplitude modulation (PAM-6/8) signals are used for investigation in a 20 km dispersion-uncompensated standard single-mode fiber (SSMF) link. With the help of a Volterra nonlinear equalizer, all three kinds of imbalances can achieve bit-error rates (BERs) below the 7% and 20% hard-decision forward error correction (HD-FEC) thresholds for PAM-6 and PAM-8 signals, respectively. The received power sensitivity is also compared at the back-to-back (BTB) case and after fiber transmission. Both numerical simulation and experimental demonstration confirm that the dispersion-induced power fading can be effectively suppressed with bias, amplitude, or skew imbalance, providing a feasible solution for transmission distance extension of C-band DD links.

Review of VCSELs for Complex Data-Format Transmission Beyond 100-Gbit/s

Persistent efforts on developing advanced and complex data algorithms to enable the vertical-cavity surface-emitting lasers (VCSELs) for data beyond 100-Gbit/s has been reviewed in this paper. This endeavor not only elevates the bit rate of the IEEE802.3 standards for data center applications but also to initiate the unification of data format for wired and wireless network coverage. To date, the highest allowable data rates of the single-VCSEL-based optical link are 80 Gbit/s for the NRZ-OOK format under back-to-back (BtB) transmission 168 Gbit/s for the PAM-4 format over 150-m OM5 MMF, and the 224 Gbit/s for the QAM-OFDM format at BtB case. For the high-level complex data-format transmission with rigorous demand on the signal-to-noise ratio (SNR), this work demonstrates the most up-to-date 32-ray quadrature amplitude modulation generalized frequency division multiplexing (32-QAM GFDM) data algorithm with optimized amplitude and K value for encoding the multimode (MM) VCSEL. To achieve the higher spectral-usage efficiency based on the SNR spectrum for maximal transmission data rate, the bit-loading discrete multi-tone (DMT) technique is implemented by rearranging each subcarrier to the appropriate QAM levels. By utilizing 32-QAM GFDM and bit-loading DMT, the maximal data rate in the BtB case could respectively achieve 119.5 Gbit/s and 130 Gbit/s when operating the MM VCSEL at 55 °C. The transmission capacity of the VCSEL operated at 55 °C would decrease to 93.8 Gbit/s for 32-QAM GFDM and 83 Gbit/s for bit-loading DMT after propagating through OM5 MMF with modal dispersion.

동기발전기-BTB 컨버터를 이용한 주파수 평활화

The synchronous generator in the power system controls the mechanical input power to regulate the frequency under normal operation, thereby delaying the speed of frequency regulation. This paper presents a frequency-regulating scheme of the synchronous generator that mitigates the frequency fluctuations caused by continuous output power fluctuations of a wind turbine generator. To achieve this, a back-to-back (BTB) converter is connected between the synchronous generator and the transformer. In the machine-side converter (MSC), to regulate the frequency within a narrow band, a control scheme that decreases the active power in the over-frequency section and increases the active power in the under-frequency section is implemented. In addition, to maintain the rotor speed of the synchronous generator within a narrow band, the speed controller adjusts its mechanical input power depending on the rotor speed variation. The power control scheme in the BTB converter uses the kinetic energy stored in the rotating masses of the synchronous generator for regulating the frequency. As a result, the rotor speed of the synchronous generator further fluctuates; however, the rotor speed controller can maintain the rotor speed within a narrow band. The simulation results in the modified IEEE 14-bus system indicate that the proposed scheme can significantly mitigate the frequency fluctuations caused by wind speed variations under normal operation.

Robust control scheme for the braking chopper of PMSG-based wind turbines–A comparative assessment

Despite the grid-integration of permanent magnet synchronous generator (PMSG) wind energy conversion systems (WECS), utilities are facing challenges that are hindering wind penetration growth. Grid-side voltage sags at the point of common coupling (PCC) are considered as one of such uncontested issues. Due to imbalance in back-to-back (BTB) converter’s dc-link input-output powers, its capacitor and power electronics are highly susceptible to PCC voltage sags. Hence, provision of wind farm grid-codes (GC) for supporting grid operations is imperative and development of strategies for realizing low-voltage ride-through capability in WECSs have emerged as an integral GC requirement. One strategy is to install a braking chopper (BC) across the dc-link. This study presents (a) an overview of conventional BC control methods, (b) design of a robust BC controller for PMSG-based WECSs, and (c) comparative performance assessment of (b) with (a) for PMSG-based WECS operations during normal and grid fault conditions. The robust BC controller takes advantage of the sliding mode method to control BC and to determine the grid-side converter active power reference set-point. This nonlinear control scheme ensures proper execution of GCs and offers reliable protection to the dc-link. Chattering phenomenon and reaching-time is reduced significantly, by further adoption of exponential reaching law. In addition to its robustness, a prominent feature of this BC controller is its applicability to present BTB control structures. The simulation results reflect on effectuality and robustness of the proposed BC controller in comparison to conventional BC control strategies.

