Hybrid Modulation Scheme Research Articles

Free Space Optics, Channel Models and Hybrid Modulation Schemes: A Review

Free space optical (FSO) communication system is a wireless communication technology which possess advantages such as huge license free bandwidth and high data carrying capacity among others. It has proved to be a viable solution towards the radio frequency spectrum crunch and hence has been applied in cellular back haul, earth to satellite links, deep space and defense among others. However, FSO performance is adversely affected by atmospheric turbulence and weather conditions. Different channel models are used to estimate the performance of FSO communication system. Meanwhile, different mitigation techniques have been proposed and analyzed over the years for improvement of FSO communication. Of late, hybrid modulation schemes have proved to be another research direction. In this article, we review the FSO channel models as well as hybrid modulation schemes that have been recently introduced to further improve the FSO performance. The conditional probability of error for the commonly used FSO modulation schemes are presented. Finally, the expressions for the bit error for the hybrid modulation schemes are also given.

A Hybrid-Bridge and Hybrid Modulation-Based Dual-Active-Bridge Converter Adapted to Wide Voltage Range

Dual-active-bridge (DAB) converter based on single-phase shift (SPS) modulation is commonly used in energy storage systems. However, if the normalized voltage gain deviates from unity, the zero-voltage-switching (ZVS) feature and efficiency of the DAB converter deteriorate. To overcome this challenge, this article proposes a hybrid-bridge and hybrid modulation-based DAB converter adapted to a wide voltage range. The primary-side hybrid bridge operates in an intermediate state between the full bridge and the half bridge. Asymmetrical pulsewidth modulation (APWM) is introduced to regulate the dc bias across the blocking capacitor in a continuous manner. Due to the synergy between the hybrid-bridge structure and hybrid modulation scheme, a general voltage match between the energy storage unit and the high-voltage bus is always achieved. Moreover, the ZVS range is extended and the circulating current is well suppressed. Therefore, efficiency is optimized over the wide voltage range. The operational principles, circuit modeling, and design considerations are detailed. A 500-W converter prototype, linking a 100–200-V energy storage unit and a 300-V dc bus, is designed and tested to validate the concept. A 97.9% peak efficiency and good overall efficiency over the wide voltage range are reported.

Hybrid Modulation Strategy for the Vienna Rectifier

The Vienna rectifier can produce three voltage levels, however only the connection to the neutral-point is fully controllable using the bidirectional switches. When the neutral-point voltage is not imposed in a phase-leg, the polarity of the pulses generated depends on the diode that is conducting in that phase-leg, which is defined by the current direction. As a result, the voltage pulses generated can go in the opposite direction to what is demanded by the controller, and the current becomes distorted. This can be mitigated by injecting a particular zero sequence so that the reference voltage is clamped to the neutral-point during the intervals, where the current of a phase-leg has the opposite sign than its reference voltage. However, the converter is at risk of entering into overmodulation when operating with high modulation indices. In this article, a hybrid modulation strategy is introduced where a calculated minimal amount of reactive current depending on the operating conditions is injected to avoid overmodulation. The reduction of reactive current required compared to the state-of-the-art solutions is up to 100% in certain operating conditions. With the proposed method, the Vienna rectifier can operate in a wide range of power factors without compromising the quality of the source currents. The concept is verified by simulation and in a 1-kW Vienna rectifier experimental setup.

An Active Hybrid Modulation Strategy for a Si/SiC Hybrid Multilevel Converter

This paper proposes a hybrid modulation strategy for a medium-voltage hybrid seven-level (7-L) converter, which consists of a three-phase silicon (Si) IGBT-based active-neutral-point-clamped (ANPC) stage and a cascaded silicon carbide (SiC) MOSFET-based H-bridge (HB) stage in each phase. Within the framework of the proposed modulation strategy, three floating HB converters are controlled using a high-frequency three-level (3-L) space-vector-modulation (SVM), while a low-frequency modulation is applied to the ANPC converter. To regulate the DC voltages across floating capacitors, a zero-sequence voltage (ZSV) injection method is adopted. The proposed method possesses three major advantages. First, it enables the simplification from a 7-L to a 3-L SVM, which significantly reduces the computational burden. Second, with all the high-frequency switching actions assigned to SiC devices in the HB stage, the power loss of the hybrid 7-L converter is reduced. Third, the hybrid modulation together with the ZSV injection enables a wider stable operating range compared to the existing 7-L SVM method. Finally, both simulation and experimental results performed on the scale-down and the full-scale 1 MVA prototype validate the effectiveness of the proposed hybrid modulation strategy.

