Showing papers on "Modulation published in 2020"

Ultra-dense optical data transmission over standard fibre with a single chip source.

Abstract: Micro-combs - optical frequency combs generated by integrated micro-cavity resonators – offer the full potential of their bulk counterparts, but in an integrated footprint. They have enabled breakthroughs in many fields including spectroscopy, microwave photonics, frequency synthesis, optical ranging, quantum sources, metrology and ultrahigh capacity data transmission. Here, by using a powerful class of micro-comb called soliton crystals, we achieve ultra-high data transmission over 75 km of standard optical fibre using a single integrated chip source. We demonstrate a line rate of 44.2 Terabits s−1 using the telecommunications C-band at 1550 nm with a spectral efficiency of 10.4 bits s−1 Hz−1. Soliton crystals exhibit robust and stable generation and operation as well as a high intrinsic efficiency that, together with an extremely low soliton micro-comb spacing of 48.9 GHz enable the use of a very high coherent data modulation format (64 QAM - quadrature amplitude modulated). This work demonstrates the capability of optical micro-combs to perform in demanding and practical optical communications networks. Microcombs provide many opportunities for integration in optical communications systems. Here, the authors implement a soliton crystal microcomb as a tool to demonstrate more than 44 Tb/s communications with high spectral efficiency.

High-performance coherent optical modulators based on thin-film lithium niobate platform

Abstract: The coherent transmission technology using digital signal processing and advanced modulation formats, is bringing networks closer to the theoretical capacity limit of optical fibres, the Shannon limit. The in-phase/quadrature electro-optic modulator that encodes information on both the amplitude and the phase of light, is one of the underpinning devices for the coherent transmission technology. Ideally, such modulator should feature a low loss, low drive voltage, large bandwidth, low chirp and compact footprint. However, these requirements have been only met on separate occasions. Here, we demonstrate integrated thin-film lithium niobate in-phase/quadrature modulators that fulfil these requirements simultaneously. The presented devices exhibit greatly improved overall performance (half-wave voltage, bandwidth and optical loss) over traditional lithium niobate counterparts, and support modulation data rate up to 320 Gbit s−1. Our devices pave new routes for future high-speed, energy-efficient, and cost-effective communication networks. In-phase/quadrature (IQ) electro-optic modulators are underpinning devices for coherent transmission technology. Here the authors present IQ modulators in the lithium-niobate-on-insulator platform, which provide improved overall performance and advanced modulation formats for future coherent transmission systems.

MIMO Transmission Through Reconfigurable Intelligent Surface: System Design, Analysis, and Implementation

Abstract: Reconfigurable intelligent surface (RIS) is a new paradigm that has great potential to achieve cost-effective, energy-efficient information modulation for wireless transmission, by the ability to change the reflection coefficients of the unit cells of a programmable metasurface. Nevertheless, the electromagnetic responses of the RISs are usually only phase-adjustable, which considerably limits the achievable rate of RIS-based transmitters. In this paper, we propose an RIS architecture to achieve amplitude-and-phase-varying modulation, which facilitates the design of multiple-input multiple-output (MIMO) quadrature amplitude modulation (QAM) transmission. The hardware constraints of the RIS and their impacts on the system design are discussed and analyzed. Furthermore, the proposed approach is evaluated using our prototype which implements the RIS-based MIMO-QAM transmission over the air in real time.

800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing

Abstract: The design of application-specific integrated circuits (ASIC) is at the core of modern ultra-high-speed transponders employing advanced digital signal processing (DSP) algorithms. This manuscript discusses the motivations for jointly utilizing transmission techniques such as probabilistic shaping and digital sub-carrier multiplexing in digital coherent optical transmissions systems. First, we describe the key-building blocks of modern high-speed DSP-based transponders working at up to 800G per wave. Second, we show the benefits of these transmission methods in terms of system level performance. Finally, we report, to the best of our knowledge, the first long-haul experimental transmission – e.g., over 1000 km – with a real-time 7 nm DSP ASIC and digital coherent optics (DCO) capable of data rates up to 1.6 Tb/s using two waves (2 × 800G).

Lithium niobate photonic-crystal electro-optic modulator

Abstract: Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V-1, a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μm3. The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s-1 with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics.

Overview of Modulation Strategies for LLC Resonant Converter

Abstract: 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.

Efficient bidirectional piezo-optomechanical transduction between microwave and optical frequency.

