Showing papers on "Modulation published in 2014"

Coherent terabit communications with microresonator Kerr frequency combs.

Abstract: Optical frequency combs have the potential to revolutionize terabit communications1. Generation of Kerr combs in nonlinear microresonators2 represents a particularly promising option3 enabling line spacings of tens of GHz. However, such combs may exhibit strong phase noise4-6, which has made high-speed data transmission impossible up to now. Here we demonstrate that systematic adjustment of pump conditions for low phase noise4,7-9 enables coherent data transmission with advanced modulation formats that pose stringent requirements on the spectral purity of the comb. In a first experiment, we encode a data stream of 392 Gbit/s on a Kerr comb using quadrature phase shift keying (QPSK) and 16-state quadrature amplitude modulation (16QAM). A second experiment demonstrates feedback-stabilization of the comb and transmission of a 1.44 Tbit/s data stream over up to 300 km. The results show that Kerr combs meet the highly demanding requirements of coherent communications and thus offer an attractive solution towards chip-scale terabit/s transceivers.

100 GHz silicon-organic hybrid modulator

Abstract: Electro-optic modulation at frequencies of 100 GHz and beyond is important for photonic-electronic signal processing at the highest speeds. To date, however, only a small number of devices exist that can operate up to this frequency. In this study, we demonstrate that this frequency range can be addressed by nanophotonic, silicon-based modulators. We exploit the ultrafast Pockels effect by using the silicon–organic hybrid (SOH) platform, which combines highly nonlinear organic molecules with silicon waveguides. Until now, the bandwidth of these devices was limited by the losses of the radiofrequency (RF) signal and the RC (resistor-capacitor) time constant of the silicon structure. The RF losses are overcome by using a device as short as 500 µm, and the RC time constant is decreased by using a highly conductive electron accumulation layer and an improved gate insulator. Using this method, we demonstrate for the first time an integrated silicon modulator with a 3dB bandwidth at an operating frequency beyond 100 GHz. Our results clearly indicate that the RC time constant is not a fundamental speed limitation of SOH devices at these frequencies. Our device has a voltage–length product of only VπL=11 V mm, which compares favorably with the best silicon-photonic modulators available today. Using cladding materials with stronger nonlinearities, the voltage–length product is expected to improve by more than an order of magnitude. Researchers have developed an integrated silicon–organic device capable of modulating light at frequencies of up to 100 GHz. The ultrafast modulator, fabricated by Luca Alloatti and co-workers, relies on the electro-optic Pockels effect that occurs in a polymer cladding covering a silicon slot waveguide. Application of an electric field to the polymer causes its refractive index to change, which in turn modifies the phase of light passing through it. The 500-µm-long device has a half-wave voltage of 22 V, meaning that this voltage is required in order to achieve a π phase shift for the output light. However, the researchers are confident that using a cladding material with a stronger nonlinearity could improve this figure of merit by a factor of ten.

Femtosecond-Long Pulse-Based Modulation for Terahertz Band Communication in Nanonetworks

Abstract: Nanonetworks consist of nano-sized communicating devices which are able to perform simple tasks at the nanoscale. Nanonetworks are the enabling technology of long-awaited applications such as advanced health monitoring systems or high-performance distributed nano-computing architectures. The peculiarities of novel plasmonic nano-transceivers and nano-antennas, which operate in the Terahertz Band (0.1-10 THz), require the development of tailored communication schemes for nanonetworks. In this paper, a modulation and channel access scheme for nanonetworks in the Terahertz Band is developed. The proposed technique is based on the transmission of one-hundred-femtosecond-long pulses by following an asymmetric On-Off Keying modulation Spread in Time (TS-OOK). The performance of TS-OOK is evaluated in terms of the achievable information rate in the single-user and the multi-user cases. An accurate Terahertz Band channel model, validated by COMSOL simulation, is used, and novel stochastic models for the molecular absorption noise in the Terahertz Band and for the multi-user interference in TS-OOK are developed. The results show that the proposed modulation can support a very large number of nano-devices simultaneously transmitting at multiple Gigabits-per-second and up to Terabits-per-second, depending on the modulation parameters and the network conditions.

A Cascaded Multilevel Inverter Based on Switched-Capacitor for High-Frequency AC Power Distribution System

Abstract: The increase of transmission frequency reveals more merits than low- or medium-frequency distribution among different kinds of power applications. High-frequency inverter serves as source side in high-frequency ac (HFAC) power distribution system (PDS). However, it is complicated to obtain a high-frequency inverter with both simple circuit topology and straightforward modulation strategy. A novel switched-capacitor-based cascaded multilevel inverter is proposed in this paper, which is constructed by a switched-capacitor frontend and H-Bridge backend. Through the conversion of series and parallel connections, the switched-capacitor frontend increases the number of voltage levels. The output harmonics and the component counter can be significantly reduced by the increasing number of voltage levels. A symmetrical triangular waveform modulation is proposed with a simple analog implementation and low modulation frequency comparing with traditional multicarrier modulation. The circuit topology, symmetrical modulation, operation cycles, Fourier analysis, parameter determination, and topology enhancement are examined. An experimental prototype with a rated output frequency of 25 kHz is implemented to compare with simulation results. The experimental results agreed very well with the simulation that confirms the feasibility of proposed multilevel inverter.