Investigation of 340-Gbps terrestrial FSO link incorporating spectral-efficient DP-QPSK-PolSK hybrid modulation scheme

A high-capacity spectral-efficient dual-polarization quadrature phase-shift keying (DP-QPSK)-polarization shift-keying (PolSK) hybrid modulation scheme for terrestrial free-space optics (FSO) transmission link is proposed and investigated. A DP-QPSK signal modulated at 300 Gbps and a PolSK signal modulated at 40 Gbps are simultaneously transmitted using a single optical carrier over the FSO link. The proposed link performance is investigated under different weather conditions, where the bit error rate metric is used to evaluate the performance of the PolSK modulated signal and the error vector magnitude parameter is used for the DP-QPSK signal. The FSO link range and the required received power are carefully explored. The conducted numerical simulations of the proposed system showed reliable 340-Gbps data transmission over link ranges varying from 1.6125 to 50 km depending on the weather conditions. The impact of the channel scintillation due to atmospheric turbulence is also investigated. The proposed high-speed FSO transmission system offers a promising solution for high-bandwidth hungry systems used for the internet of things, 5G, and smart cities. It can also be used in developing fronthaul/backhaul links for future wireless networks and optical access networks. The performance of the proposed transmission system is compared with recently published work in the literature.

An Optimized Hybrid Modulation Scheme for Reducing Conduction Losses in Dual Active Bridge Converters

A linearized hybrid modulation scheme for the dual active bridge (DAB) converter is proposed in this article. For the purpose of minimizing the conduction losses dissipated on the transformer and the power transistors, an optimal relationship function between the two control variables employed in extended phase shift (EPS) modulation can be derived. However, the obtained relationship function is a complex expression, which is not good for simple on-line control. Hence, a linearized modulation scheme is proposed in this article. This modulation scheme can achieve a quasi-minimum root-mean-square (rms) value of the leakage inductance current for the same output power. Meanwhile, the zero-voltage switching (ZVS) of the power transistors can be achieved over the whole power range. The power transfer capability is also kept the same as the optimal EPS scheme. Finally, experiments are conducted on a laboratory prototype to validate the effect of the linearized modulation scheme on the reduction of conduction losses. The experimental results present an improved converter efficiency and the realization of ZVS.

Performance assessment of hybrid PPM–BPSK–SIM based FSO communication system using time and wavelength diversity under variant atmospheric turbulence

In this paper, free space optical communication system employing a hybrid modulation technique called pulse position modulation–binary phase shift keying–subcarrier intensity modulation technique, is investigated using wavelength and time diversity, by considering log-normal and gamma–gamma probability density function as the channel-fading statistics under weak, moderate and strong turbulence. The analytical average bit error rate and closed form of expression of the proposed hybrid modulation scheme are derived. The outcomes of the simulation demonstrate the outstanding consistency of the derived closed form expression with those achieved by the Gauss–Hermite approximation and the Monte Carlo simulations.

Development of a Hybrid Cascaded Converter Based STATCOM With Reduced Switching Losses and Improved Fault Ride Through Capability

This article develops a static synchronous compensator (STATCOM) based on the hybrid cascaded converter which has the advantage of reduced number of power transistors and floating capacitors compared to the cascaded H-bridge converter. A hybrid modulation scheme is employed in the hybrid cascaded converter to reduce the switching losses while maintaining the output performance, and the corresponding dc capacitor voltage balancing control scheme is proposed. The laboratory test results are provided to verify the feasibility of the developed STATCOM and the effectiveness of the proposed scheme. Furthermore, the proposed scheme is compared with the conventional scheme to reveal the improvement in output quality, power losses, and even fault ride through capability.

