Spectral Spreading Research Articles

Enhanced Iron-Tunnel Recognition for Automotive Radars

In this paper, we propose a novel iron-tunnel recognition method to overcome the deterioration of the target or vehicle detection performance due to iron-tunnel clutters. Although a few studies on the reflection and diffraction on road surfaces have been conducted for automotive radar sensors, most of these studies did not consider the situation in which structures with larger reflection, such as an iron tunnel, are densely distributed. The proposed method measures the degree of spectral spreading through the analysis of the spectrum characteristics of the received radar signal under different road conditions. The proposed method can successfully detect iron tunnels. In addition, experimental results show that early detection and missing problem of a forward target vehicle in an iron tunnel under adaptive cruise control (ACC) is improved.

Airborne remote sensing of ocean wave directional wavenumber spectra in the marginal ice zone

AbstractInteractions between surface waves and sea ice are thought to be an important, but poorly understood, physical process in the atmosphere‐ice‐ocean system. In this work, airborne scanning lidar was used to observe ocean waves propagating into the marginal ice zone (MIZ). These represent the first direct spatial measurements of the surface wavefield in the polar MIZ. Data were compared against two attenuation models, one based on viscous dissipation and one based on scattering. Both models were capable of reproducing the measured wave energy. The observed wavenumber dependence of attenuation was found to be consistent with viscous processes, while the spectral spreading of higher wavenumbers suggested a scattering mechanism. Both models reproduced a change in peak direction due to preferential directional filtering. Floe sizes were recorded using colocated visible imagery, and their distribution was found to be consistent with ice breakup by the wavefield.

Reconfigurable Periodic Bifrequency DPWM With Custom Harmonic Reduction in DC–DC Converters

Majority of spectral spreading techniques employ chaos-based pulsewidth-modulation (PWM) schemes in a switching dc–dc converter, in which neither dynamic performance nor ripple parameter can be correctly predicted so as for efficiency. Periodic frequency modulation using a triangular carrier wave can achieve custom spectral spreading and harmonic reduction. This paper proposes a periodic bifrequency digital PWM (BF-DPWM) technique in a dc–dc converter, which uses two discrete frequencies $f_{\rm 1}$ and $f_{\rm 2}$ as guided by a modulating signal $f_{\rm m}$ . Fourier analysis shows that $f_{\rm 1}$ and $f_{\rm 2}$ do not exist; however, they contribute to power spectrum of $f_{\rm m}$ . This method achieves considerable harmonic reduction and improved DPWM resolution with an insignificant ripple impact, compared to a dither-based periodic modulation scheme. Design guidelines using simulation study are presented. A buck converter prototype has been made, and different control schemes are implemented using a field-programmable gate array device. Test results show that the BF-DPWM has insignificant impacts on transient performance. Finally, the proposed scheme is extended to the triangular-wave modulation scheme, which results in further harmonic reduction.

Generalised random switching circuits: A time‐domain technique

A probabilistic solution of the time-domain performance of an arbitrary switching circuit driven by a generalised random switching sequence is presented. A generalised random switching sequence is the superset of all clock-based random pulse-width modulation schemes and has the best possible spectral spreading. The switching circuits considered are linear and time invariant, have a constant DC input and contain at least one ideal switch. The mean of the probability density updates exactly like a discrete time solution to state-space averaging, with probability replacing the duty cycle. The covariance matrix of the probability density updates using a difference equation. The value of this work is in the prediction of the statistics of transients for randomly switched power converters. A boost converter is presented as an example.

Performance Analysis of PAPR Reduction in 4×4 Spatially Multiplexed MIMO-OFDM System using SLM and Optimum-PTS Techniques

Multiple-input Multiple-output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) is a promising candidate for 4G high-data-rate broadband wireless communications. However, MIMO-OFDM inherited the problem of high Peak-to-Average Power Ratio (PAPR) from OFDM which leads to in-band distortion, undesired spectral spreading, low power efficiency and increases the cost of the RF power amplifier. Many PAPR reduction techniques were developed in last two decades to reduce the PAPR of OFDM, among them Partial Transmit Sequence (PTS) and Selected Mapping (SLM) are the important ones. This paper proposes two PAPR reduction technique; SLM and Optimum -PTS for reducing the PAPR and does performance comparison of these two techniques on the basis of reduction in PAPR level, BER and the number of redundant bits required. It shows that the use of Optimum-PTS technique with QPSK modulation comprising of 128 subcarriers provides better PAPR reduction and good BER as compared to SLM.

