Estimation techniques for GMSK using linear detectors in satellite communications

This paper describes data-aided signal level and noise variance estimators for Gaussian minimum shift keying (GMSK) when the observations are limited to the output of a filter matched to the first pulse-amplitude modulation (PAM) pulse in the equivalent PAM representation. The estimators are based on the maximum likelihood (ML) principle and assume burst-mode transmission with known timing and a block of L₀ known bits. While it is well known that ML estimators are asymptotically unbiased and efficient, the analysis quantifies the rate at which the estimators approach these asymptotic properties. It is shown that the carrier phase, amplitude, and noise variance estimators are unbiased and can achieve their corresponding Cramer-Rao bounds with modest combinations of signal-to-noise ratio and observation length. The estimates are used to estimate the signal-to-noise ratio. It is shown that the mean squared error performance of the ratio increases with signal-to-noise ratio while the mean squared error performance of the ratio in decibels decreases with signal-to-noise ratio. Simulation results are provided to confirm the accuracy of the analytic results.     

Rectangular-Wave Modulation

Rectangular-wave modulated pulse lengths are expressed as a function of the modulation index. The theoretical expressions are experimentally verified. An experimental measurement of signal-to-noise ratio for rectangular-wave modulation confirms the theoretical analysis. Jelonek's signal-to-noise ratio formula for pulse-length modulation is also found applicable to rectangular-wave modulation. It is concluded that rectangular-wave modulation is closed-loop pulse-length modulation.     

Output Signal-to-Noise Ratio for a Random-Access Repeater Link with an Ideal Hard Limiter

The acquisition detectability parameter, output signal-to-noise ratio, has been computed for unrestricted random access through an ideal hard limiter. The method involves multidimensional Fourier series and numerical integration. The results depend on the number of active users, total number of users, input signal-to-noise ratio, address code length, cross-correlation properties of code, and other parameters. The method provides a tool for synthesis of randomaccess networks, i.e., satellite repeaters for land, sea, or air-borne transmitters, or mobile radio relays concerned with accidental or intentional interference.     

基于Walsh基和二值离散余弦基单像素成像的对比分析

为提升单像素成像的测量效率,对Walsh基和二值离散余弦基单像素成像进行了对比研究。针对Walsh变换和抖动二值化后的离散余弦变换,提出了一种借鉴机器学习的排序方法,即通过分别统计80000幅图像的Walsh测量基和离散余弦测量基各自相应测量权重绝对值,按权重由大到小排序得到了两种具有先验知识的测量基优化排序。按两种测量基优化排序,研究了在相同压缩采样下的单像素成像。利用随机选取的500幅图像,定量分析了成像的峰值信噪比和结构相似度。研究发现,两类测量基在压缩采样为5%时,500幅图像平均峰值信噪比可达20dB,平均结构相似度达0.5以上;在采样率1%~10%的区间,二值化的离散余弦基重建结果占优;在采样率10%~20%的区间,两种测量基效果相当;在采样率大于20%时,Walsh基占优。该结论为单像素成像技术实现快速、高信噪比成像提供了一种方法,对推进单像素成像测量基的选取具有较好的参考价值。     

Closed-Form Expressions for Noncoherent Radar Integration Gain and Collapsing Loss

Closed-form formulas allow rapid determination of noncoherent integration gain and integration loss when the single-sample IF signal-to-noise ratio (SNR) is known. In addition, if the required SNR is known for any number of integrated pulses, the required SNR for any other number is easily determined. A closed-form expression is given for radar collapsing loss, expressed in terms of the equivalent integrated signal-to-noise ratio required to produce a given combination of false-alarm and detection probabilities. Alternatively, the single-sample signal-to-noise ratio of a set of samples may be used together with the closed-form expression for integration gain to get the equivalent integrated signal-to-noise ratio.     

Error Performance of Two-Channel Phase-Measurement Systems

The problem of measuring a constant phase difference between two sinusoids which have been corrupted by two additive noise processes is investigated. For the case in which both noise processes are Gaussian and the signal-to-noise ratio is high, the variance of the time-averaged phase-difference estimate is found as a single integral over the noise power spectral densities and cross spectral density. Arbitrary cross correlation is allowed between the two noise processes. Two cases of practical interest are considered: 1) the noise processes have identical, rectangular power spectral densities and are statistically independent; 2) an idealized radio direction finding situation in which two spatially separated sensors are immersed in an isotropic, planar noise field. For the statistically independent case, a universal curve is presented which permits determination of phase-estimate standard deviation for arbitrary signal-to-noise ratio and for a wide range of bandwidths and integration times.     