Integrated Bridgeless-Boost Nonresonant Half-Bridge Converter Employing Hybrid Modulation Strategy for LED Driver Applications

A single-stage high-power-factor LED driver based on the integration of the bridgeless boost rectifier with the nonresonant half-bridge converter (BLB-HB-NR) employing a hybrid modulation strategy is introduced in this article. A key feature of the proposal is that the system can operate with universal input voltage range and dimming from 100% to 10% of rated output power while keeping the dc-link voltage limited to a safe level. In the proposed hybrid scheme, the LEDs current is regulated by pulse-frequency-modulation (PFM) while the dc-link is controlled using an asymmetrical-pulsewidth-modulation strategy. The system owns important operational and constitutive features, such as reduced component count, high-power factor and soft switching on the main switches, thus a high-efficiency operation is achieved. Steady-state and dynamic analyses are discussed in the article and a complete control strategy is also elaborated. Finally, experimental results for a universal input voltage, 42 W output power prototype demonstrates the feasibility of the proposed lighting system.

Error performance analysis of PPM-and FSK-based hybrid modulation scheme for FSO satellite downlink

To enhance the error performance of satellite downlink optical wireless communication system, a hybrid modulation scheme called PPM-FSK-SIM is proposed in this study. It is based on the concept of pulse position modulation (PPM), M-ary frequency shift keying (MFSK) and subcarrier intensity modulation (SIM). The analytical bit error rate (BER) of the proposed hybrid scheme is derived, in the presence of atmospheric turbulence with avalanche photodiode (APD) detection by using Meijer-G, which stands out as a reliable and efficient analytical tool that replaces the time-consuming Monte-Carlo simulation method to target a desirable BER. It is analysed over additive white gaussian noise (AWGN), log-normal and gamma–gamma channel models for geostationary satellite to Earth (downlink) communication. As compared to PPM and minimum shift keying (MSK), PPM-MFSK-SIM gives better error rate performance. The scheme is power efficient making it a favourable modulation technique for satellite downlink communication.

Efficiency-Optimized Modulation Scheme for Three-Phase Dual-Active-Bridge DC–DC Converter

The three-phase dual-active-bridge (3p-DAB) converter features isolated bidirectional power flow, soft-switching capability, reduced transformer, and filter volume. Thus, it is a promising topology for high-power dc-dc conversion. However, when it operates under light load or far from the unitary voltage ratio, the resulting converter efficiency is degraded by the loss of soft-switching capability or increased rms current. To overcome this issue, this article presents an optimized hybrid modulation scheme designed to minimize the total power losses of a 3p-DAB converter under load and voltage variations. An optimization scheme, including switching losses, conduction losses, and core losses, selects the best operation mode for a desired operating point between a novel zero-vector modulation and the traditional phase-shift modulation. An experimental prototype operating at a switching frequency of 100 kHz and a maximum output power of 1 kW was built to validate the theoretical analysis. The results show substantial gain in the converter efficiency, especially at low power or nonunity voltage ratio operating points.

Hybrid modulation strategy of three-phase dual-active-bridge converter to improve power conversion efficiency under light load conditions in LVDC applications

A hybrid control strategy for the three-phase dual-active-bridge (3P-DAB) converter of LVDC applications is presented to improve the power conversion efficiency under light load conditions. The 3P-DAB converter is an attractive topology for high-power applications such as railway traction and aircraft due to its inherent the zero voltage switching (ZVS) capability and reduction of conduction loss due to an interleaved structure with seamless bi-directional power transitions. However, conventional phase shift modulation (PSM) applied to a 3P-DAB converter has disadvantages such as ZVS failure under light load conditions. In this paper, an asymmetrical pulse width modulation is inserted into PSM to replace the existing modulation algorithm of a 3P-DAB converter with a simple control approach. In the proposed control, the transferred power can be calculated according to the mode changes. In addition, the soft switching area of the power switches can be expanded by the proposed hybrid control strategy. Finally, experimental results validate the proposed hybrid modulation strategy using a 3-kW prototype 3P-DAB converter.