Performance Analysis of Adaptive Clipping Technique for Reduction of PAPR in Alamouti Coded MIMO-OFDM Systems

Like single-input single-output orthogonal frequency division multiplexing (SISO-OFDM), multi-input multi-output (MIMO) OFDM systems also suffer from the problem of high peak to average power ratio (PAPR). To account for this issue, amplitude clipping is one of the simplest techniques employed to reduce PAPR. However, it leads to additional distortion. In this paper, adaptive clipping technique is proposed to reduce PAPR of Alamouti coded MIMO-OFDM systems. In this scheme, successive peaks are clipped according to an adaptive algorithm. Simulation results reveal that the proposed clipping technique provides effective reduction in PAPR and better bit error rate (BER) performance. BER performance and PAPR reduction capability of the proposed algorithm in MIMO-OFDM systems are also compared with SISO-OFDM systems. Power spectral density (PSD) for conventionally clipped, adaptively clipped, and normal SISO-OFDM systems is analyzed and plotted. It can be revealed from the PSD plot that the spectral spreading due to the proposed scheme is less than the conventional clipping scheme.

On the kurtosis of deep-water gravity waves

In this paper, we revisit Janssen’s (J. Phys. Oceanogr., vol. 33 (4), 2003, pp. 863–884) formulation for the dynamic excess kurtosis of weakly nonlinear gravity waves in deep water. For narrowband directional spectra, the formulation is given by a sixfold integral that depends upon the Benjamin–Feir index and the parameter $R={\it\sigma}_{{\it\theta}}^{2}/2{\it\nu}^{2}$, a measure of short-crestedness for the dominant waves, with ${\it\nu}$ and ${\it\sigma}_{{\it\theta}}$ denoting spectral bandwidth and angular spreading. Our refinement leads to a new analytical solution for the dynamic kurtosis of narrowband directional waves described with a Gaussian-type spectrum. For multidirectional or short-crested seas initially homogeneous and Gaussian, in a focusing (defocusing) regime dynamic kurtosis grows initially, attaining a positive maximum (negative minimum) at the intrinsic time scale ${\it\tau}_{c}={\it\nu}^{2}{\it\omega}_{0}t_{c}=1/\sqrt{3R}$, or $t_{c}/T_{0}\approx 0.13/{\it\nu}{\it\sigma}_{{\it\theta}}$, where ${\it\omega}_{0}=2{\rm\pi}/T_{0}$ denotes the dominant angular frequency. Eventually the dynamic excess kurtosis tends monotonically to zero as the wave field reaches a quasi-equilibrium state characterized by nonlinearities mainly due to bound harmonics. Quasi-resonant interactions are dominant only in unidirectional or long-crested seas where the longer-time dynamic kurtosis can be larger than that induced by bound harmonics, especially as the Benjamin–Feir index increases. Finally, we discuss the implication of these results for the prediction of rogue waves.

Offset Phase Shift Keying Modulation in Multiple-Input Multiple-Output Spatial Multiplexing

The increasing demand for multimedia data transmission in mobile wireless communication poses a challenge to reliable signal reception. In order to have good signal quality, a robust digital modulation scheme is required at the transmitter. However, the conventional M-ary Phase Shift Keying (MPSK) commonly used in multiple-input multiple-output (MIMO) communication systems with non-linear radio frequency (RF) power amplifiers causes a relative increase in intercarrier interference (ICI). This paper presents a development of offset-MPSK (O-MPSK) modulation scheme in MIMO spatial multiplexing over the Rayleigh fading channel. The O-MPSK modulation schemes were developed for 4, 8, 16, 32 and 64 constellation sizes. The development of the O-MPSK was done by shifting the phase of the conventional QPSK, 8-PSK, 16-PSK, 32-PSK and 64-PSK by an odd multiple of pi (π) to give -QPSK, -8PSK, -16PSK, -32PSK and -64PSK, respectively, with a view to reducing the spectral spreading in the power amplifiers at the receiver of a MIMO system. The MIMO techniques used was MIMO Spatial Multiplexing (MIMO–SM). The system models were developed around these schemes and later simulated using MATLAB application toolkit. The performances of the O-MPSK schemes were evaluated using bit error rate (BER) at signal-to-noise ratio (SNR) range of 0 to 20 dB and compared with the conventional MPSK schemes. The results obtained for all the SNRs in MIMO-SM showed that mean BER of 0.0024, 0.0040, 0.0085, 0.0183 and 0.036 were obtained for -QPSK, -8PSK, -16PSK, -32PSK and -64PSK respectively as against mean BER of 0.0025, 0.0044, 0.0088, 0.0178 and 0.0358 obtained for conventional QPSK, 8PSK, 16PSK, 32PSK and 64PSK respectively. The mean BER values obtained reveal that the developed O-MPSK outperforms the conventional MPSK due to the relatively lower BER of O-MPSK schemes compared with the MPSK schemes. This is as a result of the reduction in the amplitude variations and spectral spreading at the receiver of the MIMO system.

