Signal Spectrum Width Research Articles

Digital Beamforming Features in Subaperture Antenna Arrays of Highly Informative Space Radars with Aperture Synthesis

It is shown that during digital beamforming in a subaperture antenna array of highly informative space radars with synthetic aperture, it is necessary to provide a procedure for equalizing the time delays of the subaperture output signals for transmission and reception. It has been established that losses in the signal-to-noise ratio on a radar image of a point target for a typical aperture size at an elevation angle of 0.7 m and a signal spectrum width of 600 MHz reach 2.1 and 7.5 dB for off-nadir angles of 21° and 50°, respectively. Recommendations are formulated for equalizing time delays for transmission and reception based on spectral processing. Simulation results are presented.

ВИМІРЮВАННЯ ОПОРНОЇ ЧАСТОТИ ВУЗЬКОСМУГОВОГО РАДІОСИГНАЛУ ОБМЕЖЕНОЇ ТРИВАЛОСТІ

The work concerns analog-digital systems that work with radio signals emitted and received by the antenna, and these signals have a limited duration, ie are pulsed. The propagation conditions of such signals and the processes of formation of echo signals affect their amplitude, frequency and phase characteristics in such a way that they form classical narrowband signals. The operation of the system involves the determination of certain parameters of echo signals, and taking into account their pulse nature for such a definition is given a limited time interval. This means that the procedure for determining the parameters must meet the criterion of high speed, and therefore differ from the traditional, built on the use of phase-locked loop. The article is devoted to solving the problem of measuring the reference frequency of a pulsed narrowband radio signal. By analyzing the results of experimental studies, two types of errors in measuring the reference frequency of a narrowband radio signal were identified and the causes of their occurrence were established. These errors are provoked by the peculiarities of the structure of the narrowband signal. The reason for the error of the 1st type, the absolute value of which correlates with the duration of the half-cycle of the reference frequency, is the so-called phase jumps at the point of change of the sign of the bypass. The frequency of such errors within the duration of the echo signal is generally low, although it increases with increasing signal spectrum width. Type 2 errors occur due to the appearance of zones with almost completely suppressed signal amplitude, which may be the result of intrapulse interference and / or signal attenuation. Such errors also occur more often if the spectrum width is larger. A method for measuring the reference frequency of a narrowband pulsed radio signal is proposed, which is based on counting the number of half-cycles of the reference frequency on a time-limited measurement interval and removing from this procedure areas with completely suppressed signal amplitude. An auxiliary highly stable reference frequency is used to establish the numerical value of the reference frequency. The block diagram of the frequency meter and the algorithm of its operation are given, the implementation of which avoids these errors.

Анализ влияния быстрых и медленных мультипликативных помех на искажения диаграммы направленности системы "решетка-приемник".

The effect of fast and slow multiplicative noise on the distortion of the beam pattern of the «array-receiver» system in the conditions of the smoothing effect of the receiver and taking into account the time of relative lag from the grating elements was analyzed. Expressions are obtained for instantaneous beam patterns of the «array-receiver» system in the absence and presence of multiplicative noise. It is shown that in the case of action of multiplicative noise the beam pattern of the «array-receiver» system is obtained by averaging the instantaneous pattern on the grating correctness at a time interval equal to the signal duration. Influence of periodic multiplicative noise on distortion of beam pattern of system «array-receiver» is investigted. It is shown that for all signals except frequency-modulated, the beam pattern expression is maximized at the time t0=0. It is also shown that the beam pattern distortion of the «array-receiver» system when exposed to periodic multiplicative noise is less than for the separately considered phased antenna array. The degree of reduction of distortion from smoothing action of matched receiver filter is determined; and the distortion is less, the greater the ratio of the spectrum width of the noise modulation function to the signal spectrum width. Analysis of the influence of fluctuation multiplicative noise on the distortion of beam pattern of «array-receiver» system for the case of matching with signal of receiver filter is performed. The power-average beam pattern of the «array-receiver» system is determined through the energy spectrum of the noise modulation function. It is noted that multiplicative noise, all other things being equal, has less effect on the beam-receiver pattern of the system when using broadband pulse signals without intra-pulse modulation and at the same time resolution in range. It is noted that in case of deep phase distortions for gratings with reversible phases, the distortion of the beam pattern of the «array-receiver» system caused by multiplicative noise is significantly more than those distortions associated with the extremity of the signal spectrum width. The newly smoothing effect of the receiver reduces the expansion of the beam lobe caused by multiplicative noise. An expression is obtained for the average power of the beam pattern of the «array-receiver» system with normally distributed phase distortions of the signal. The effect of the signal duration on beam distortions caused by multiplicative noise was quantified.