OFDM With Interleaved Subcarrier-Index Modulation

Abstract: Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a recently developed technique which modulates part of the information bits using the indices of OFDM subcarriers. In this letter, a simple and efficient subcarrier-level interleaving scheme is introduced to improve the performance of conventional OFDM-IM through enlarging the Euclidean distances among the modulated symbols. Both theoretical analysis and simulation results are presented to show that the proposed OFDM with interleaved sucarrier-index modulation (OFDM-ISIM) can achieve better system performance than conventional OFDM-IM and OFDM with low-order modulation schemes such as binary phase shift keying, quadrature phase shift keying and 16 quadrature amplitude modulation.

Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for “smart window” applications

Abstract: The application of a thin film of poly(N-isopropylacrylmide) hydrogel to thermochromic smart windows has been investigated. As the thickness of the hydrogel increased from 26 to 200 μm and the testing temperature increased from 20 to 60 °C, the solar modulating ability (ΔTsol) increased and the luminous transmittance (Tlum) decreased. Compared with the best reported results for the most studied inorganic VO2 thermochromic coatings, an unprecedented good combination of near-doubled average Tlum (70.7% vs. 42.8%), higher ΔTsol (25.5% vs. 22.3%) and lower transition temperature (∼32 °C vs. ∼68 °C) could be achieved by hydrogels with an optimized thickness. Good durability and reversibility were established with the correct sealing of the sandwich structure.

Low-loss, silicon integrated, aluminum nitride photonic circuits and their use for electro-optic signal processing

Abstract: Photonic miniaturization requires seamless integration of linear and nonlinear optical components to achieve passive and active functions simultaneously. Among the available material systems, silicon photonics holds immense promise for optical signal processing and on-chip optical networks. However, silicon is limited to wavelengths above 1100 nm and does not provide the desired lowest order optical nonlinearity for active signal processing. Here we report the integration of aluminum nitride (AlN) films on silicon substrates to bring active functionalities to chip-scale photonics. Using CMOS-compatible sputtered thin films we fabricate AlN-on-insulator waveguides that exhibit low propagation loss (0.6 dB/cm). Exploiting AlN's inherent Pockels effect we demonstrate electro-optic modulation up to 4.5 Gb/s with very low energy consumption (down to 10 fJ/bit). The ultra-wide transparency window of AlN devices also enables high speed modulation at visible wavelengths. Our low cost, wideband, carrier-free photonic circuits hold promise for ultra-low power and high speed signal processing at the microprocessor chip level.

Adaptive OFDM Modulation for Underwater Acoustic Communications: Design Considerations and Experimental Results

Abstract: In this paper, we explore design aspects of adaptive modulation based on orthogonal frequency-division multiplexing (OFDM) for underwater acoustic (UWA) communications, and study its performance using real-time at-sea experiments. Our design criterion is to maximize the system throughput under a target average bit error rate (BER). We consider two different schemes based on the level of adaptivity: in the first scheme, only the modulation levels are adjusted while the power is allocated uniformly across the subcarriers, whereas in the second scheme, both the modulation levels and the power are adjusted adaptively. For both schemes we linearly predict the channel one travel time ahead so as to improve the performance in the presence of a long propagation delay. The system design assumes a feedback link from the receiver that is exploited in two forms: one that conveys the modulation alphabet and quantized power levels to be used for each subcarrier, and the other that conveys a quantized estimate of the sparse channel impulse response. The second approach is shown to be advantageous, as it requires significantly fewer feedback bits for the same system throughput. The effectiveness of the proposed adaptive schemes is demonstrated using computer simulations, real channel measurements recorded in shallow water off the western coast of Kauai, HI, USA, in June 2008, and real-time at-sea experiments conducted at the same location in July 2011. We note that this is the first paper that presents adaptive modulation results for UWA links with real-time at-sea experiments.