A study of three-dimensional optical code-division multiple-access for optical fiber sensor networks

This study proposes a novel optical fiber sensor (OFS) network using three-dimensional (3-D) wavelength/time/spatial optical code-division multiple-access (OCDMA) scheme. The proposed 3-D modified quadratic congruence/M-matrix (MQC/M-matrix) coding scheme overcome the restriction of single pulse per row inherent in traditional three-dimensional codes and maintain a high signal-to-noise ratio even when source power is low. The 3-D system is implemented using optical switches (OSW), fiber-Bragg gratings (FBGs), optical splitter and optical combiner. The noises of phase-induced intensity noise (PIIN) and multiple access interference (MAI) in the decoding mechanism can be suppressed by using a spectral spreading scheme and balance-detection in the receivers. By constructing 3-D codes using bipolar pseudorandom (PN) codes rather than unipolar codes provides a significant increase in the maximum permissible number of active sensor nodes.

Optimized Designing of Optical Local Area Network With Optical Code Division Multiple Access Incorporating Forward Error Correction

Optical Code-Division Multiple-Access (OCDMA) have flourished as successful schemes for expanding the transmission capacity as well as enhancing the security for OCDMA. Optical code-division multiple-access is proposed as a natural solution to achieving asynchronous, high-speed connectivity in a local area network environment. Optical CDMA is shown to be competitive with other networking technologies such as WDMA and TDMA, but has the benefit of more flexibility, simpler protocols, and no need for centralized network control. The purpose of this paper is to review the literature on OCDMA overlay systems, including our approach of one dimensional coding of SAC OCDMA. Furthermore, an essential background of OCDMA, recent coding techniques and security issues are also presented.The limitations of one-dimensional optical orthogonal codes for CDMA have motivated the idea of spectral spreading in both the temporal and wavelength domains. If the constraints on constant weight in these two-dimensional codes are relaxed, differentiated levels of service at the physical layer become possible. Areas for further research are suggested which may allow quality of service levels to be guaranteed at the physical layer.

Publisher's Note: Spectral analysis and slow spreading dynamics on complex networks [Phys. Rev. E88, 032109 (2013)

Publisher's Note: Spectral analysis and slow spreading dynamics on complex networks [Phys. Rev. E<b>88</b>, 032109 (2013)

Polarization and modal filters based on W-fibers Panda for fiber-optic gyroscopes and high-power fiber lasers

Polarizing Panda-type W-fibers, practical realizations are presented for fiber-optic gyroscope (FOG) sensing coil (500-m length, 8-μm fundamental mode field diameter) and for polarization/modal filtering at the FOG loop interferometer input (∼1-m length, 10-μm fundamental mode field diameter). Using these fibers, two different mathematical models for light propagation are tested and applied for further dichroism spectral band spreading of polarizing W-fibers with fundamental mode field diameter (MFD) of 6 to 10 μm for FOG coil and at least with MFD=8 to 10 μm for input filtering in loop interferometer. Also, some fibers with large MFD are considered, which may be useful for high-power fiber lasers, requiring the low nonlinear effects. One is a practically realized single-mode 200-m W-fiber Panda with dichroism and MFD=12.4 μm at 1.06 μm, which is bend resistant for 60-mm diameters. For further MFD increase a straight 100-mm length modal and polarization filters are suggested with MFD=30 to 70 μm at 1.06 μm, based on a pure silica core fiber, narrow/shallow fluorine cladding, and stress-applying parts. The latter are used for core refractive index regulation by birefringence (W-profile forming), which is more precise than with core germanium doping.

