Arbitrary pulse shape synthesis via nonuniform transmission lines

A discrete inverse scattering technique is used to define the impedance profile for a nonuniform transmission line which reflects an arbitrary waveform. Initially charged nonuniform lines, switched out into a general load, can also be synthesized by this method, and are discussed. The direct or layer peeling algorithm is applied to generate profiles which are subsequently analyzed using the one-dimensional finite difference method and fabricated in stripline. Excitation for the nonuniform line is done by using a charged line connected to a photoconductive Si switch triggered by a mode-locked YLF laser. Several lines were fabricated relevant to amplitude modulation of the master oscillator laser pulse for fusion experiments. Using the layer peeling method, a complex high-voltage pulse shape for use in laser fusion experiments is synthesized to an extraordinary degree of precision. It is possible to generate any arbitrary pulse shape by reflecting a step pulse off a synthesized nonuniform transmission line provided the power spectrum of the reflected pulse does not exceed that of the input pulse at any frequency     

Robust transmit pulse design

The problem of designing a transmit pulse such that after transmission over a dispersive channel the received pulse fits in a prescribed template can be formulated as a quadratic programming problem with affine semi-infinite constraints. In constrained optimisation, the optimal solution invariably lies on the boundary of the feasible set, and consequently perturbations on the optimal solution or the constraints means that the received pulses may no longer fit in the template. Perturbations to the optimal transmit pulse and the constraints arise, in practice, from errors in the implementation and uncertainty in the channel parameter, respectively. A robust formulation is presented which ensures that the constraints are satisfied even in the presence of implementation errors and channel parameter uncertainty. The technique developed is applied to determine the optimal transmit pulse shape to be programmed on a commercial T1 (1.544 Mbit/s) line interface unit.     

Maximation of the signal-to-interference ratio for a doubly spread target: Problems in nonlinear programming

Three optimization problems concerning the maximization of the signal-to-interference ratio for a doubly spread target via signal design are expressed in terms of equivalent nonlinear programming problems defined on a real space by restricting the transmit and processing waveforms to be complex weighted, uniformly spaced pulse trains. Each subpulse can be different in shape and occupy the entire interpulse spacing interval. The approach taken is analogous to the Rayleigh-Ritz technique. The first two optimization problems involve maximization with respect to the complex weights, The third problem involves maximization with respect to the subpulse parameters (e.g., frequency deviation, swept bandwidth, etc.) and allows one to find optimum frequency hop codes.One need not develop algorithms to solve these problems, but rather, one can simply use standard computer programs or methods which are available for solving nonlinear programming problems.     

Land mine detection using a ground-penetrating radar based onresistively loaded Vee dipoles

Resistively loaded Vee dipoles are considered for use in a short-pulse ground penetrating radar (GPR) used to detect buried antipersonnel land mines. First, a study is made to select a short pulse to radiate that is most appropriate for the problem. A simple one-dimensional (1-D) analysis of some representative soils and a land mine is used to select a radiated pulse similar in shape to a differentiated Gaussian pulse with a spectral peak at 4 GHz. Based on previous studies, the conductivity of the arms of the Vee dipole is linearly tapered from the feed to the open ends. A fully three-dimensional (3-D) finite-difference time domain (FDTD) model is developed and used to simulate the GPR land mine detection problem. Using this model, a resistively loaded Vee dipole is selected and evaluated. Parametric studies related to the problem are conducted including: varying the height of the Vee above the ground, varying the position of the land mine both laterally and in depth, and examining the effects of the geometry of the land mine on the received signal. Environmental conditions are examined including signal returns from rocks and variations in the shape of the surface of the ground. The FDTD results are validated by comparisons with experimental data. These studies demonstrate that resistively loaded Vee dipoles can greatly reduce clutter related to the antenna, making the task of distinguishing land mines (targets) much easier     

Transform-limited optical pulse generation up to 20-GHz repetition rate by a sinusoidally driven InGaAsP electroabsorption modulator