Information Transmission Using the Nonlinear Fourier Transform, Part III: Spectrum Modulation

Abstract: Motivated by the looming capacity crunch in fiber-optic networks, information transmission over such systems is revisited. Among numerous distortions, interchannel interference in multiuser wavelength-division multiplexing (WDM) is identified as the seemingly intractable factor limiting the achievable rate at high launch power. However, this distortion and similar ones arising from nonlinearity are primarily due to the use of methods suited for linear systems, namely WDM and linear pulse-train transmission, for the nonlinear optical channel. Exploiting the integrability of the nonlinear Schrodinger (NLS) equation, a nonlinear frequency-division multiplexing (NFDM) scheme is presented, which directly modulates noninteracting signal degrees-of-freedom under NLS propagation. The main distinction between this and previous methods is that NFDM is able to cope with the nonlinearity, and thus, as the signal power or transmission distance is increased, the new method does not suffer from the deterministic crosstalk between signal components, which has degraded the performance of previous approaches. In this paper, emphasis is placed on modulation of the discrete component of the nonlinear Fourier transform of the signal and some simple examples of achievable spectral efficiencies are provided.

Directional Modulation Based on 4-D Antenna Arrays

Abstract: Four-dimensional (4-D) antenna arrays are formed by introducing a fourth dimension, time, into traditional antenna arrays. In this paper, a time-modulated 4-D array with constant instantaneous directivity is proposed for directional modulation. The main idea is that the 4-D array transmits correct signal without time modulation in the desired direction, while transmitting time-modulated signals in other directions. As longs as the time modulation frequency is less than the bandwidth of the transmitted signal, the time-modulated signals cannot be demodulated correctly due to the aliasing effect, implying that time-modulated signals go distorted. Thus, the 4-D array can be used to transmit direction-dependent signals in secure wireless communications. The proposed idea is verified by experiments based on AM signal transmission through the 4-D array. Moreover, BPSK signal transmission through the 4-D array is studied and the bit error rate (BER) performance is investigated. Simulation results show that the BERs of time-modulated BPSK (TM-BPSK) signals transmitted through the sidelobes of the 4-D array are much higher than those of BPSK signals and almost keep unchanged even under higher SNR. Finally, two enhanced methods are presented to improve the feasibility of directional modulation by using random time sequences and random time modulation frequency.

4-D Arrays as Enabling Technology for Cognitive Radio Systems

Abstract: Time-modulation (TM) in four-dimensional (4-D) arrays is implemented by using a set of radio-frequency switches in the beam forming network to modulate, by means of periodic pulse sequences, the static excitations and thus control the antenna radiation features. The on-off reconfiguration of the switches, that can be easily implemented via software, unavoidably generates harmonic radiations that can be suitably exploited for multiple channel communication purposes. As a matter of fact, harmonic beams can be synthesized having different spatial distribution and shapes in order to receive signals arriving on the antenna from different directions. Similarly, the capability to generate a field having different frequency and spatial distribution implies that the signal transmitted by time-modulated 4-D arrays is direction-dependent. Accordingly, such a feature is also exploited to implement a secure communication scheme directly at the physical layer. Thanks to the easy software-based reconfigurability, the multiple harmonic beamforming, and the security capability, 4-D arrays can be considered as an enabling technology for future cognitive radio systems. In this paper, these potentialities of time-modulated 4-D arrays are presented and their effectiveness is supported by a set of representative numerical simulation results.

Low-Complexity Signal Detection for Generalized Spatial Modulation

Abstract: Generalized spatial modulation (GSM), in which only part of the transmit antennas are activated in each time slot, was recently proposed as a trade-off between spatial modulation (SM) and vertical Bell laboratories space-time (V-BLAST). Although the maximum likelihood (ML) detector is able to achieve the optimal performance, its exhaustive search leads to intractable computational complexity. In this letter, an efficient signal detection algorithm, termed ordered block minimum mean-squared error (OB-MMSE), is proposed for achieving near-ML performance with low complexity. On one hand, an ordering algorithm is proposed to sort the possible transmit antenna combinations (TACs). On the other hand, the possible signal vector for each ordered TAC is detected by block-MMSE equalization. Simulation results show that the proposed OB-MMSE detector provides similar performance to ML with near 80% reduction in complexity.

Low-bias terahertz amplitude modulator based on split-ring resonators and graphene.

Abstract: Split-ring resonators represent the ideal route to achieve optical control of the incident light at THz frequencies. These subwavelength metamaterial elements exhibit broad resonances that can be easily tuned lithographically. We have realized a design based on the interplay between the resonances of metallic split rings and the electronic properties of monolayer graphene integrated in a single device. By varying the major carrier concentration of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 and 3.1 THz, achieving a maximum modulation depth of 18%, with a bias as low as 0.5 V.

