Fluctuations of noiselike signals reflected from a rough surface at the output of a correlation receiver

The frequency and time averaging of the fluctuations that occur in the cross-correlation function of a radiated noiselike acoustic signal with the signal received after its reflection from a rough water surface is considered. The variance and temporal correlation function are calculated for the output effect of a correlation receiver for different ratios between the averaging time and the time correlation interval of fluctuations, the band width of the radiated signal, and the frequency correlation interval of the transfer function fluctuations.     

Diversity reception of hydroacoustic signals from the results of experiments in the Black Sea

The results are presented of an experimental study of spatial and frequency correlation of amplitude fluctuations of hydroacoustic signals on two paths of the Black Sea under conditions of a thermal underwater sound channel and tonal-continuous emission at frequencies of 4 and 1.5 kHz. The emitters were located in the coastal wedge on the bottom slope at depths of 35 and 60 m; a receiver system was submerged to a depth of 50–60 m off the side of a ship drifting at various distances in the open sea. Data are presented on the spatial (horizontal and vertical) and frequency intervals of fluctuation correlation on the first path extending 100 km at an emission frequency of 4 kHz under conditions of dominant fast fluctuations, as well as on the second path extending 300 km at an omission frequency of 1.5 kHz under conditions of dominant slow fluctuations. The results of experiments are used to estimate the efficiency of space-diversity and frequency-diversity reception of acoustic signals in application to information transmission over a hydroacoustic channel.     

Influence of the source bandwidth on the observation of microwave photon echoes

The fluctuations of the source frequency are shown to be responsible of an extra damping of the optical precession and of the photon echoes observed in the experiments involving many shots and an averaging of the detected signals. Assuming a phase diffusing model and various profiles for the frequency power spectrum, the r.m.s. accumulated phase error is calculated and two asymptotic behaviours are pointed out according to the relative values of the sequence duration and of the frequency correlation time. The calculation is well supported by a photon echo experiment at a 3-mm wavelength.     

Intensity fluctuations in a limited optical beam

An experimental study was made of the dispersion and correlation functions for intensity fluctuations of laser radiation at distances of 0.5, 2.4, and 7.0 km. The averaging due to the receiver aperture is less important than predicted by theory. The correlation time for the intensity fluctuations in the strong-fluctuation range is a weak function of the path length. At receiver-aperture sizes much greater than the size of the laser beam, large intensity fluctuations are observed, and the probability distribution for the intensity fluctuations is approximately normal.Translated from Izvestiya VUZ. Fizika, No. 2, pp. 52–56, February, 1970.     

Influence of fluctuations of the weight coefficients on the characteristics of adaptive antenna arrays

We analyze theoretically the influence of the weight-coefficient fluctuations on the characteristics of adaptive antenna arrays (AAAs) with gradient adjustment algorithms under the assumption that the useful and noise signals have different correlation times. We obtain expressions for the correlation function and the spectral power density of the AAA output signal, find the average radiation pattern (RP), and derive an expression for the correlation matrix of the weight-coefficient fluctuations. It is shown that the weight-coefficient fluctuations bring about distortions of the useful signal and decrease its output power compared with the case of no fluctuations. In the frequency spectrum, this “overcompensation” phenomenon results in the depression of the output signal spectrum of the antenna array in the frequency band where high-power noise exists. As an example, we calculate the spectral power density of the output signal for an AAA with single linear constraints and plot the fluctuation RP and the variance of the weight-coefficient fluctuations versus the noise arrival angle. N. I. Lobachevsky State University, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 1, pp. 83–92, September 2000.     

Narrowband heterodyne reception using unstabilized sources

Source power for laboratory-type experiments is often limited at millimeter and submillimeter wavelengths. This requires the use of sensitive receiving equipment. Cooled square-law detectors and narrowband heterodyne detectors are examples. We have developed a coherent mechanical frequency shifter, which makes possible narrowband heterodyne detection without the need for highly frequency stable sources. Identical frequency and phase fluctuations of the transmit and local oscillator signals derived from one source are eliminated at the intermediate frequency. The widely tunable frequency shifter, designed for a 637GHz scale-model radar, was tested in a 140GHz non-radar configuration. We investigated the receiver's minimum coherently resolvable bandwidth and its sensitivity. Several types of sources were compared for the effect of local oscillator amplitude noise on receiver sensitivity at low intermediate frequency.     

Detection of new acoustic signals from the human head

New acoustic signals generated in the human head have been found using piezoelectric transducers of longitudinal acoustic oscillations; these signals clearly manifest themselves in recording from the temporal regions and are less pronounced when recording from the forehead. They have a form of 4-ms pulses with a repetition frequency varying from 60 to 120 ms. The signal level exceeds that of thermal acoustic fluctuations by about two orders of magnitude. The signals are formed when the subject of an experiment is relaxed; i.e., the human is at rest with closed eyes and is about to fall asleep. The repetition frequency of these signals is close to the alpha-rhythm frequencies; however, there is no exact correlation between these types of signals at long (of about minute) time intervals. The signals recorded from different temples generally have no strong synchronization. Since some parameters of these signals are close to those of electroencephalographic signals, the former can be referred to as acoustoencephalographic signals.     