Abstract: Efficient interconversion of both classical and quantum information between microwave and optical frequency is an important engineering challenge. The optomechanical approach with gigahertz-frequency mechanical devices has the potential to be extremely efficient due to the large optomechanical response of common materials, and the ability to localize mechanical energy into a micron-scale volume. However, existing demonstrations suffer from some combination of low optical quality factor, low electrical-to-mechanical transduction efficiency, and low optomechanical interaction rate. Here we demonstrate an on-chip piezo-optomechanical transducer that systematically addresses all these challenges to achieve nearly three orders of magnitude improvement in conversion efficiency over previous work. Our modulator demonstrates acousto-optic modulation with $${V}_{\pi }$$ = 0.02 V. We show bidirectional conversion efficiency of $$1{0}^{-5}$$ with 3.3 μW red-detuned optical pump, and $$5.5 \%$$ with 323 μW blue-detuned pump. Further study of quantum transduction at millikelvin temperatures is required to understand how the efficiency and added noise are affected by reduced mechanical dissipation, thermal conductivity, and thermal capacity. Current optomechanical implementations of microwave and optical frequency interconversion are lacking in efficiency and interaction strength. The authors design and demonstrate an on-chip piezo-optomechanical solution which overcomes several technical barriers to reach several orders of magnitude improvement in efficiency.

An Overview of Signal Processing Techniques for Terahertz Communications

Abstract: Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advancements in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and the corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress paves the way for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques with an emphasis on ultra-massive multiple-input multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, which are vital to overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

Realization of Multi-Modulation Schemes for Wireless Communication by Time-Domain Digital Coding Metasurface

Abstract: Metasurfaces are well-designed artificial periodic or quasiperiodic structures to achieve customized electromagnetic responses. Based on the time-domain digital coding metasurface (TDCM) with dynamic reflection or transmission characteristics, one can realize direct information modulation on the metasurface, which has been used in wireless communications. In comparison with the classical architectures of wireless communication systems, the TDCM has advantages for simple architecture, easy manufacture, and low cost. However, the wireless communication systems based on the TDCM are still limited by low conversion efficiency, spectrum pollution, and hard to implement high-order modulation schemes. To solve these limitations, we propose a new route to achieve multi-modulation schemes in wireless communications by regulating the reflection phases of the TDCM in a nonlinear way. By carefully designing the regulation voltages applied to the diodes on TDCM, arbitrary constellation diagrams are successfully synthesized. The novel systems can implement various modulation schemes such as quadrature phase shift keying (QPSK), 8PSK, and 16QAM with good communication quality, high stability, and free scheme switching. The presented method may find important applications in modern wireless communication technologies.

Conventional/phase Shift Dual Drive Mach-Zehnder Modulation Measured Type Based Radio Over Fiber Systems

Abstract: This work clarified the conventional/phase shift dual Mach–Zehnder modulation measured technique for upgrading radio over fiber (ROF) systems. Four levels of pulse amplitude modulation and non-return to zero code scheme are employed in different previous studies for upgrading ROF systems. The possible transmission distance is extended to 150 km and a bit rate of 40 Gb/s for achieved acceptable max. Q factor of 13.7498 and minimum bit error rates of 1.82×10–43. The total optical received power is measured at fiber cable length for maximum distance. Also, in the same way, the signal power levels are measured after fiber optic cable and avalanche photo detector optical receiver. The proposed model has presented the enhancement percentage ratio of 13.654% over the previous models in signal quality enhancement.

Different modulation schemes for direct and external modulators based on various laser sources

Abstract: Different types of laser source modulation techniques have been used in various applications depending on the objective. As optical systems extract the laws and the best solutions from experiments and simulations, the present study uses simulation software with different modulation types so the output signals can be compared. The modulators used are Mach-Zehnder, which is an external modulator, and electro-absorption modulator and laser rate equation modulator, which are direct modulators. All these types have an optical link multimode (MM) fiber with a photodiode in the receiver end that can be modeled. The input and output signals are analyzed using different types of modulations.

Laser measured rate equations with various transmission coders for optimum of data transmission error rates

Abstract: The present study has outlined laser-measured rate equations with various transmission coders for optimum data transmission error rates. Various modulation transmission coders are employed, such as a pulse position modulation coder, a differential pulse intensity modulation coder, and a four band/five band modulation transmission coder, in order to create optimized data rates of up to 40 GB/s for a fiber extension length of up to 100 km. This study has emphasized the important role of pulse position modulation transmission coders, which exhibit superior performance in max. Q parameter and min. data error rates, even for high data rate transmission.