CPM-Based Spread Spectrum Systems for Multi-User Communications

We propose a new spread spectrum (SS) system based on continuous-phase modulations (CPMs). The main idea is to exploit the sequence of modulation indices of a multi-h CPM as a frequency-hopping (FH) sequence. Spectral spreading, flatness and smoothness can be easily achieved by an appropriate choice of the maximum value of the modulation index and of the length of the index sequence. We will show that the proposed CPM-based spread spectrum system achieves an overall spectral efficiency larger than that of a single-user single-h CPM transmission even when a single-user detector is employed at the receiver. It also outperforms other solutions in the literature. In addition, we will also derive some suboptimal multi-user detectors.

Spatio Temporal Analysis of Vegetation by Vegetation Indices from Multi-dates Satellite Images: Application to a Semi Arid Area in ALGERIA

The vegetation index is considered a good indicator of vegetation behavior and can contribute to explain the evolution of vegetation. Hence the problem of our research: to what extent the analysis of the spatial and temporal evolution of vegetation index on a semi-arid region, allows us to understand its evolution and its relationship with the phenomenon of urbanization?The objectives of the study is to analyze the spatio-temporal variations of the vegetation dynamics from a series of available multi-dates satellite images (MSS, LANDSAT ETM +) covering the Laghouat city which is a semi arid area of Algeria (from 1987 to 2006) and to describe the context of overall vegetation indices (NDVI, SAVI, TSAVI) to make a comparative study of fourths vegetation indices, to choose the best index according to the study area using.There are two approaches that reduce the specific contribution of the ground so that the sensitivity of the index for the vegetation: One is based on a modified formulation of the normalized difference vegetation index (NDVI)differentiation respective contributions of red and near infrared bands i.e. SAVI index. On comparing the (NDVI) index with (TVI) which is designed to eliminate negative values and stabilize the variance, we note a clear description of the spectral behavior of vegetation by (TVI) according to its spectral spreading of vegetation histogram which is more important than the (NDVI).As for the (SAVI) index designed for low vegetation cover by chlorophyll is used to reduce the spectral contribution of soil best suited to the semi-arid region. We note an improvement in spectral spreading vegetation compared to the NDVI and TVI, thus improving the description of the vegetation index SAVI. By adopting the adjusted vegetation index SAVI ground and classifying thematic images as following: the 1987 blue, green 2001 and the 2006 red and superposition of three layers, one obtains a thematic map representing the vegetation change from 1987 to 2006. The choice of these three dates specifically related to the fact that the analysis is sensitive to vegetation season (vegetation different from one season to another). To make a comparative study one must choose images, the acquisition dates of which are very close (the same month). This diachronic study (spatio-temporal) allows us the analysis of the rate/rhythm of the extension and the distribution of vegetation.

Optimal Code Set Selection and Security Issues in Spectral Phase-Encoded Time Spreading (SPECTS) OCDMA Systems

In this paper, we perform a thorough analysis of a spectral phase-encoded time spreading optical code division multiple access (SPECTS-OCDMA) system based on Walsh-Hadamard (W-H) codes aiming not only at finding optimal code-set selections but also at assessing its loss of security due to crosstalk. We prove that an inadequate choice of codes can make the crosstalk between active users to become large enough so as to cause the data from the user of interest to be detected by other user. The proposed algorithm for code optimization targets code sets that produce minimum bit error rate (BER) among all codes for a specific number of simultaneous users. This methodology allows us to find optimal code sets for any OCDMA system, regardless the code family used and the number of active users. This procedure is crucial for circumventing the unexpected lack of security due to crosstalk. We also show that a SPECTS-OCDMA system based on W-H 32(64) fundamentally limits the number of simultaneous users to 4(8) with no security violation due to crosstalk. More importantly, we prove that only a small fraction of the available code sets is actually immune to crosstalk with acceptable BER (<;10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> ), i.e., approximately 0.5% for W-H 32 with four simultaneous users, and about 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> % for W-H 64 with eight simultaneous users.