0.34-THz Wireless Link Based on High-Order Modulation for Future Wireless Local Area Network Applications

Abstract: This paper presents a 0.34-THz wireless link for future wireless local area networks (WLANs), which is based on high order 16-quadrature amplitude modulation (16QAM). The system adopts super heterodyne transceivers and parallel digital signal-processing techniques. The 0.34-THz transceiver consists of a 0.34-THz subharmonic mixer, a 0.34-THz waveguide H-ladder bandpass filter, and a 0.17-THz multiplier chain. Two 0.34-THz Cassegrain antennas with 48.4-dBi gain have been developed to extend the transmission distance. Based on a 32-way parallel signal processing, we have successfully realized the 3-Gb/s, 16QAM real-time modulator and demodulator. The measured data indicate that the lowest bit error rate of the 0.34-THz, 3-Gb/s data link is 1.784×10-10 over a 50.0-m line-of-sight channel. The maximum received energy per bit to noise power spectral density ratio ( Eb/N0) is 23.8 dB, while the output power of transmitter is -17.5 dBm and the noise temperature of receiver is 5227 K. In addition, this paper presents a 0.34-THz WLAN prototype based on IEEE 802.11b/g protocol. The WLAN prototype, which consists of an access point and two terminal nodes, achieves a transmission data rate of 6.536 Mb/s over 1.15 m by using rectangular horn antennas.

New Modulation Strategy to Balance the Neutral-Point Voltage for Three-Level Neutral-Clamped Inverter Systems

Abstract: This paper proposes a new modulation strategy that balances the neutral-point voltage for three-level neutral-clamped inverter systems. The proposed modulation replaces the P-type or N-type small switching states with other switching states that do not affect the neutral-point voltage. The zero and medium switching states are employed to help the neutral-point voltage balancing. This method little bit increases the switching events and output total harmonic distortion. However, this method has a strong balancing ability at all regions. Further, it is very simple to implement in both space vector modulation and carrier-based PWM methods. Simulation and experimental results verify the validity and feasibility of the proposed new modulation strategy.

High-dimensional modulation for coherent optical communications systems.

Abstract: In this paper, we examine the performance of several modulation formats in more than four dimensions for coherent optical communications systems. We compare two high-dimensional modulation design methodologies based on spherical cutting of lattices and block coding of a ‘base constellation’ of binary phase shift keying (BPSK) on each dimension. The performances of modulation formats generated with these methodologies is analyzed in the asymptotic signal-to-noise ratio regime and for an additive white Gaussian noise (AWGN) channel. We then study the application of both types of high-dimensional modulation formats to standard single-mode fiber (SSMF) transmission systems. For modulation with spectral efficiencies comparable to dual-polarization (DP-) BPSK, polarization-switched quaternary phase shift keying (PS-QPSK) and DP-QPSK, we demonstrate SNR gains of up to 3 dB, 0.9 dB and 1 dB respectively, at a BER of 10−3.

50-Gb/s Direct Conversion QPSK Modulator and Demodulator MMICs for Terahertz Communications at 300 GHz

Abstract: We demonstrate direct quadrature modulator and demodulator monolithic microwave integrated circuits for future terahertz communications at 300 GHz based on the quadrature phase-shift keying (QPSK) modulation format. For the modulating and demodulating signal, we employed half-Gilbert cell mixers, which provide balanced signaling and moderate performance in conversion efficiency with a simple circuit configuration. In order to maintain the balance performance of the modulator and demodulator, passive baluns and couplers are implemented with thin-film microstrip lines, which exhibit less insertion loss than inverted microstrip lines (IMSLs), while the active mixers are based on IMSLs for short interconnections. The half-Gilbert-cell mixers have a wide enough operation bandwidth for high-throughput communications of more than 10% at 300 GHz. According to the static constellation of the modulator, imbalance is expected to be less than approximately ±0.6 dB ∠4°. A nonchip back-to-back experiment was conducted at up to 60 Gb/s, and 50-Gb/s operation was verified with a low bit error rate on the order of 10-8 or less. The results demonstrate that the QPSK modulation scheme can be applied to double the data rate at terahertz frequencies.

Pseudo-random binary sequence phase modulation for narrow linewidth, kilowatt, monolithic fiber amplifiers.

Abstract: We report on pseudo random binary sequence (PRBS) phase modulation for narrow-linewidth, kilowatt-class, monolithic (all-fiber) amplifiers. Stimulated Brillouin scattering (SBS) threshold enhancement factors for different patterns of PRBS modulated fiber amplifiers were experimentally analyzed and agreed well with the theoretical predictions. We also examined seeding of the SBS process by phase modulated signals when the effective linewidth is on the same order as the Brillouin shift frequency. Here ~30% variations in SBS power thresholds were observed from small tunings of the modulation frequency. In addition, a 3 GHz PRBS modulated, 1.17 kW fiber amplifier was demonstrated. Near diffraction-limited beam quality was achieved (M2 = 1.2) with an optical pump efficiency of 83%. Overall, the improved SBS suppression and narrow linewidth achieved through PRBS modulation can have a significant impact on the beam combining of kilowatt class fiber lasers.