Chaotic RNA and DNA for security OFDM-WDM-PON and dynamic key agreement.

A chaotic ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) encryption scheme is firstly proposed for security OFDM-WDM-PON in this paper. We adopt a dynamic key agreement based on the messenger RNA (mRNA) codebook to distribute the key, and the security and randomness of this key are enhanced by a pre-sharing key parameter set instead of transmission of a key directly. Also, the security key can be dynamically updated in real-time according to the needs of the users. The real (I) and imaginary (Q) parts of the QAM symbol matrix after modulation are encrypted by the correspondence between transfer RNA (tRNA) and amino acids and the selection mapping of DNA base complementary rules. Also, we add cubic permutation to ensure all data security encryption. The encrypted signals of 35.29 Gb/s on different wavelength channels are successfully demonstrated over a 25-km standard single-mode fiber (SSMF) and a back-to-back (BTB) system. It is proved that the proposed security OFDM-WDM-PON encryption scheme is compatible with the traditional WDM system, which can make full use of bandwidth resources and enhance the security with a large key space.

DC‐side current compensation control in the rectifier terminal for power variations in back‐to‐back converters

Back-to-back (BTB) converters, which consist of an AC/DC rectifier cascaded with a DC/AC inverter, have been widely adopted in the interconnection of power systems. The DC link, being the interface between the rectifier and inverter terminals, is susceptible to disturbances, particularly from power fluctuations. Regardless of the magnitude of power or changes in the power flow direction, stability degradation will occur. To lessen the influence of power, a DC-link current compensation control scheme is proposed and applied to the rectifier terminal. The proposed scheme reduces the DC link impedance gain of the rectifier terminal to balance the impedance at both ends, and thus enhance stability and controllability. The scheme is applied to a BTB converter to improve the system's resistance to disturbance, regardless of variations in power magnitude or power flow direction. The accuracy of the theoretical analysis and the effectiveness of the proposed scheme are verified using simulations and a scaled-down experimental setup.

0.36 THz photonic terahertz-wave generation by simple optical frequency comb

A novel 0.36 THz photonic-assisted terahertz-wave (THz-wave) signal generation scheme is demonstrated via simulation. We employ only one single phase modulator (PM) as optical frequency comb generator to get the desired optical carries with high-frequency stability. We get two target optical carriers of symmetrical orders (−6th, +6th) chosen from the generated optical frequency comb via wavelength selector switch (WSS). One optical carrier from one port is acted as optical local oscillator (LO) for heterodyne beating. And one optical carrier from the other port is sent into the Mach Zendell modulator (MZM), which is modulated with a 2-Gband 16 quadrature amplitude modulation (16QAM) baseband signal or 4-Gbaud quadrature phase shift keying (QPSK) baseband signal generated in the MATLAB. Employing our proposed 0.36 THz photonic-assisted THz-wave signal generation scheme, we demonstrate the performance of 4-Gband vector QPSK signal or 2-Gbaud vector 16 QAM signal at back to back (BTB) transmission case or 10 km single-mode fiber transmission case. The bit error ratio (BER) under aforementioned conditions in every case is under the hard decision forward error correction (HD-FEC) threshold of 3.8e−3. To our knowledge, it’s the first time to utilize only one PM as optical frequency comb generator in the photonic-assisted THz-wave signal generation.

Polar coded probabilistic shaping PAM8 based on many-to-one mapping for short-reach optical interconnection.

In this paper, a polar coded probabilistic shaping (PS) 8-ary pulse amplitude modulation (PAM8) based on many-to-one (MTO) mapping is investigated for short-reach optical interconnection. By ingeniously assigning parity bits to ambiguities positions, no extra PS redundancy and no complex distribution matcher are required in the scheme comparing to traditional probabilistic amplitude shaping (PAS). The noise distributions after different transmission distances are studied and an optimal clock recovery method for PS signal is proposed to degrade the impact of severe eye skew effect on BER performance. The experimental results show that up to 1.2 dB and 0.8 dB shaping gains are respectively achieved over back-to-back (BTB) and 2-km standard single mode fiber (SSMF) transmission. With the help of the proposed optimal clock recovery method in the PS scheme, the shaping gain is improved from 0.15 dB to 0.4 dB after 10-km transmission. Moreover, compared to low-density parity-check (LDPC) code, the polar coded PS-PAM8 can provide an additional coding gain of 2.2 dB with code length of 256, which proves the performance superiority of polar code in short code length. Therefore, the proposed polar coded PS-PAM8 with low complexity and satisfactory BER performance is believed to be an alternative solution for the cost-sensitive short-reach optical interconnection.