The research on 220GHz multicarrier high-speed communication system

Abstract: This paper presents our investigation into a 220 GHz multicarrier high-speed communication system based on solid state transceivers. The proposed system has eased the demand of high sampling rate analog-to-digital converter (ADC) by providing several signal carriers in microwave band and converting them to 220 GHz channel. The system consists of a set of 220 GHz solid-state transceiver with 2 signal carriers, two base-bands for 4 GSPS ADCs. It has achieved 12.8 Gbps rate real-time signal transmission using 16QAM modulation over a distance of 20 m without any other auxiliary equipment or test instruments. The baseband algorithm overcomes the problem of frequency difference generates by non-coherent structure, which guarantees the feasibility of long-distance transmission application. Most importantly, the proposed system has already carried out multi-channel 8K video parallel transmission through switch equipment, which shows the multicarrier high-speed communication system in submillimeter wave has great application prospects. To the best of the authors' knowledge, this is the first all-solid-state electronics multicarrier communication system in submillimeter and terahertz band.

High speed optical switching gain based EDFA model with 30 Gb/s NRZ modulation code in optical systems

Abstract: This study presents high speed optical switching gain based Erbium doped fiber amplifier model. By using the proposed model the optical fiber loss can be minimized. The system is stabilized with the power budget of 25.875 mW a long 75 km as a length of optical fiber in this study can be verified. The modulation rate of 10 Gb/s can be upgrade up to reach 30 Gb/s. The suitable power for the optical transmitter is −2.440 dBm and NRZ modulation code is verified. The receiver sensitivity can be upgraded with the minimum bit error rate and max Q factor are 1.806 e−009 and 5.899.

Optical data transmission at 44Tb/s and 10 bits/s/Hz over the C-band with standard fibre and a single micro-comb source

Abstract: Micro-combs [1 - 4], optical frequency combs generated by integrated micro-cavity resonators, offer the full potential of their bulk counterparts [5,6], but in an integrated footprint. The discovery of temporal soliton states (DKS dissipative Kerr solitons) [4,7-11] as a means of modelocking microcombs has enabled breakthroughs in many fields including spectroscopy [12,13], microwave photonics [14], frequency synthesis [15], optical ranging [16,17], quantum sources [18,19], metrology [20,21] and more. One of their most promising applications has been optical fibre communications where they have enabled massively parallel ultrahigh capacity multiplexed data transmission [22,23]. Here, by using a new and powerful class of microcomb called soliton crystals [11], we achieve unprecedented data transmission over standard optical fibre using a single integrated chip source. We demonstrate a line rate of 44.2 Terabits per second using the telecommunications C band at 1550nm with a spectral efficiency, a critically important performance metric, of 10.4 bits/s/Hz. Soliton crystals exhibit robust and stable generation and operation as well as a high intrinsic efficiency that, together with a low soliton microcomb spacing of 48.9 GHz enable the use of a very high coherent data modulation format of 64 QAM (quadrature amplitude modulated). We demonstrate error free transmission over 75 km of standard optical fibre in the laboratory as well as in a field trial over an installed metropolitan optical fibre network. These experiments were greatly aided by the ability of the soliton crystals to operate without stabilization or feedback control. This work demonstrates the capability of optical soliton crystal microcombs to perform in demanding and practical optical communications networks.

Signal and System Design for Wireless Power Transfer: Prototype, Experiment and Validation

Abstract: A new line of research on communications and signals design for Wireless Power Transfer (WPT) has recently emerged in the communication literature. Promising signal strategies to maximize the power transfer efficiency of WPT rely on (energy) beamforming, waveform, modulation and transmit diversity, and a combination thereof. To a great extent, the study of those strategies has so far been limited to theoretical performance analysis. In this paper, we study the real over-the-air performance of all the aforementioned signal strategies for WPT. To that end, we have designed, prototyped and experimented an innovative radiative WPT architecture based on Software-Defined Radio (SDR) that can operate in open-loop and closed-loop (with channel acquisition at the transmitter) modes. The prototype consists of three important blocks, namely the channel estimator, the signal generator, and the energy harvester. The experiments have been conducted in a variety of deployments, including frequency flat and frequency selective channels, under static and mobility conditions. Experiments highlight that a channel-adaptive WPT architecture based on joint beamforming and waveform design offers significant performance improvements in harvested DC power over conventional single-antenna/multi-antenna continuous wave systems. The experimental results fully validate the observations predicted from the theoretical signal designs and confirm the crucial and beneficial role played by the energy harvester nonlinearity.