THE FORMATION METHOD OF COMPLEX SIGNALS ENSEMBLES BY FREQUENCY FILTRATION OF PSEUDO-RANDOM SEQUENCES WITH LOW INTERACTION IN THE TIME DOMAIN

Context. The problem of forming complex signal ensembles on the basis of frequency band filtering and research of their properties is considered. The object of research is the process of synthesis of signal ensembles based on frequency filtering of pseudo-random sequences of short video pulses with low interaction in the time domain.Objective. It is to form complex signal ensembles with satisfactory values of intercorrelation properties, which are close to the signals with minimal energy interaction. Method. The results of the application of forming complex signal ensembles method by frequency filtering of pseudo-random sequences with low interaction in the time domain are presented. As a result of the spectral band selection of the studied pseudorandom short video pulse sequences due to the use of bandpass filters based on the Chebyshev filter of the first kind, new samples of sequences with spectrum restriction are obtained. By applying intercorrelation analysis to the obtained sequence samples, the values of the maximum emissions of the side lobes of the cross-correlation functions (CCF) for all possible signal pairs are estimated. If the values of the maximum emissions of the side lobes of the CCF signals exceed the limit values, the sequence of the analyzed pair with a smaller value of the number of pulses is removed from the ensemble. In case of satisfactory value – the received signals are accepted for the signal ensemble formation with the minimum power interaction. Thus, a new set of values of the maximum emissions of the side lobes of the CCF is formed. This approach increases the number of signals in ensembles with satisfactory values of statistical characteristics with limited signal spectrum width, and the correlation properties of such sequences approach the signals with minimal energy interaction, which reduces the level of multiple access interference. As a result, complex signal ensembles obtained by frequency filtering should be used in cognitive radio systems with code division multiplexing.Results. Based on the software implementation of the method of forming complex signal ensembles by frequency filtering of pseudo-random sequences with low interaction in the time domain, signals with satisfactory values of statistical characteristics with limited signal spectrum width with intercorrelation properties close to signals with minimal energy interaction and higher ensemble volume were selected.Conclusions. The application of frequency filtering to pseudo-random sequences of short video pulses with a low level of crosscorrelation allows to obtain complex signal ensembles, which will be similar in correlation properties to sequences with minimal energy interaction. It will reduce the level of multiple access interference. The analysis revealed that the use of frequency filtering of sequences will slightly worsen the mutual correlation properties of signals, possibly due to suboptimal synthesis of values of maximum emission levels of side lobes of CCF signals, but, nevertheless, it is possible to use such signals in modern cognitive systems radio access multiple access with code division multiplexing.

Application of Radar Signal Spectrum Width Estimation to Detect Vertical Airflows

Application of Radar Signal Spectrum Width Estimation to Detect Vertical Airflows

The bandwidth-duration product of the received radar signal as a criterion for observing wind field inhomogeneity

The coherent mode meteorological radars applicability to recognizing weather hazard associated with wind fields inhomogeneity is analyzed. It is noted that for the aviation weather service the estimates of the received reflections spectral characteristics prove to be little informative. Problems that hinder the use of these phenomena detection results have been identified. It is proposed that new methods based on estimates of the reflected signal spectrum width and methods based on the detection of the complexity of the received signal, which is the sum of two reflections from different air masses with different characteristics, should be used for this purpose. As an indicator of this complication, it is assumed to use the bandwidth-duration product, which is a product of its energy spectrum width by the interval of its amplitude correlation. The theoretical calculation results of the bandwidth-duration product, which spectrum consists of two Gaussian form components, for different values of average frequency and/or width, are given. The received data allow asserting that the bandwidth-duration product practically does not change at small differences of Doppler shifts of signal components, but very sensitively reacts to a difference in a spectrum width, i.e. in the most interesting cases for a solved problem.