Performance Characteristics and Metrics for Intra-Pulse Radar-Embedded Communication

Low probability of intercept (LPI) communication generally relies on the presence of noise to obfuscate a covert signal through the use of spectral spreading or hopping. In contrast, this paper addresses the use of ambient interference from other man-made emissions as a means to mask the presence of covert communication. Specifically, the high power, wide bandwidth, and repeating structure of pulsed radar systems provide an advantageous framework within which to embed a communication signal. The operating paradigm considered here is that of an RF tag/transponder that is illuminated by the radar and intends to covertly communicate with the radar or some other desired receiver while being masked by the ambient radar backscatter to avoid detection by an intercept receiver. Communication takes place on an intra-pulse (or individual pulse) basis to maximize the data rate. The impact of multipath, and its exploitation using time reversal to achieve spatio-temporal focusing, is considered. The processing gain for the destination receiver and intercept receiver are derived analytically and subsequently used to optimize the parameterization of communication symbol design.

Angular diagram of broadband emission of millimeter-sized water droplets exposed to gigawatt femtosecond laser pulses

We report on the experiments on the interaction of gigawatt femtosecond laser pulses with suspended millimeter-sized water droplets. The transparent droplets experienced laser-induced breakdown and explosive boiling up and emitted a broadband radiation. This radiation covers the spectral range from 450 to 1100 nm and consists of the spectrum of laser pulse scattered and transformed by the droplet due to self-phase modulation and plasma emission produced in water during photoionization. The droplet emission spectrum showed remarkable broadening at all viewing angles and is maximal in the direction of the laser exit from the droplet. The enlargement of the droplet results in additional spectral spreading of the emitted radiation. The depth and amount of laser pulse spectral self-transformations upon propagation through the water droplet are simulated by means of numerical calculations.

Stochastic Asymmetry Properties of 3D Gauss-Lagrange Ocean Waves with Directional Spreading

In the stochastic Lagrange model for ocean waves the vertical and horizontal location of surface water particles are modeled as correlated Gaussian processes. In this article we investigate the statistical properties of wave characteristics related to wave asymmetry in the 3D Lagrange model. We present a modification of the original Lagrange model that can produce front-back asymmetry both of the space waves, i.e. observation of the sea surface at a fixed time, and of the time waves, observed at a fixed measuring station. The results, which are based on a multivariate form of Rice's formula for the expected number of level crossings, are given in the form of the cumulative distribution functions for the slopes observed either by asynchronous sampling in space, or at synchronous sampling at upcrossings and down-crossings, respectively, of a specified fixed level. The theory is illustrated in a numerical section, showing how the degree of wave asymmetry depends on the directional spectral spreading and on the mean wave direction. It is seen that the asymmetry is more accentuated for high waves, a fact that may be of importance in safety analysis of capsizing risk.

Refinements of sea state statistics for marine renewables: A case study from simultaneous buoy measurements in Portugal

This paper presents a refined description of the wave climate observed in a Portuguese location for marine energy projects. The data used in this case study were derived from 3-month in situ measurements carried out using three directional buoys that have been deployed simultaneously – yet independently (various buoy types and water depths) – on the Atlantic coast in the neighbourhood of Figueira da Foz. Two ways of reporting the statistics were adopted. In the first one, each recorded sea state was summarized as a set of global parameters accounting for energy, mean frequency and direction, spectral bandwidth and directional spreading related to the main ongoing wave field considered as unimodal. In the second one, each sea state has been decomposed into its own wave components – swells and wind-sea – which are characterised separately and individually by the same set of parameters as previously. Besides the adopted data processing techniques and the illustration of such thorough wave climate description modes, the paper also addresses the advantages and limits of each adopted parameterization in the frame of simultaneous and independent in situ wave measurements.

Designing transmitters for spectral conformity: power amplifier design issues and strategies

Spectral constraints placed on radar systems by regulatory agencies require the design of highly linear amplifiers. Spectral spreading in power amplifiers is a result of transistors operated in the non-linear regime to optimise efficiency. Various methods are employed by power amplifier designers to maximise both linearity and efficiency. The methods of predistortion, feedforward, envelope tracking, Doherty and ‘linear amplification using non-linear components’ are discussed in this article. Trade-offs and challenges inherent in these design approaches are surveyed.