Hybrid Beamforming for Terahertz Multi-Carrier Systems Over Frequency Selective Fading

Abstract: We propose novel hybrid beamforming schemes for the terahertz (THz) wireless system where a multi-antenna base station (BS) communicates with a multi-antenna user over frequency selective fading. Here, we assume that the BS employs sub-connected hybrid beamforming and multi-carrier modulation to deliver ultra high data rate. We consider a three-dimensional wideband THz channel by incorporating the joint effect of molecular absorption, high sparsity, and multi-path fading, and consider the carrier frequency offset in multi-carrier systems. With this model, we first propose a two-stage wideband hybrid beamforming scheme which includes a beamsteering codebook searching algorithm for analog beamforming and a regularized channel inversion method for digital beamforming. We then propose a novel wideband hybrid beamforming scheme with two digital beamformers. In this scheme, an additional digital beamformer is developed to compensate for the performance loss caused by the constant-amplitude hardware constraints and the difference of channel matrices among subcarriers. Furthermore, we consider imperfect channel state information (CSI) and propose a probabilistic robust hybrid beamforming scheme to combat channel estimation errors. Numerical results demonstrate the benefits of our proposed schemes for the sake of practical implementation, especially considering its high spectral efficiency, low complexity, and robustness against imperfect CSI.

80-GHz Bandwidth and 1.5-V V π InP-Based IQ Modulator

Abstract: We report a promising IQ optical modulator for beyond 100-GBd transmitter. By introducing both a new n-i-p-n heterostructure and an optimized capacitance-loaded traveling-wave electrode (CL-TWE), high-frequency electrical losses of the modulator can be drastically reduced. As a result, we extended an electro-optic (EO) bandwidth without degrading other properties, such as half-wave voltage (Vπ) and optical losses. The 3-dB EO bandwidth of the 1.5-V Vπ modulator reaches 80 GHz. Furthermore, we demonstrated up to 128-GBd IQ modulations by co-assembling with an ultra-broadband InP-based driver IC.

Design and Implementation of New Multilevel Inverter Topology for Trinary Sequence Using Unipolar Pulsewidth Modulation

Abstract: This paper proposes a new multilevel inverter (MLI) topology that utilizes trinary sequence for the dc sources. It gives maximum output voltage level with minimum dc source and switch count when compared to other sequences, such as symmetric, natural, binary, and quasi-linear. This is due to the fact that the trinary sequence generates of all additive and subtractive combinations of input dc levels in the output voltage waveform. The concept is implemented on a 9-level asymmetric MLI using only four active devices. Multicarrier unipolar pulsewidth modulation technique is adopted to create the switching pulses. Theoretical calculation of total harmonic distortion in both voltage and current waveforms has been performed using asymptotic time domain formula. These values are compared with simulation and experimental values for different modulation indices. Power loss calculation for proposed topology is discussed with appropriate mathematical equations.

Ultrafast all-optical switching enabled by epsilon-near-zero-tailored absorption in metal-insulator nanocavities

Abstract: Ultrafast control of light−matter interactions is fundamental in view of new technological frontiers of information processing However, conventional optical elements are either static or feature switching speeds that are extremely low with respect to the time scales at which it is possible to control light Here, we exploit the artificial epsilon-near-zero (ENZ) modes of a metal-insulator-metal nanocavity to tailor the linear photon absorption of our system and realize a nondegenerate all-optical ultrafast modulation of the reflectance at a specific wavelength Optical pumping of the system at its high energy ENZ mode leads to a strong redshift of the low energy mode because of the transient increase of the local dielectric function, which leads to a sub-3-ps control of the reflectance at a specific wavelength with a relative modulation depth approaching 120% All-optical switching allows control of one optical signal using another, holding potential to overcome the limitations of electrical switches via ultrafast manipulation of light In this work, sub-3 ps all-optical switching is achieved in an epsilon-near-zero nanocavity, exhibiting a relative modulation depth of 120% at a specific wavelength

Linear and Nonlinear Frequency-Division Multiplexing

Abstract: Two signal multiplexing schemes for optical fiber communication are considered: Wavelength-division multiplexing (WDM) and nonlinear frequency-division multiplexing (NFDM), based on the nonlinear Fourier transform. Achievable information rates (AIRs) of NFDM and WDM are compared in a network scenario with an ideal lossless model of the optical fiber in the defocusing regime. It is shown that the NFDM AIR is greater than the WDM AIR subject to a bandwidth and average power constraint, in a representative system with one symbol per user. The improvement results from nonlinear signal multiplexing.