Statistical Estimates of the Accuracy of Mono-Pulse Direction Finding of Sources of Radioemission by Dual-Channel Phase Devices

Formulation of the problem in general. The practice of radio monitoring indicates that information should be obtained on a time-to-real scale, with maximum speed and minimum time loss; the accuracy of the estimation of the parameters of the radioemission sources and their bearings should be maximized; the probability of recognizing and classifying sources and objects is no less than a given one. This confirms the existence of a problematic situation, which is caused by the existing contradiction between modern requirements for ensuring high speed, accuracy and reliability in the process of obtaining information, data and information and technical capabilities of modern radio monitoring facilities. Analysis of recent researches and publications. Currently, mono-pulse routing techniques are mainly designed to monitor radar signals where most of the signal parameters are known (example, carrier frequency, duration, period or frequency of pulse repetition, signal spectrum width, frequency deviation, signal to noise ratio). But in the conditions of partial or complete uncertainty of parameters, mono-pulse radar methods lose their efficiency and become unsuitable. Presenting the main material. The statistical estimates of the accuracy of the mono-pulse two-channel phasometric direction finding method, which provides unbiased, efficient and optimal estimates with a given probability and minimal mathematical expectation and variance, are calculated. The gain in the accuracy of direction finding when using two-phase phase direction finders will be determined by the ratio of the mean squared errors of the coarse and precise direction channels and equal to the inverse of the magnitudes of the phasometric bases of both channels. Conclusion. The use of mono-impulse dual-channel phase-finding method and defining the parameters of radioemission sources ensure unbiased, efficient and optimal estimates with a given probability and minimal mathematical expectation and variance. The minimization of the mathematical expectation and variance of the statistical estimates of the parameters of radio sources when using the two-channel phasometric method can be assured by rational choice of the magnitude of the phasometric bases of coarse and accurate channels and depends on the signal-to-noise ratio at the input of the direction finder and the accuracy of the carrier frequency measurement. The perspectives of future researches. One of the directions of further researches can be considered the solution of the problem of development of the combined amplitude-phase method (method) of bearing with synthesis of structural circuits of devices conducting mono-impulse bearing of sources of radio emission of telecommunication networks, which provide optimal estimation of relative radiation analysis evaluation of their quality indicators.

Influence of Tight Optical Filtering on Long-Haul Transmission Performance of Several Advanced Signaling Formats

In this paper, the influence of tight optical filtering (TOF) on dispersion-managed long-haul (LH) transmission performance of advanced signaling formats is analyzed through extensive numerical simulation. The investigation is performed for duobinary, carrier-suppressed-return-to-zero (CS-RZ), and duobinary CS-RZ (DCS-RZ) signaling formats, which were suggested for LH ultradense wavelength division multiplexing systems previously. The simultaneous variation of pre-, inline, and postcompensation of dispersion amounts is performed to find out the optimum dispersion map of each signaling format with and without TOF, and their dispersion tolerances. The tolerance to fiber nonlinearity of each signaling format is also analyzed. It is shown that the TOF affects differently the LH transmission performance of the three signaling formats. The TOF reduces the dispersion tolerances of the duobinary signaling format, but improves the maximum -factor and tolerance to nonlinear fiber effects due to the eye-opening improvement. It improves the tolerance to inline dispersion and fiber nonlinearity of the CS-RZ signaling format, but it reduces remarkably the eye opening leading to a significant -factor degradation. It improves also the maximum -factor and the total residual dispersion tolerance of DCS-RZ format due to the reduction of the signal spectrum width, but it worsens the tolerance to pre- and inline dispersion, and fiber nonlinearity.

Limitations imposed by Rayleigh backscattering in closely interleaved, bidirectional WDM transmission systems

By deriving signal-backscattering and backscattering-backscattering beat noises, this letter quantifies the performance impairment due to backscattering produced by closely spaced counterpropagating signals. Experiments using 10-Gb/s data signals verify the theoretical analysis. The interference between signal and backscattering induces significant impairment when their channel separation is less than the sum of the data rate and signal spectrum width. The spectral efficiency limitation imposed by backscattering is clarified for closely spaced, interleaved bidirectional transmission systems.