DRU Based Low Frequency AC Transmission Scheme for Offshore Wind Farm Integration

The low frequency AC (LFAC) technology is found to be a competitive solution for offshore wind farm integration. This article proposed an LFAC system with a hybrid onshore back-to-back (BtB) converter, for which the diode-based rectifier unit (DRU) and modular multilevel converter (MMC) based inverter are adopted. Then, this article also proposed corresponding control strategies for offshore wind turbines, where the DC link voltage control is implemented on the machine side converter, and the AC voltage of the offshore grid is controlled by the grid side converter based on the fixed reference frame. The small signal analysis for the whole system was carried out based on the equivalent circuit, and the small-signal stability is proven by calculated eigenvalues. And the system responses to typical operating conditions are investigated by time-domain simulations in PSCAD. Thus, the feasibility of the proposed scheme is verified.

Multilayer Dynamic Encryption for Security OFDM-PON Using DNA-Reconstructed Chaotic Sequences Under Cryptanalysis

In this paper, a multilayer dynamic encryption scheme using deoxyribonucleic acid reconstructed chaotic sequences (DNA-RCS) under cryptanalysis is firstly proposed, which aims at enhancing the security of orthogonal frequency division multiplexing passive optical network (OFDM-PON). We adopt DNA coding to reconstruct chaotic sequences, the selected coding rules and the number of chaotic sequence blocks divided are then random, the randomness and security of encryption sequences are improved. The transmitted signal is encrypted in two layers. The first layer is hybrid chaotic permutation and diffusion. Each symbol can be encrypted by the combination of a single non-repetitive permutation and plaintext-related diffusion. It makes encryption not only depend on the chaotic sequences but also relate to the order of permutation. The second layer is a dynamic Josephus permutation. By taking the unit as the permutation object, the scrambling efficiency is increased. Also, the counting period is randomly selected, which can enhance the security of the system. The number of tests needed to break a secure transmission for an attacker can reach up to $3.096\times 10^{106}$ . An encryption signal with 22.06Gb/s is successfully demonstrated over a 25-km standard single-mode fiber (SSMF) and a back-to-back (BTB) system. It is proved that the proposed scheme does not degrade the system performance and can effectively resist various attacks by the performance analysis model based on cryptanalysis.

PID-type terminal sliding mode control for permanent magnet synchronous generator based enhanced wind energy conversion systems

The provision of wind farm (WF) grid-codes (GC) in sustaining grid operations has become imperative, especially for WFs with permanent-magnet synchronous generator (PMSG) wind energy conversion system. Numerous techniques are developed for executing GC requirements in the event of grid faults. Among the methods, an intriguing strategy is to enhance the performance of back-to-back (BTB) converter controllers. In this research, the PID-type terminal sliding mode control (PID-TSMC) scheme is implemented for both machine-side and grid-side converter modified controllers of BTB-converter, to reinforce the nonlinear relationship among the state-variable and control input. The application of this control scheme decreases the response time and improves the robustness of the BTB-converter controllers to parameters uncertainty and external disturbances. The grid-side converter tracks the maximum power point, contributing to the generator active power output rapid decrease during faults. This frees up converter capacity for injecting GC-compliant reactive current into grid. Besides, the machine-side converter regulates dc-link voltage, in which its variations during external disturbances decrease substantially with the PID-TSMC. The discussions over the simulations contemplate on the robustness and efficiency of the implemented PID-TSMC strategy in comparison to other BTB-converter control strategies.

10 Gb/s Bidirectional Transmission with an Optimized SOA and a SOA-EAM Based ONU

We investigated the application of a semiconductor optical amplifier (SOA) and an SOA electro-absorption modulator (SOA-EAM) as attractive, low-cost solutions in passive optical networks (PONs). The main characteristics of an SOA with optimal performance for phase and amplitude modulation were tested. Additionally, a 10 Gb/s bidirectional transmission with an optical network unit (ONU) transmitter integrated with a distributed feedback (DFB) laser, electro-absorption modulator (EAM), and SOA was designed. The upstream (US) and downstream (DS) receiver sensitivities at the forward error correction (FEC) level reached −29.5 dBm and −33.5 dBm for back-to-back (BtB) fiber and −28.9 dBm and −33.1 dBm for 20 km fiber. For multichannel transmission, the US receiver sensitivities reached −28.8 dBm and −28.2 dBm for BtB and 20 km fibers, and the DS receiver sensitivities reached −33 dBm and −32.6 dBm for BtB and 20 km fibers, respectively.