Enhanced Single-Sideband Time-Modulated Phased Array With Lower Sideband Level and Loss

Abstract: This article presents an enhanced single-sideband time-modulated phased array (ESTMPA) using modulating pulses with stepped waveforms. Based on the in-phase/quadrature (I/Q) complex modulation technique, this phase-only weighting array generates a scanning beam at the 1st sideband. The proposed modulating pulses realized through a reconfigurable power divider in I/Q time modulator can avoid the power loss from the switches during switch-OFF state and eliminate the maximum undesired sideband—the 5th harmonic in STMPA. As a result, it brings a power spectrum with less undesired sidebands, lower sideband level (−16.9 dB), higher harmonic efficiency (94.96%), and wider allowable signal bandwidth (eight times as wide as that of the conventional time modulated array). To experimentally verify the feasibility of the proposed design, a wideband enhanced I/Q time modulator and its corresponding eight-element ESTMPA are designed and manufactured. A detailed study on the effect of the magnitude and phase deviations in the circuit and the transition period of modulating pulse are presented. The measured results of power spectrum and radiation pattern have a good agreement with the simulated ones.

A Generalized Carrier-Overlapped PWM Method for Neutral-Point-Clamped Multilevel Converters

Abstract: The neutral-point voltage unbalance problem holds back the extensive application of neutral-point-clamped (NPC) multilevel converters with more than three voltage levels in industry. Traditional phase-disposition pulsewidth modulation (PDPWM) or nearest-three-vector (NTV) modulation cannot achieve the voltage balance over the full modulation indexes and load power factors when the voltage level is higher than three. To solve this problem, a novel generalized carrier-based PWM method, named carrier-overlapped PWM (COPWM), for NPC multilevel converters is proposed in this article, which is proven to satisfy the volt–second balance principle. With this modulation method, the average values of all the neutral-point currents are equal to zero in a fundamental period, and therefore, all the dc-link capacitor voltages can be naturally balanced in the ideal and steady-state conditions. In order to simplify its implementation, an equivalent multireference modulation method with only one triangular carrier is also derived. Simulation and experimental results on a five-level diode-clamped inverter are presented to verify the proposed modulation method.

A WDM PAM4 FSO–UWOC Integrated System With a Channel Capacity of 100 Gb/s

Abstract: A wavelength-division-multiplexing (WDM) four-level pulse amplitude modulation (PAM4) free-space optical (FSO)–underwater wireless optical communication (UWOC) integrated system with a channel capacity of 100 Gb/s is proposed and attainably demonstrated. Analytic results reveal that 1.8-GHz 405-nm blue-violet-light and 1.7-GHz 450-nm blue-light laser diodes (LDs) with two-stage light injection and optoelectronic feedback techniques are competently adopted for 100 Gb/s PAM4 signal transmission through a 500-m free-space transmission with 5-m clear ocean underwater link. Combining dual-wavelength WDM scenario with PAM4 modulation, the channel capacity of FSO–UWOC integrated systems is significantly enhanced with an aggregate transmission rate of 100 Gb/s (25 Gbaud PAM4/wavelength × 2 wavelengths). With doublet lenses in FSO, laser beam reducer and transmissive spatial light modulator in UWOC, a sufficiently low bit error rate of 10−9 and acceptable PAM4 eye diagrams are acquired. This demonstrated 100 Gb/s PAM4 FSO–UWOC integrated system with a WDM scenario is advantageous for the enhancement of a high-speed optical wireless link with long-reach transmission.

Hybrid integrated photonics using bulk acoustic resonators.

Abstract: Integrated photonic devices based on Si3N4 waveguides allow for the exploitation of nonlinear frequency conversion, exhibit low propagation loss, and have led to advances in compact atomic clocks, ultrafast ranging, and spectroscopy. Yet, the lack of Pockels effect presents a major challenge to achieve high-speed modulation of Si3N4. Here, microwave-frequency acousto-optic modulation is realized by exciting high-overtone bulk acoustic wave resonances (HBAR) in the photonic stack. Although HBAR is ubiquitously used in modern communication and superconducting circuits, this is the first time it has been incorporated on a photonic integrated chip. The tight vertical acoustic confinement releases the lateral design of freedom, and enables negligible cross-talk and preserving low optical loss. This hybrid HBAR nanophotonic platform can find immediate applications in topological photonics with synthetic dimensions, compact opto-electronic oscillators, and microwave-to-optical converters. As an application, a Si3N4-based optical isolator is demonstrated by spatiotemporal modulation, with over 17 dB isolation achieved. Here, the authors demonstrate acousto-optic modulation of silicon nitride microring resonators using high-overtone bulk acoustic wave resonances, allowing modulation in the GHz range via acoustic waves. As an application, an optical isolator is demonstrated with 17 dB non-reciprocity.