The authors propose and demonstrate a simple technique for generating a transform-limited optical short pulse with variable repetition rates by using a sinusoidally driven InGaAsP electroabsorption modulator without any optical resonators. Using the pulse compression effect due to nonlinear attenuation characteristics of the modulator, a transform-limited optical pulse can be generated just with sinusoidal modulation. Theoretical calculations and experimental results show that the pulse shape is very close to the sech/sup 2/ shape, and the pulse width can be easily varied by varying the bias and modulation voltages. Transform-limited optical pulse generation with a minimum pulse width of 11 ps was achieved up to 20-GHz repetition rate. The time-bandwidth product was as small as 0.32.<>     

Charge-driven HF transistor-tuned power amplifier

A new approach to the design of HF transistor-tuned power amplifiers is proposed that uses the concepts of charge control. A circuit arrangement and design technique is described that allows the performance of tuned power amplifiers operating at high frequencies to be specified and predicted accurately. The collector current of a transistor is, within limits, proportional to the charge stored in the base. A technique is described that defines this stored charge by a sinusoidal drive voltage across a capacitor in series with the base. Sinusoidally varying charge in phase with the drive voltage is injected into the base and approximately forms a truncated sinusoid of collector current with a predictable and controlled conduction angle and height. Dynamic collector characteristics are described that relate the stored base charge to the collector voltage and current. From these, or by direct measurement, are obtained dynamic constant current curves that are plotted on the graph of base charge versus collector voltage. This graph enables the collector current pulse shape to be predicted, and thus the difficult design problem where the collector current pulse shape is unknown is overcome. An amplifier is designed using this technique, and design and experimental results are in good agreement.     

A Class of MSK Baseband Pulse Formats with Sharp Spectral Roll-Off

Minimum frequency-shift-keying (MSK) is a very attractive modulation technique because of its spectral properties and the constant envelope property of the signal, but out-of-band energy spillover must sometimes be minimized to control co-channel interference. This problem has recently received attention in published articles and, since the pulse shape is the main parameter, many different MSK pulse formats have been described. In this note, a general shaping function, which allows various asymptotic spectral fall-off characteristics, is presented; then DSFSK (Double Sine Frequency Shift Keying) is derived from it, and its spectrum is computed, analyzed and compared with Rabzel's results.     

Effect of group velocity mismatch on the measurement of ultrashort optical pulses via second harmonic generation

The use of second harmonic generation as a technique for optical pulse width measurements is analyzed to determine the effects of both phase velocity and group velocity mismatch between fundamental and second harmonic fields. An expression for the time average second harmonic energy, which except for special cases differs from the intensity autocorrelation function, is derived. For Type I phase matching, the measurement yields an apparent correlation width which can be either shorter or longer than the actual intensity correlation width, depending on the specific pulse shape. When the group velocity mismatch is nonzero, the measurement becomes sensitive to the phase matching condition. Two special pulse shapes for which the measurement is independent of group and phase velocity mismatch are the Gaussian and the single-sided exponential.     

A new approach for evaluating the performance of a symbol timingrecovery system employing a general type of nonlinearity

A new technique is presented for evaluating the jitter performance of a symbol timing recovery (STR) subsystem for digital data transmission systems. The STR system consists of any even-symmetric zero-memory nonlinear device followed by a narrowband filter tuned to the pulse repetition frequency. Exact analytical expressions are derived for the mean and the mean-square values of the timing wave, based on iterative computations of high-order moments of the input signal. Then, the root mean-square (RMS) jitter performance is determined as a function of various system parameters such as the power series expansion of the zero-memory nonlinear device, the rolloff factor of the input pulse shape, and the postfiltering. Finally, the numerical results obtained from some specific examples serve to illustrate several aspects of the timing recovery problem     