A survey of low-switching frequency modulation techniques for medium-voltage multilevel converters

Abstract: Multilevel converters (MLCs) have emerged as standard power electronic converters for medium-voltage high-power industrial applications. Owing to dominating device switching losses in high-power applications, it is preferable to use low device switching frequency (LDSF) modulation techniques. Then, it is possible to achieve higher device utilization, higher converter efficiency, and reduced cooling requirements. However, there exists a tradeoff between device switching frequency and harmonic distortion of converter output currents. Therefore, the main challenge for LDSF modulation techniques is to minimize the harmonic distortion of the output currents. The goal of this paper is to provide a review of various LDSF modulation techniques proposed in the literature and also discuss in detail about one of the emerging LDSF control techniques known as synchronous optimal pulsewidth modulation. Finally, challenges to LDSF modulation techniques for emerging multilevel topologies and future trends in applications of MLCs are discussed to motivate further research, to enhance the proposed LDSF techniques, and to explore for new alternatives.

DC-Link Voltage Balancing for a Three-Level Electric Vehicle Traction Inverter Using an Innovative Switching Sequence Control Scheme

Abstract: This paper presents an advanced switching sequence for space-vector pulsewidth modulation (SV-PWM)-based three level neutral-point clamped inverter. The developed scheme helps to reduce the number of converter switching sequences, compared with the conventional SV-PWM strategy, and keeps the voltage difference between the two dc-link capacitors at the desired voltage level. The developed test bench is utilized for a permanent magnet synchronous machine (PMSM) drive for electric vehicle applications. The proposed strategy is compared with the performance of a PI controller-based voltage balancing strategy. The proposed control strategy is based on the nearest three-vector (N3V) scheme, with a hysteresis control of the dc-link capacitor voltage difference. Conventional N3V scheme uses a higher number of switching sequences, which makes the switching losses higher. In addition, these switching sequences are not same for all subsectors. This makes the switching frequency to vary extensively. In the proposed control strategy, a reduced number of switching sequences are used, and they are same for all subsectors. This makes the system operate with constant switching frequency. Detailed simulation studies are performed to verify the performance of the proposed control strategy. The performance-based test results are then compared with those of a PI controller-based strategy. Experimental test results show significant improvement in the performance of the PMSM with respect to dc-link capacitor voltage variation as well as wide speed and torque range of machine operation.

A Wideband 2 $\times$ 13-bit All-Digital I/Q RF-DAC

Abstract: This paper presents a wideband 2 ×13-bit in-phase/quadrature-phase (I/Q) RF digital-to-analog converter-based all-digital modulator realized in 65-nm CMOS. The isolation between I and Q paths is guaranteed employing 25% duty-cycle differential quadrature clocks. With a 1.3-V supply and an on-chip power combiner, the digital I/Q transmitter provides more than 21-dBm RF output power within a frequency range of 1.36-2.51 GHz. The peak RF output power, overall system, and drain efficiencies of the modulator are 22.8 dBm, 34%, and 42%, respectively. The measured static noise floor is below -160 dBc/Hz. The digital I/Q RF modulator demonstrates an IQ image rejection and local oscillator leakage of -65 and -68 dBc, respectively. It could be linearized using either of the two digital predistortion (DPD) approaches: a memoryless polynomial or a lookup table. Its linearity is examined using single-carrier 4/16/64/256/1024 quadrature amplitude modulation (QAM), as well as multi-carrier 256-QAM orthogonal frequency-division multiplexing baseband signals while their related modulation bandwidth can be as high as 154 MHz. Employing DPD improves the third-order intermodulation product (IM3) by more than 25 dB, while the measured error vector magnitude for a “single-carrier 22-MHz 64-QAM” signal is better than -28 dB.

Coherent control of light interaction with graphene

Abstract: We report the experimental observation of all-optical modulation of light in a graphene film. The graphene film is scanned across a standing wave formed by two counter-propagating laser beams in a Sagnac interferometer. Through a coherent absorption process the on-axis transmission is modulated with close to 80% efficiency. Furthermore, we observe modulation of the scattered energy by mapping the off-axis scattered optical signal: scattering is minimized at a node of the standing wave pattern and maximized at an antinode. The results highlight the possibility to switch and modulate any given optical interaction with deeply sub-wavelength films.

100 Gb/s Intensity Modulation and Direct Detection

Abstract: Recent advances in short reach 100 Gb/s intensity modulation and directed detection systems are briefly reviewed. As an illustrative example of using digital signal processing enabled transmitters and receivers to allow relatively modest symbol rates, the generation and detection of 112 Gb/s 16-QAM half-cycle Nyquist subcarrier modulation are considered using dual-polarization, single-carrier and single-polarization, dual-carrier implementations. High bandwidth directly modulated passive feedback lasers are used to generate the optical signals and pre-amplified receivers are used to detect the received signals in a back-to-back system and after transmission over 4 km of single-mode fiber.