Transmitter and Receiver Window Designs for Orthogonal Time Frequency Space Modulation

Abstract: In this paper, we investigate the impacts of transmitter and receiver windows on the performance of orthogonal time-frequency space (OTFS) modulation and propose window designs to improve the OTFS channel estimation and data detection performance. In particular, assuming ideal pulse shaping filters at the transceiver, we derive the impacts of windowing on the effective channel and its estimation performance in the delay-Doppler (DD) domain, the total average transmit power and the effective noise covariance matrix. When the channel state information (CSI) is available at the transceiver, we analyze the minimum squared error (MSE) of data detection and propose an optimal transmitter window to minimize the detection MSE. The proposed optimal transmitter window is interpreted as a mercury/water-filling power allocation scheme, where the mercury is firstly filled before pouring water to pre-equalize the TF domain channels. When the CSI is not available at the transmitter but can be estimated at the receiver, we propose to apply a Dolph-Chebyshev (DC) window at either the transmitter or the receiver, which can effectively enhance the sparsity of the effective channel in the DD domain. Thanks to the enhanced DD domain channel sparsity, the channel spread due to the fractional Doppler is significantly reduced, which leads to a lower error floor in both channel estimation and data detection compared with that of rectangular window. Simulation results verify the accuracy of the obtained analytical results and confirm the superiority of the proposed window designs in improving the channel estimation and data detection performance over the conventional rectangular window design.

A Simplified Space Vector Pulsewidth Modulation Scheme for Three-Phase Cascaded H-Bridge Inverters

Abstract: A simplified space vector pulsewidth modulation (SVPWM) for three-phase cascaded H-bridge (CHB) inverters is presented in this article. Treating each unit as a three-level inverter and adopting serial calculation mode, a CHB inverter is modulated unit by unit using three-level SVPWM. Duty cycles of real sector are obtained by mapping duty cycles of sector 1, in which the calculation of three-level SVPWM is done. The process of implementing multilevel SVPWM is simplified to the process of implementing three-level SVPWM. By reusing field-programmable gate array (FPGA) chip resource that is used for the calculation of three-level SVPWM, the presented SVPWM can be easily adopted for a CHB inverter with different number of units, while the FPGA chip resource utilization is reduced significantly. In addition, the presented SVPWM provides an effective switching frequency higher than the switching frequency of IGBTs. Simulation and experimental results are provided to verify the feasibility of the presented SVPWM.

Blind Modulation Classification for Asynchronous OFDM Systems Over Unknown Signal Parameters and Channel Statistics

Abstract: Blind modulation classification (MC) is an integral part of designing an adaptive or intelligent transceiver for future wireless communications. However, till date, only a few works have been reported in the literature for blind MC of orthogonal frequency division multiplexing (OFDM) system over frequency-selective fading environment. In this paper, a blind MC algorithm has been proposed and implemented over National Instruments (NI) testbed setup for linearly modulated signals of OFDM system by using discrete Fourier transform (DFT) and normalized fourth-order cumulant. The proposed MC algorithm works in the presence of synchronization errors, i.e., frequency, timing, and phase offsets and without the prior information about the signal parameters and channel statistics. To nullify the effect of timing offset in the feature extraction process, a statistical average has been taken over OFDM symbols after introducing uniformly distributed random timing offsets in each of the OFDM symbols. In this work, we have classified a more extensive pool of modulation formats for OFDM signal, i.e., binary phase-shift keying (BPSK), quadrature PSK (QPSK), offset QPSK (OQPSK), minimum shift keying (MSK), and 16 quadrature amplitude modulation (16-QAM). Classification is performed in two stages. At the first stage, a normalized fourth-order cumulant is used on the DFT of the received OFDM signal to classify OQPSK, MSK, and 16-QAM modulation formats. At the second stage, first we compute the DFT of the square of the received OFDM signal and then a normalized fourth-order cumulant is used to classify BPSK and QPSK modulation formats. The success rate of the proposed MC algorithm is evaluated through analytical and Monte Carlo simulations and compared with existing methods. Finally, the work is validated by providing an experimental setup on NI hardware over an indoor propagation environment.