Integration of fiber and FSO network with fault-protection for optical access network

In this paper, we proposed and experimentally demonstrated the integration of optical fiber and free-space optic (FSO) with fault protection for the optical access network. The proposed architecture has a fault protection or self-healing function when a link failure is occurred in the network. Thus, the proposed self-healing architecture is a promising network for improved reliability and survivability of optical network especially for the remote area when the fiber is broken or cut between each remote node (RN) and optical network unit (ONU). Moreover integration of optical fiber with the FSO network is used to increase the coverage range of the broadband wireless network and improve the flexibility of the network. In the experiment, the signal is generated by using the signal generator, and propagates through a hybrid of 25 km SMF, and FSO link and then transmitted into each ONU. Finally, we compare and evaluate the signal transmission performance of channels in terms of bit error rate (BER) and error vector magnitude (EVM) after back-to-back (BTB) transmission, 25 km SMF transmission, and after a hybrid of 25 km SMF with FSO link transmission. The experimental result proves that the hybrid of 25 km SMF with FSO link increases the communication coverage range with a reasonable performance.

Optimal Planning and Operation of Multi-Frequency HVac Transmission Systems

Low-frequency high-voltage alternating-current (LF-HVac) transmission scheme has been recently proposed as an alternative solution to conventional 50/60-Hz HVac and high-voltage direct-current (HVdc) schemes for bulk power transfer. This paper proposes an optimal planning and operation for loss minimization in a multi-frequency HVac transmission system. In such a system, conventional HVac and LF-HVac grids are interconnected using back-to-back (BTB) converters. The dependence of system MW losses on converter dispatch as well as the operating voltage and frequency in the LF-HVac is discussed and compared with that of HVdc transmission. Based on the results of the loss analysis, multi-objective optimization formulations for both planning and operation stages are proposed. The planning phase decides a suitable voltage level for the LF-HVac grid, while the operation phase determines the optimal operating frequency and power dispatch of BTB converters, generators, and shunt capacitors. A solution approach that effectively handles the variations of transmission line parameters with the rated voltage and operating frequency in the LF-HVac grid is proposed. The proposed solutions of the planning and operation stages are evaluated using a multi-frequency HVac system. The results show a significant loss reduction and improved voltage regulation during a 24-hour simulation.

An Improved Direct Power Control for Doubly Fed Induction Generator

A novel voltage-modulated direct power control (VM-DPC) is first designed for a back-to-back (BTB) converter in the doubly fed induction generator (DFIG) system. The proposed VM-DPC is built in the stator stationary reference frame (αβ). The proposed method uses a simple feed-forward and feedback structure without a phase-locked loop and the Park transformation. Therefore, it can be easily implemented in the BTB converter. Another essential advantage of the proposed VM-DPC is that it can transform the closed-loop DFIG system into a linear-time-invariant one, which can be analyzed and designed through multiple linear control techniques. The proposed method guarantees exponential stability in the stiff grid as well as in weak-grid integration, which is proved based on eigenvalue analysis. Simulation results demonstrate that the proposed VM-DPC has a faster transient response than conventional vector-oriented control (VOC). Also, it maintains a satisfactory steady-state performance at the same level as the VOC. The robustness of proposed VM-DPC against distorted voltage conditions and parameter mismatch is also tested. Finally, the proposed VM-DPC control strategy is validated in an experimental hardware prototype of a 7.5-kW DFIG system operating in real time.

Temperature and Noise Dependence of Tri-Mode VCSEL Carried 120-Gbit/s QAM-OFDM Data in Back-to-Back and OM5-MMF Links

The transmission performances of the tri-mode vertical cavity surface-emitting laser (VCSEL) carrying 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM OFDM) data streams at a data rate beyond 100 Gbit/s are demonstrated in OM5 and OM4 multi-mode fibers (MMFs) over 100 m. By using the double oxide-confined aperture with a diameter of 5 μm, the tri-mode VCSEL provides three main transverse modes at a power of 2 mW and a quantum efficiency of 0.4 mW/mA, in which these main transverse modes individually contribute to 53.1%, 37.6%, and 9.3% of the output power from the tri-mode VCSEL for delivering the QAM-OFDM data. With optimizing the bias current of the tri-mode VCSEL to 8 mA (32 Ith) and pre-leveling the QAM-OFDM subcarrier amplitudes at a slope of 0.5 dB/GHz in the back-to-back (BtB) transmission case, the raw data rate up to 120 Gbit/s can be achieved under the FEC receiving criterion. In particular, the performance of the tri-mode VCSEL operating at the temperature of up to 85°C in BtB case is also demonstrated, while its lasing spectrum translates from tri-mode to quasi-single-mode because of the heating induced carrier dissipation and index reprofiling process. After propagating through the 100-m OM5-MMF, the pre-leveled 16-QAM OFDM data carried by the tri-mode VCSEL guarantees the 104-Gbit/s FEC-certified transmission with a receiving power penalty of 3.8 dB.