Statistical Properties of Timing Jitter in a PAM Timing Recovery Scheme

A new technique is presented for evaluating the performance of a popular type of timing recovery circuit for baseband synchronous pulse amplitude modulation (PAM) data signals. The timing circuit consists of a square-law device followed by a narrowband filter tuned to the pulse repetition frequency along with provision for reshaping the pulses entering the timing path (prefiltering). The output of the timing circuit is a nearly sinusoidal timing wave whose zero crossings indicate the appropriate sampling instants for demodulation of the PAM signal. For a random data sequence, the timing wave exhibits phase fluctuations which are strongly dependent on the pulse shapes entering the timing path and the passband shape of the narrow-band filter. Expressions for rms phase fluctuation in the timing wave as a function of the prefiltering and postfiltering characteristics of the filters preceding and following the square-law device are presented. These expressions have a form which is especially suitable for studying the case where the baseband PAM signal is band-limited to frequencies less than the pulse repetition frequency. A condition on prefiltering and postfiltering which gives error-free timing recovery is presented. Results obtained from some specific examples serve to illustrate several aspects of the timing recovery problem.     

The WSSUS Pulse Design Problem in Multicarrier Transmission

Optimal link adaption to the scattering function of wide-sense stationary uncorrelated scattering (WSSUS) mobile communication channels is still an unsolved problem despite its importance for next-generation system design. In multicarrier transmission, such link adaption is performed by pulse shaping, i.e., by properly adjusting the transmit and receive filters. For example, pulse-shaped offset-quadrature amplitude modulation (OQAM) systems have recently been shown to have superior performance over standard cyclic prefix orthogonal frequency-division multiplexing (OFDM) (while operating at higher spectral efficiency). In this paper, we establish a general mathematical framework for joint transmitter and receiver pulse shape optimization for so-called Weyl-Heisenberg or Gabor signaling with respect to the scattering function of the WSSUS channel. In our framework, the pulse shape optimization problem is translated to an optimization problem over trace class operators which, in turn, is related to fidelity optimization in quantum information processing. By convexity relaxation, the problem is shown to be equivalent to a convex constraint quasi-convex maximization problem thereby revealing the nonconvex nature of the overall WSSUS pulse design problem. We present several iterative algorithms for optimization providing applicable results even for large-scale problem constellations. We show that with transmitter-side knowledge of the channel statistics a gain of 3-6 dB in signal-to-interference-and-noise-ratio (SINR) can be expected.     

深孔激光加工研究

研究了深孔形成的物理过程,提出了一种新型的深孔加工的激光脉冲波形,用光锥方程和能量平衡关系式推导出孔深、孔径及有关激光参数,与实验结果基本相符。     

Ultrahigh-speed photography of picosecond light pulses

A new technique for the display of picosecond light pulses is presented. Ultrashort (6 ps) green light pulses passing through a light-scattering medium are photographed from the side by a camera positioned behind a shutter of 10-ps framing time. The shutter is an ultrafast Kerr cell driven by infrared pulses 8 ps in duration. Color photographs show a bright spot on a dark background, revealing the unambiguous presence of well-isolated picosecond light pulses. The shape of the spot is the result of a convolution involving the three-dimensional shape of the green pulse and the time transmission function of the shutter, this function being dictated by the shape of the infrared pulse. The experiment indicates that a new technique for visualizing light pulses consists in simply observing their flight through a scattering medium from behind an ultrafast shutter having a framing time equal to the time resolution desired. The new technique has many advantages over the two-photon fluorescence display technique, such as higher sensitivity, wider spectral range, and easier interpretation. The ultrafast camera also can be used for the photographic measurement of ultrashort relaxation times in dielectrics and in fluorescent dyes.     