超燃冲压发动机燃烧室出口温度场分布CARS测量

针对超燃冲压发动机研究中对燃烧室出口温度场的测量需求以及暂冲式超燃冲压发动机燃烧台架试验中的应用难点,开发了适用于瞬态燃烧场温度测量的单脉冲相干反斯托克斯拉曼反射（CARS）系统及CARS光谱计算和温度反演软件CARSCF。采用USED相位匹配方式来降低湍流影响,结合多尺度小波分析方法来实现CARS光谱降噪处理,提高信噪比。在暂冲式脉冲燃烧风洞上开展了来流马赫数2.6条件下超燃冲压发动机燃烧室出口温度测量试验,获取了超声速来流（冷态）建立、H₂点火加热空气、建立超声速燃烧流场直至试验结束过程中的燃烧室出口温度,以及煤油/空气燃烧时燃烧室出口温度场分布。结果显示,超声速冷流时温度处于低温（约205K）状态,随着H₂点火加热来流空气,来流温度上升至853K;随着煤油/Air点火,温度急剧上升,稳定燃烧状态下燃烧流场温度为1970K±144K。燃烧室出口截面温度场分布测量结果显示,高温区位于燃烧室出口截面上侧区域,而燃烧室出口截面上中间区域的温度低于上下两侧。燃烧室出口温度分布CARS测量结果与火焰自发光成像结果一致,表明单脉冲CARS技术用于瞬态燃烧流场温度测量的可行性。     

一种改进的直方图概率多假设多目标跟踪方法

直方图概率多假设跟踪（H-PMHT）方法及其变形泊松分布直方图概率多假设跟踪（P-HPMHT）方法的一个主要缺点是其量测模型仅考虑了背景杂波而没有考虑传感器噪声,从而导致在低信噪比条件下检测概率较低。针对这一问题,提出了一种带传感器噪声模型的H-PMHT方法,通过将传感器噪声引入量测模型,从而明显提高了对低信噪比目标的跟踪检测能力。该方法的计算量与目标数保持线性关系,仍然适用于目标数目较多的情况。仿真实验表明：该方法在误跟踪比率为1‰,信噪比为6 dB时,检测比率可提升近20%,信噪比为3 dB时,可提升近10%。     

Detection Probabilities for Log-Normally Distributed Signals

The amplitude and power of a large family of radio signals are observed to have log-normal probability density functions. Among these are signals propagated through random inhomogeneous media, a notable example being low frequency atmospheric radio noise. Of greater importance are certain radar targets that have been observed to have essentially log-normal density functions. Both ships and space vehicles may fall into this category. Curves of probability of detection vs. signal-to-noise ratio for the case of log-normal signals in Gaussian noise have been computed and are presented in this paper. The curves apply for square-law detection with varying degrees of postdetection linear integration. Both fully correlated and completely uncorrelated fluctuating signals are considered. It is shown that for log-normal signal distributions having large variances, the probability of detection differs significantly from that obtained using curves based on an assumed Rayleigh signal distribution.     

Effects of CW Interference on Narrow-Band Second-Order Phase-Lock Loops

This paper describes an experimental study of the effect of continuous wave (CW) interference and white noise on a second-order phase-lock loop. The reciprocal of the loop mean-square phase error is used as an index of performance, and the effect of interference levels that do not cause cycle skipping or loss of lock is described in terms of this index. Loop thresholds are determined by measurement of cycle-skipping rates. Stationary or slowly-sweeping CW interference caused a degradation in loop threshold of roughly 3 dB for every 6 dB of interference power above the noise power level. The effective loop signal-to-noise ratio was decreased approximately 1 dB at interference-to-noise power ratios of -3 dB. Interference levels equal to the signal level consistently caused loss of lock, regardless of the loop signal-to-noise ratio.     

Improved Range Resolution Filters for Rectangular-Pulse Radar Systems

The performance of several new clutter-reduction filters suitable for rectangular-pulse radar systems is investigated. The new filters consist of various approximations and modifications of two filters known to be optimal for certain criteria: the well-known Urkowitz filter which optiizes the clutter improvement ratio, and the newer sidelobe reduction filter which minimizes output noise power subject to peak sidelobe constaints. The new filters are compared usig five basic criteria: clutter improvement ratio, signal-to-noise ratio, sidelobe peak ratio, pulse compression ratio, and filter complexity. The results are summarized in tabular and graphical form.     

The Moon as Source for G/T Measurements

For large-aperture antennas, it is customary to utilize radio stars in order to determine the receiving gain to temperature ratio. In the case of small-aperture antennas, which not only have reduced gain but usually higher system noise temperature as well, the y factors obtained from the radio star measurements are so small that the measurement error is intolerable. The moon, on the other hand, provides a power flux density higher by at least one order of magnitude compared to the strongest radio star, and the resulting y factors are usable. G/T ratios determined from moon measurements agree well with expected values.     