Correlation receiver of below-noise pulsed signals based on parametric interactions of spin waves in magnetic films

A correlation receiver capable of receiving pulsed microwave signals having amplitudes substantially below the noise level is proposed and experimentally realized. The device is based on the parametric interaction of two contra-propagating spin waves excited by the received microwave signal and the reference pulsed signal, containing information about the received signal shape, in a ferrite film waveguide. The output nonlinear correlation signal having the doubled carrier frequency and the signal-to-noise ratio enhanced by several orders of magnitude is received by a dielectric resonator coupled to an output microwave transmission line.     

Spectral correlation characteristics of an adaptive Antenna array tuned by the Hebb algorithm with allowance for the weight-vector fluctuations

In this paper, we present the results of statistical analysis of the characteristics of an adaptive antenna array tuned by the Hebb algorithm with allowance for the weight-vector fluctuations in the case where signals with different levels of autocorrelatedness of the input-signal readouts arrive at the antenna array. Expressions for the correlation function of the output signal and the power spectral density at the antenna-array output are obtained. It is shown that fluctuations lead to distortions of the output signal of the adaptive antenna array and appearance of additional terms in the formulas for these characteristics.     

Calibration of hydrophones in a field with continuous radiation in a reverberating pool

We examine a method for estimating the transient impedance of an emitter and a receiver in a free field based on sliding complex averaging of the frequency dependence of the transient impedance of the emitter and the receiver obtained under continuous emission in an undamped pool. The method makes it possible to weaken the influence of reflected signals and to obtain frequency characteristics almost coinciding with measured ones in free-field conditions. We present results from experimental studies of the method and its application for calibration of hydrophones by field with the reciprocity method in a reverberating field.     

Some effects of partial averaging of signal fluctuations in a random medium

The concept of partial averaging of signal fluctuations in a stretched turbulent medium is considered. Such uncontrollable averaging over time, frequency, or space arises from the mutual influence of signals scattered by irregularities of different scales (or different scattering layers) on fluctuations of the received signal intensity. The scintillation index of a partially averaged field is defined for the averaging action of a radiation source of finite angular dimensions. The proposed concept and obtained results may be useful in many applications as a development of the statistical theory of wave propagation in random media.Scientific-Research Institute of Radiophysics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 6, pp. 754–762, June, 1994.     

浅海中孤立子内波引起的声能量起伏

当孤立子内波的波阵面与声传播路径所成角度较大时,简正波耦合是导致声信号起伏的主要因素。研究了浅海中孤立子内波引起的声能量起伏规律,给出声场起伏的耦合简正波表达式,并使用抛物方程模型进行仿真。数值分析表明,接收点声强随时间变化呈准周期性。在频谱图中能够得到声强起伏的主导频率,主导频率与孤立子内波沿声传播路径的移动速度成正比,与无扰动波导中简正波在距离上的干涉周期(对应于射线理论中临界声线的跨度)成反比,与孤立子内波的形状无关。此外,对声强频谱的垂直结构进行了分析,该结构与对声场起伏起主要作用简正波的本征函数相关。      

系统增益对光混沌通信系统性能影响的研究

针对强注入开环结构的外腔反馈半导体激光器混沌通信系统,数值计算得到系统增益与注入系数的定量关系曲线.对比是否使用系统增益对接收系统输出进行校正两种情况下系统的解调性能,表明使用增益校正的相减解调法获得的信息更接近原始信息.通过采用不同频率的正弦信号进行调制,表明信息频率对系统增益没有影响;用不同调制方法对0.2?Gbit/s数字信号加密,结果表明系统增益与信号类型及调制方法无关. 关键词： 光混沌通信 强注入 系统增益 同步     

大气湍流对空间光耦合至单模光纤的影响

 根据弱湍理论，考虑湍流强度闪烁、孔径平均效应、湍流波前畸变及耦合系统跟踪误差的影响，导出了空间光到单模光纤的平均耦合效率及耦合功率起伏方差模型。根据Hufnagel-Valley 5/7折射率结构模型，对下行连接及上行连接的平均耦合效率及耦合功率起伏进行了研究。仿真结果表明，对于下行连接，随着地面接收天线孔径的增大，相对功率起伏先是由于孔径平均效应而减小，而后由于湍流波前畸变的影响而逐渐增大；对于上行连接，可忽略湍流波前畸变及孔径平均效应的影响。     

Effect of the ion background on the spectral and amplitude characteristics of the output signal from the oscillator with the turbulent electron beam

The effect of the residual gas pressure on the spectral and amplitude characteristics of the output signal from the oscillator with the turbulent electron beam is examined. It is shown that ions resulting from gas ionization in the drift region promote formation of denser and more compact space charge bunches. Denser bunches increase the output signal power and aid in appearance of higher-order components in the spectrum, thus extending the working range to a higher-frequency region. When the device operates in the regime of noiselike signal generation, ions contribute to a decrease in the irregularity of the output power spectrum to about 0.3 dB in the working frequency range up to two octaves.     