Underwater visible light communication at 3.24 Gb/s using novel two-dimensional bit allocation.

Underwater visible light communication (UVLC) systems suffer from a strong nonlinear effect and high inter-symbol interference (ISI). In this study, to improve the performance of a UVLC system under such conditions, we propose a novel nonlinear hybrid modulation scheme named two-dimensional bit allocation (2DBA). By comparing the performance of 2DBA with the famous Levin-Campello (LC) algorithm and the quadrature amplitude modulation (QAM)-based time-domain hybrid modulation (TDHQ) algorithm, we have proved by analysis and experiment that 2DBA can outperform the power allocation-based LC algorithm and the TDHQ algorithm below the 3.8×10-3 hard decision forward error correction threshold (HD-FEC) when the system has a severe nonlinear effect and ISI. The data rate 3.24 Gb/s of 2DBA is measured after 1.2 m underwater transmission; as far as we know, this is the highest data rate reported in a blue LED chip based UVLC system.

Overview of Modulation Strategies for LLC Resonant Converter

In this article, based on the LLC resonant converter fundamental harmonic analysis model, the modulation strategies for LLC resonant converter can be categorized into the following three groups: 1) input voltage fundamental harmonic modulation; 2) resonant tank elements modulation; and 3) secondary equivalent impedance modulation. Operational principles and control diagrams for different modulation strategies are given. Comprehensive comparisons between these modulation strategies in terms of topology complexity, control complexity, and voltage gain range are presented with respect to the same system specifications. The advantages and disadvantages for each modulation strategy are summarized to provide guidance for engineers when analyzing and designing an LLC resonant converter. The hybrid modulation strategies are categorized into different groups based on the specific applications. Brief introductions of these hybrid modulation strategies are presented. Future trends regarding the modulation strategies of LLC resonant converters are presented.

Dual-Mode Modulation Scheme With Seamless Transition for a Tunable Immittance-Based DAB Converter Featuring High-Efficiency Performance Over Whole Output Power Range

Switching loss and conduction loss are two main loss mechanisms in dual-active-bridge (DAB) converter and the degree to which they affect the efficiency of DAB converter depends on the chosen power modulation scheme. In general, no single modulation scheme will perform optimally under all operating conditions. In this article, a hybrid modulation scheme and a new DAB converter topology designed for its realization are presented. The DAB converter is designed to switch between two operation modes—dynamic frequency matching (DFM) for medium- to full-load condition and enhanced dual-phase-shift (EDPS) modulation for light-load condition. Under DFM modulation, a switch-controlled capacitor is used to tune the resonant frequency of an $LCL$ immittance network such that minimum rms current is guaranteed by unity-power-factor operation under different load conditions, leading to zero backflow power, zero circulating current, and zero-voltage switching (ZVS) operation of all switches. Under EDPS modulation, zero backflow power, and ZVS operation of all switches are achieved. These advantageous features are critical to ensuring a high-efficiency operation of DAB converter. The proposed solution is experimentally verified using a 1.5-kW hardware prototype with efficiency exceeding 96% for all output power levels.

A New Hybrid Multilevel Inverter with Extended Number of Voltage Steps

This paper presents and studies an improved multilevel inverter topology to produce higher voltage levels with reduced switching devices at the inverter output. The suggested topology is based on SCSS multilevel inverter. In this topology, DC sources are divided into two groups, inner group and outer group. The function of the outer dc source group is to create the square waveforms that are close in nature to the required output waveforms. However, the inner group is used to increase the number of the DC voltage levels to produce an output voltage close to the desired wave. This task is accomplished by arithmetic operation (addition or subtraction) of the value of the inner DC source with the blocks created by the outer group, respectively. A hybrid modulation scheme has been utilized to substantially reduce harmonics distortion. The performance of the proposed topology is then compared with other conventional and concurrent structures of multilevel inverter as well as evaluated by simulation results.