Adapting Transmitter Power and Modulation Format to Improve Optical Network Performance Utilizing the Gaussian Noise Model of Nonlinear Impairments

Abstract: This paper serves to highlight the gains in SNR margin and/or data capacity that can be achieved through a proper optimization of the transceiver parameters, for example, launch power, modulation format, and channel allocation. A simple quality of transmission estimator is described that allows a rapid estimation of the signal quality based on ASE noise and nonlinear interference utilizing the Gaussian noise model. The quality of transmission estimator was used to optimize the SNR and maximise the data throughput of transmission signals in a point-to-point link by adjusting the launch power and modulation format. In a three-node network, the launch power and channel allocation were adjusted to minimise the overall effect of nonlinear interference. This paper goes on to show that by optimizing the transceiver modulation format as part of the channel allocation and routing problem gains in network data throughput can be achieved for the 14-node NSF mesh network.

Simultaneous optical performance monitoring and modulation format/bit-rate identification using principal component analysis

Abstract: We propose a novel technique for simultaneous multi-impairment monitoring and autonomous bit-rate and modulation format identification (BR-MFI) in next-generation heterogeneous fiber-optic communication networks by using principal component analysis-based pattern recognition on asynchronous delay-tap plots. The results of numerical simulations performed for three commonly used modulation formats at two different bit-rates each demonstrate simultaneous and independent monitoring of optical signal-to-noise ratio, chromatic dispersion, and differential group delay with mean errors of 1 dB, 4 ps/nm, and 1.6 ps, respectively, without knowing the signal's bit-rate and modulation format. Similarly, the results for joint BR-MFI validate accurate identification of all the bit-rates and modulation formats despite the presence of various network impairments. The effects of fiber nonli-nearity and transmitter variations on the performance of the proposed technique are also investigated.

Avoiding spectral efficiency loss in unipolar OFDM for optical wireless communication

Abstract: Unipolar orthogonal frequency division multiplexing (U-OFDM) has recently been introduced for intensity modulation and direct detection (IM/DD) systems. The scheme achieves higher power efficiency than the conventional direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) at the expense of half the spectral efficiency for the same M-ary quadrature amplitude modulation (M-QAM) order. This paper presents a modulation approach which doubles the spectral efficiency of U-OFDM and still allows it to achieve better performance in terms of both electrical power and optical power dissipation compared to DCO-OFDM. The simulation results and the theoretical analysis suggest that the performance improvement of the proposed scheme over DCO-OFDM increases with the modulation order. This trend is different from the inherently unipolar state-of-the-art techniques such as U-OFDM, asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse-amplitude-modulated discrete multitone modulation (PAM-DMT). It is typical for these schemes to exhibit a loss in the power efficiency as the spectral efficiency is increased. The novel approach is very promising for the achievement of high data rates in IM/DD systems. To the best of the authors' knowledge, this is the first design of a strictly unipolar orthogonal frequency division multiplexing (OFDM) scheme which requires no biasing and is able to demonstrate significant energy advantage over DCO-OFDM without sacrificing spectral efficiency.

High-contrast electro-optic modulation of spatial light induced by graphene-integrated Fabry-Pérot microcavity

Abstract: We have proposed a graphene-integrated Fabry-Perot microcavity for efficient modulation of spatial light. A simplified theoretical model is established to analyze the performance of our system, and the calculated results agree well with the simulation results. It is shown that the plasmon-induced transparency (PIT) effect is achieved in the proposed microcavity and the central frequency of PIT window can be dynamically tuned by gate voltages. In particular, the PIT spectra exhibit extremely large modulation depths (∼90%) across a broad range of frequencies. The proposed ultracompact configuration demonstrates a type of cavity-induced high-contrast and frequency-selective electro-optic modulators, offering opportunities in exploiting active chip-integrated high-performance devices operating at frequencies from terahertz to mid-infrared.

Optical Single-Sideband Modulation Based on a Dual-Drive MZM and a 120° Hybrid Coupler

Abstract: A novel scheme for implementing high-performance optical single-sideband (OSSB) modulation based on a dual-drive Mach-Zehnder modulator (MZM) and a 120° hybrid coupler is proposed and demonstrated. A RF signal is divided by the 120° hybrid coupler into two parts with equal powers and a phase difference of 120°, and then, led to the two RF ports of the dual-drive MZM. With a proper dc bias, an OSSB signal with the -1st and +2nd-order sidebands (or the +1st and -2nd-order sidebands) suppressed is generated. A numerical simulation and a proof-of-concept experiment are carried out. As compared with the conventional OSSB modulation based on a 90° hybrid coupler, the suppression of the +2nd (or -2nd)-order sideband improves evidently the performance when the OSSB modulation is applied in a radio-over-fiber system or an optical vector network analyzer.