Frequency correlation of fluctuations of short-wave signals in the theory of refractive scattering of radiowaves in the ionosphere

We consider the problem of frequency correlation of phase fluctuations of HF signals reflecte from a turbulent ionospheric layer with an arbitrary profile of electron density in vertical and oblique sounding of the ionosphere. Expressions for evaluation of the coherence band of HF signals reflected from a randomly inhomogeneous ionosphere are obtained. Calculation results are compared with corresponding calculations of frequency correlation of fluctuations of HF radiowaves in Buker's theory of refractive scattering of radiowaves. It is shown, in particular, that the influence of splitting of rays for waves with different frequencies in an inhomogeneous ionosphere on the frequency distortions of HF signals prevails compared with the phenomenon of frequency decorrelation of fluctuating HF signals propagating in a free space beyond a turbulent ionospheric layer.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 9, pp. 893–900, September, 1995.     

Direction finding and suppression of vector-scalar sound signals in shallow water taking into account their correlation and mode structure

The correlation of low-frequency sound signals from towed tonal low-frequency sources at the output of the scalar and vector channels is studied in shallow water. The correlation of the scalar field and signal received by a horizontally oriented vector receiver on average is 0.92–0.99; correlation with the signal received by a vertical vector receiver decreases to 0.66–85. When scalar fields or horizontal projections of the vibration velocity vector with application of the aperture synthesis algorithm are used, 3–5 normal waves are isolated; when the vertical component is used, 7–9 modes. It is demonstrated that the high signal correlation ensures direction-finding accuracy and suppression of strongly noise-emitting moving sources by 20–30 dB or more if the cardioid is directed at the source according to the zone of the minimum.     

Complex calibration of a pressure gradient receiver using the reciprocity method procedure

Absolute complex free-field calibration of a pressure gradient receiver is considered. The calibration is carried out in a reflecting water tank using the reciprocity method procedure during radiation of continuous signals with linear free-field frequency modulation. To obtain the free-field frequency dependences, complex moving weighted averaging is used. It is shown that this method, developed for calibration of sound pressure receivers, allows effective recovery and measurement of the vector component of a direct acoustic wave in the presence of reflections. The method makes it possible to measure the modulus and phase angle of the complex sensitivity and reveal the insufficiencies of an experimental pressure gradient receiver.     

Testing underwater bottom-moored antenna arrays in the sea and in a man-made lake

Measurements of response, gain, and noise immunity are carried out for an underwater compensated additive receiving array with randomly spaced hydrophones that is moored at the bottom of a man-made lake with multimode sound propagation. The in-sea locating ability of a similar array is demonstrated with the sources of noiselike signals at frequencies of 5–100 Hz. A dedicated numerical processor is developed and tested for processing the signals received by a random underwater array.     

Sound fluctuations caused by internal waves in a shallow sea

Estimates are presented for the fluctuations of the parameters of low-frequency sound fields in shallow-water regions of the Barents Sea, in the presence of seasonal internal gravity waves. The objective of the experiments is to reveal the main mechanisms that govern the sound fluctuations and their statistical parameters on paths of moderate lengths (50–60 to 100–120 km). Another objective is to determine the features of the sound interaction with internal waves for the sound speed profile of the summer—autumn type for which the water stratification is most pronounced. As the probing signals, continuous tonal ones produced by bottommoored sources at the frequencies about 100 and 300 Hz are used along with the 1/3-octave noise signals with the central frequency 1000 Hz, which are generated by a source deployed from a vessel. For the signal reception, both fixed bottom-moored hydrophones and a vertical chain of hydrophones are used, the chain also being deployed from the vessel. The water temperature, the salinity, and the thermocline displacements are monitored with standard hydrographic sensors. The following main results are presented: the estimate of the degree of correlation between the sound fluctuations and the parameters of the water layer, the comparison of the fluctuations in the signal amplitude envelope with the data obtained in other regions, and the estimate of the statistical parameters of the signal amplitude fluctuations, including their dependence on the path length. One more result consists in the proof of the wave nature of the interaction of sound and internal waves, which manifests itself in a strong dependence of the sound interaction with internal waves of discrete frequencies on the frequency of the probing signal and on the angle at which these wave beams intersect. An attempt is made to explain the observed phenomena by the synchronism in the interacting sound and gravity waves. The data obtained can be used to analyze and compare the fluctuations of the sound fields in the ocean, especially in shallow-water regions.