RE Pulse Shapes for Selective Excitation in NMR Imaging

It is shown that the requirements of an RF pulse needed to produce a slice for use in NMR imaging approximate the requirements of the windows used for calculating the discrete Fourier transform (DFT). This approach gives an interesting insight into the physical background of pulse shape tailoring. One of the more successful DFT windows is the 3 term Blackman-Harris window, which is tailored to produce minimum sidelobes. The results of applying radio frequency (RF) pulses modulated to give this window shape is given. The pulse shape produces a uniform magnetization over a range of frequencies with a quasi-linear phase shift making it suitable for producing a slice for NMR imaging when an echo is produced via an inverse magnetic field gradient.     

Transmission of a Gaussian pulse in single-mode fiber systems

A formula for the calculation of the transmitted pulse shape through single-mode fiber systems has been derived by using the linear superposition method. It is then applied to the transmission of a Gaussian pulse in the systems with monochromatic, single-mode, and multilongitudinal-mode laser sources. Numerical results are obtained for the various envelopes of multilongitudinal laser spectra. Applicability of the theory to the power spectra of any shape, effects of side modes of a single-mode laser on pulse transmission, and the possibility of developing a technique for obtaining the chromatic dispersion of a fiber are pointed out.     

Power spectrum of unbalanced NRZ and biphase signals in thepresence of data asymmetry

The impact of data unbalance and asymmetry on the power spectral density of nonreturn-to-zero (NRZ) and biphase baseband modulations is presented. Previously reported results for this problem assumed an incorrect model for the shape and number of elementary pulse shapes that characterize an arbitrary random data stream and thus led to erroneous computations of these power spectra. This paper provides the correct analytical model and then use it to obtain theoretical power spectrum results that are in full agreement with those obtained from computer simulation     

Statistical discriminant analysis of arrhythmias using intracardialelectrograms

The problem of classifying ventricular arrhythmias from intracardial electrograms is considered. Standard statistical discrimination procedures are applied using a simple parametric model for the shape of the pulse near its peak. This approach makes simultaneous use of the model parameters, has well known statistical properties, and involves computations that can be carried out efficiently. Preliminary analyses of real data sets, using both linear and quadratic discrimination functions, yield promising results     

Pulse Shape Design for Teletext Data Transmission

This paper studies the problem of designing a suitable pulse shape for teletext data transmission. The following four criteria are used: 1) Nyquist I criterion, 2) Nyquist II criterion, 3) degree of overshoots in the channel signal, and 4) robustness to sampling phase jitter. For system bandwidths less than the inverse-baud rate, it is not possible to satisfy all these criteria simultaneously; tradeoffs that have to be made are illustrated. Several candidate pulse shapes are given and a composite criterion developed. A pulse shape, which satisfies the Nyquist I criterion and is closest to satisfying the Nyquist II criterion, in a sum-of-squares-of-deviations sense, is recommended.     

一种降低OFDM信号PAPR的新方法

正交频分复用(OFDM)技术的一个缺点是信号的高峰值平均功率比(PAPR)大大降低了系统中线性功放的效率。本文提出一种新的基于Nyquist脉冲成形的PAPR抑制方法。这种方法基于选择适当的Nyquist脉冲波形集合对OFDM的各个子载波进行脉冲成形。分析论证了该方法的PAPR上限值和所用的脉冲成形波形集合。仿真结果表明该方法能有效降低任意子载波数目的OFDM信号的PAPR值,而且与已有方法相比应用更为简单。因此脉冲成形技术不仅能对传输信号进行频谱成形来提高系统的频带利用率,而且还可以减小信号的PAPR值。     

Active picosecond optical pulse compression in semiconductoroptical amplifiers

A novel technique is proposed to compress and amplify a weak picosecond optical pulse by utilizing a copropagating intense pump pulse in semiconductor optical amplifiers. It is shown that, simply by adjusting the time delay between the two pulses, a controlled compression of the pulsewidth by up to a factor of ~4 with pulse energy amplification of up to ~16 dB can be achieved simultaneously. Furthermore, a compressed pulse shape with strongly damped wings is also observed. The shortest achievable pulsewidth is demonstrated to be dependent on the frequency detuning and pump pulse properties