All-Digital I/Q RF-DAC

Abstract: Due to the severe cost pressure of consumer electronics, a migration to an advanced nanoscale CMOS processes, which is primarily developed for fast and low-power digital circuits operating at low supply voltages, is necessary, but it forces wireless RF transceivers to exploit more and more digital circuitry These basic CMOS properties tend to coerce the design of wireless functions towards the digital domain where transistors are utilized as switches rather than current sources Within the past decade, there have been tremendous efforts towards implementing fully-digital or digitally-intensive RF transmitters in which they demonstrate transmitter designs that operate from baseband up to the pre-power amplifier (PA) stage entirely in the digital domain In view of this digitalization, the RF transmitter modulator, being the nearest to the antenna as it converts digital baseband modulation samples into an RF waveform, is considered the most critical building block of the transmitter, and it can be in the form of either a polar, Cartesian (I/Q), or an outphasing topology For wide modulation bandwidths, due to their direct linear summation of the in-phase (I) and quadrature-phase (Q) signals and thus the avoidance of the bandwidth expansion, Cartesian modulators are substantiated as the most appropriate choice over their polar or outphasing counterparts Since the effective modulating sample resolution is the utmost important parameter as it directly impacts the achievable dynamic range, linearity, error vector magnitude (EVM), noise floor, and out-of-band spectral emission, this thesis proposes a wideband, high-resolution, all-digital orthogonal I/Q radio-frequency digital-to-analog (RF-DAC) Chapter 1 briefly provides an overview of the conventional RF radio building blocks It is discussed that contemporary RF transceivers must support most of multi-mode/multiband communication standards such as Wi-Fi, Bluetooth, and Fourth Generation (4G) of 3GPP cellular In Chapter 2, four types of RF transmitter architectures have been briefly described The analog I/Q modulators are the most straightforward and widely employed RF transmitters They are later replaced by analog polar counterparts to address their poor power efficiency and noise performance On the other hand, in the analog polar RF transmitters, their related amplitude and phase signals must be aligned or spectral regrowth is inevitable Utilizing digitally intensive polar RF transmitters mitigates the latter alignment issue Nonetheless, polar transmitters suffer from an additional issue that is related to their nonlinear conversion of in-phase and quadrature-phase signals into the amplitude and phase representation Therefore, the polar RF transmitters are not able to manage very large baseband bandwidth of the most stringent communication standards, therefore, reusing I/Q modulators based on digitally intensive implementation appears to be a reasonable approach to resolve this issue The digital I/Q RF transmitters, however, suffer again from inadequate power efficiency Moreover, the combination of in-phase and quadrature phase paths must be orthogonal to produce an undistorted-upconverted-modulated RF signal In Chapter 3, a novel all-digital I/Q RF modulator is described Employing an upconverting RF clock with a 25% duty cycle ensures the orthogonal summation of Ipath and Qpath, which avoids nonlinear signal distortion It was clarified that electric summing of I and Q digital unit array switches is the most appropriate I/Q orthogonal summation approach Moreover, to address all four quadrants of the constellation diagram, the differential quadrature upconverting RF clocks must be utilized In addition, it was explained that employing switches instead of utilizing current sources leads to superior noise performance of the all-digital I/Q transmitter In Chapter 4, a novel 2×3-bit all-digital I/Q (Cartesian) RF transmit modulator is implemented which operates as an RF-DAC The modulator performs based on the concept of orthogonal summing, which is introduced and elaborated in Chapter 3 It is based on a time-division duplexing (TDD) manner of an orthogonal I/Q addition By employing this method, a very simple and compact design featuring high-output power, power-efficiency and low-EVM has been realized The resolution of the experimental RF-DAC presented in this work is only 3-bit (including one sign bit), but it will be demonstrated in the following chapters that the resolution can be increased to 8–12 bits in an unequivocal manner for utilization in multi-standard wireless applications In Chapter 5, the system design considerations of the proposed high-resolution, wideband all-digital I/Q RF-DAC are discussed It is demonstrated that the upsampling clock frequency (fCKR), DRAC resolution (Nb), and memory length (lmem) are three important parameters that affect the dynamic performance of the proposed RF-DAC Based on system level simulation results and the limitation in implementing the RF-DAC test-chip, they are designated as fCKR=300 MHz, Nb=12 bit, and lmem=8 k-word The effect of these parameters on the in-band as well as out-of-band performance of RF-DAC are investigated It is concluded that exploiting 13 bits of resolution for quadrature baseband signals is sufficient to meet the most stringent communication requirements In Chapter 6, the theory and the design procedure of an innovative, differential, orthogonal power combining network, which is employed in the proposed all-digital modulator, is thoroughly explained It is demonstrated that, in order to maintain an orthogonal operation between the in-phase and quadrature-phase paths, the effect of the power combiner on the in-phase and quadrature-phase paths must be considered, otherwise, the linear summation will not occur As a result, the EVM and linearity performance will diminish The power combiner consists of a transformer balun as well as its related programmable primary and secondary shunt capacitors In order to achieve high efficiency at full power of operation, a class-E type matching network is adopted and subsequently modified in order to obtain a minimum modulation error A switchable cascode structure is exploited to mitigate a reliability issue as well as to perform a mixer operation Moreover, utilizing a switchable cascode structure also improves the isolation between quadrature paths Furthermore, it is explained that the power combiner efficiency is primarily related to the transformer balun efficiency A procedure is introduced in order to design an efficient, compact balun transformer Also, it is explained that the RF-DAC operates as a class-B power amplifier at the power back-off levels As a result, its performance in the power back-off region is lowered In Chapter 7, the implemented wideband, 2×13-bit I/Q RF-DAC-based all-digital modulator realized in 65-nm CMOS is presented Employing the orthogonal I/Q combining approach which is proposed in Chapter 3 guarantees the isolation between in-phase and quadrature-phase paths The 4×f0 off-chip single-ended clock is converted to a differential version employing an on-chip transformer The wide swing, low phase noise, high-speed dividers are incorporated to translate the 4×f0 differential clock to the fundamental frequency of f0 In the meantime, the complementary quadrature sign bit is used to address four quadrants of the related constellation diagram The 25% differential quadrature clocks are generated using logic-AND operation between 2×f0 differential clock and f0 differential quadrature clocks The 12-bit DRAC is implemented employing a segmentation approach, which consists of 256 MSB and 16 LSB thermometer unit cells The layout arrangement of the DRAC unit cell proves to be very crucial It was concluded that the vertical layout would be the most appropriate selection The LO leakage and I/Q image rejection technique as well as two DPD memoryless techniques of AM-AM/AM-PM and constellation mapping are introduced, which will be extensively utilized in the measurement segment In Chapter 8, the high-resolution wideband 2×13-bit all-digital I/Q transmitter, which was introduced in Chapter 7, is thoroughly measured First, the chip is tested in continuouswave mode operation It is demonstrated that, with a 13V supply and, of course, an on-chip power combiner, the RF-DAC chip generates more than 21dBm RF output power within a frequency range of 136–251 GHz The peak RF output power, overall system, and drain energy efficiencies of the modulator are 228 dBm, 34%, and 42%, respectively The measured static noise floor is below -160 dBc/Hz The digital I/Q RF modulator demonstrates an IQ image rejection and LO leakage of -65 dBc and -68 dBc, respectively The RF-DAC could be linearized employing either of the two digital predistortion (DPD) approaches: memoryless polynomial or a lookup table Its linearity is examined utilizing 4/16/64/256/1024-QAM baseband signals while their related modulation bandwidth can be as high as 154 MHz Using AM-AM/AM-PM DPD improves the linearity by more than 25 dB while the measured EVM is better than -28 dB Moreover, the constellation-mapping DPD is applied to the RF-DAC which improves linearity by more than 19 dB These numbers indicate that this innovative concept is a viable option for the next generations of multi band/multi-standard transmitters The realized demonstrator can perform as an energy-efficient RF-DAC in a stand-alone digital transmitter directly (eg, for WLAN) or as a pre-driver for high-power basestation PAs Chapter 9 draws the conclusions of the this thesis work and provides recommendations for future research and directions in the field of all-digital RF transmitters for wireless communication applications