Modified Modulation Strategy of 1: 1: 2 Asymmetric Nine-Level Inverter and Its Power Balance Method

The three unit nine-level inverter can output more voltage levels with fewer h-bridge units, while having better output waveform quality. However, in the conventional hybrid frequency modulation strategy, only one low-voltage unit adopts pulse width modulation (PWM), which causes the problem of switching loss and uneven heat distribution between the two low-voltage units. At the same time, the output power of the conventional modulation strategy is unbalanced. Aiming to resolve the above problems, a modified hybrid modulation strategy and a power balance control method under the strategy is proposed in this paper. The modulation strategy achieves output power balance between the three units and an even distribution of switching losses between the two low voltage units while maintaining the same output power quality. Simulation and experimental results verify the feasibility of the modulation strategy.

Simulation of WDM-ROF-PON selective access link system based on hybrid modulation of MZM and OFM

Based on the introduction of the principle of the optical wireless system, the WDM-ROF based on hybrid modulation of MZM and OFM is designed WDM-ROF-PON system architecture, using simulation software to build the system to verify the rationality of the MZM and OFM hybrid modulation system structure; and put forward an MZM and OFM hybrid modulation scheme, using IRZ code in the downlink data stream, with better extinction ratio and dispersion tolerance, reducing the cost of the user receiving equipment, improving the system transmission quality. The simulation results show that the scheme of MZM and OFM hybrid modulation WDM-of-pon network architecture is reasonable and practical.

Deep Learning for Optical Vehicular Communication

In this paper, we consider the current status and technical issues involved in the use of optical camera communication (OCC)/visible light communication (VLC) technologies in vehicular communication systems. Hybrid spatial phase-shift keying was introduced in IEEE 802.15.7-2018 as the standard hybrid modulation scheme for vehicular OCC/VLC systems. We herein propose a functional communication system architecture for vehicular systems based on this hybrid waveform, and we also present state-of-the-art research work on an artificial intelligence (AI)-based vehicular OCC system. Every AI module within the proposed system architecture is discussed in detail. Finally, our experimental procedures and results are analyzed to evaluate the performance of the proposed system over a complex channel model in a vehicular environment. We effectively employed the popular You Only Look Once version 2 object detection algorithm for real-time region-of-interest tracking in city driving (at a vehicular velocity of around 30 km/h and highway night driving (at a vehicular velocity of > 60 km/h) scenarios. Moreover, our novel neural-network-based decoder and AI-based error correction proved effective in improving the data decoding accuracy, resulting in a best-case reduction of 2.2 and 9.0 dB, respectively, in the signal-to-noise ratio needed to achieve the desired bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> in a vehicular OCC/VLC system.

Square geometrical shaping 128QAM based time domain hybrid modulation in visible light communication system

We proposed two kinds of visible light communication(VLC) systems which respectively based on 64QAM/square geometrical shaping(SGS) 128QAM time domain hybrid modulation scheme (SGSHY) and 64QAM/128QAM time domain hybrid modulation scheme (REGHY). These two systems can operate around specific forward error correction (FEC) threshold and maximize the achievable information rate (AIR) of the system. The principles of SGSHY and REGHY are proposed in detail, which has very low computation complexity compared with probabilistic shaping. The SGSHY outperforms REGHY at high peak to peak voltage (Vpp). Experimental results show that at high Vpp like 1.4 V, which means the system is suffering from high nonlinear distortion, the AIR of SGSHY outperforms that of REGHY by 0.12Gb/s at the 2×10–2 FEC threshold. The AIR of the REGHY is at most 0.36Gb/s higher than that of 64QAM at 0.8V Vpp and 7% FEC threshold, while the (achievable information rate) AIR of SGSHY is at most 0.40Gb/s higher than that of 64QAM at 1.4 V Vpp and 20% FEC threshold.