Phase-Shift-Keyed Signal Detection with Noisy Reference Signals

This paper derives and graphically illustrates the performance characteristics of Phase-Shift-Keyed communication systems where the receiver's phase reference is noisy and derived from the observed waveform by means of a narrow-band tracking filter (a phase-locked loop). In particular, two phase measurement methods are considered. One method requires the transmission of an auxiliary carrier (in practice, this signal is usually referred to as the sync subcarrier). This carrier is tracked at the receiver by means of a phase-locked loop, and the output of this loop is used as a reference signal for performing a coherent detection. The second method is self-synchronizing in that the reference signal is derived from the modulated data signal by means of a squaring-loop. The statistics (and their properties) of the differenced-correlator outputs are derived and graphically illustrated as a function of the signal-to-noise ratio existing in the tracking filter's loop bandwidth and the signal-to-noise ratio in the data channel. Conclusions of these results as well as design trends are presented.     

MMW system trade-offs

A brief review is given of the current state of knowledge of millimeter-wave atmospheric propagation and clutter characteristics and the detection performance of two air-to-ground fire-control systems evaluated in terms of their ability to detect a 40-m² target in the presence of atmospheric attenuation and three types of clutter. Generally, the 35-GHz system considered performed better in terms of signal-to-noise ratio performance and in signal-to-clutter ratio performance for light clutter. In heavy clutter, the 95-GHz system performed significantly better than the 35-GHz system. For 300-m altitude with refrozen snow clutter background, neither system developed a large enough signal-to-clutter radio to detect the target reliably     

白天大气层内星敏感器观星能力分析

利用星敏感器进行白天大气层内观星,首先要解决的问题是强天空背景的干扰。通过对白天天空背景和不同光谱恒星的特点进行分析,提出了光谱滤波和偏振成像加形态学滤波和多帧图像累加的技术路线,综合利用光学方法和图像处理方法提高观测对比度和信噪比。给出了白天观星的信噪比模型,并在不同天空背景偏振度条件下,得出了G、K、M光谱恒星的0至5等星的单帧图像信噪比和对应的使CCD(Charge Coupled Device,电荷耦合器件)接近饱和的极限曝光时间。结果表明:星等值越低,天空背景偏振度越大,则信噪比越高;同等条件下,信噪比由小到大依次为G、K、M光谱恒星;而对于低信噪比的图像,通过多帧累加后可满足观测的要求。     

Effects of Random Implementation Errors in Matched Filters for Binary Phase Code Pulse Signals

The effects of implementation errors in tapped delay line filters on the peak-to-sidelobe ratio and the signal-to-noise ratio of binary PSK pulse compression codes are considered. As error model the delay elements and weighting factors of such filters are assumed to have an error distribution with known mean and variance.     

影象云纹仪及其在航空工程测量中的应用

 本文介绍影象云纹仪的光学原理与设计、结构的特点,结合实例说明了在航空工程测量中的应用。 影象云纹仪是一种非相干光光学图像测量仪器,利用光栅与其影象的干涉形成等高云纹,从而测量物体的三维形状和离面位移。文中用付里叶光学推导了影象云纹光场强度分布,得到云纹标高计算公式;创造了一种制作云纹光栅的金属模板直接拷贝法,研制出的光栅节距误差小,对比度高;仪器光栅主方向往复运动提高了图象的信噪比,沿主光轴的微动平移可实现分数级条纹补偿,并可进行凹凸形态的判定。 文中还给出了薄壁结构失稳扩展和屈曲形态的云纹图,以及旋翼叶片的等高云纹图。     

On the Mean Frequency Measurement System Using Correlation Detection

This paper is concerned with the problem of measuring the mean frequency of the power spectrum of a zero-mean, stationary, narrowband Gaussian random signal in the presence of additive Gaussian noise. Signal-to-noise ratios at the output of the mean frequency measurement system using correlation detection are analyzed in terms of input signal-to-noise ratio, input signal and noise bandwidths, and integration time. The results obtained are verified experimentally, and a comparison with a conventional zero-crossing detector is also made.     

The Response of Hard-Limiting Bandpass Limiters to PM Signals

The signal-to-noise ratio of the output of a hard-limiting bandpass limiter to a PM signal is calculated by using the probability density function of the random phase variable. The signal-to-noise ratio transfer characteristics are plotted for comparison.     

Signal-to-Noise Ratio in the Bandpass Output of a Discriminator

A formula is derived for the output signal-to-noise power ratio from a limiter-discriminator that is followed by an ideal (rectangular) bandpass filter. This signal-to-noise ratio is shown to be inversely proportional to the difference between the cube of the upper cutoff frequency and the cube of the lower cutoff frequency. The formula commonly utilized by designers for the discriminator output signal-to-noise ratio was derived for the case of a lowpass filter on the discriminator output. The bandpass and low-pass formulas are compared to indicate 1) the advantage of bandpass filtering for the discriminator output, and 2) the error incurred if the designer employs the low-pass formula to evaluate the performance of a bandpass filter design. Finally, the accuracy of a narrow-band approximation for the bandpass formula is evaluated.