Noncontact detection of fatigue cracks by laser nonlinear wave modulation spectroscopy (LNWMS)

Abstract: Nonlinear wave modulation spectroscopy (NWMS) has been used to detect nonlinear ultrasonic signatures produced by fatigue cracks in materials. It is done by generating ultrasonic waves at two different frequencies and measuring their modulation. A proper choice of two distinct frequencies for a given structure plays a significant role in NWMS. This paper, instead of using input signals at two distinct frequencies, takes only one broadband pulse signal as the driving input, which can be generated by a laser beam. With a broadband excitation, material nonlinearity exhibits modulation at multiple peaks in a spectral plot due to interactions among various input frequency components of the broadband input. A feature called sideband peak count (SPC), which is defined as the ratio of the number of sideband (modulation) peaks over a moving threshold to the total peak number in the specified frequency band, is extracted from the spectral plot to measure the degree of material nonlinearity. The basic premise of the proposed laser nonlinear wave modulation spectroscopy (LNWMS) is that this SPC value will rise as the level of material nonlinearity increases. A noncontact laser ultrasonic system has been built for LNWMS measurement by integrating and synchronizing a Q-switched Nd:YAG laser for ultrasonic wave generation and a laser Doppler vibrometer for ultrasonic wave detection. The proposed LNWMS technique has been successfully tested for detecting fatigue cracks in metallic plates and aircraft fitting-lugs having complex geometries.