Optimal Resolution of Rectangular Pulses in Noise

Optimal detection of rectangular pulses in noise is considered, subject to a sidelobe constraint which ensures adequate resolution capabilities, and a new sidelobe reduction filter is derived. Tests in the laboratory and on a Westinghouse AN/TPS-27 search radar system em indicate that use of the new filter substantially improves both resolution and clutter performance over such standard techniques as fast time constant (FTC), delay line differentiator (DLD), and pulse length discriminator (PLD).     

Detection Performance of the Cell Averaging LOG/CFAR Receiver

The cell averaging LOG/CFAR receiver is a special implementation of a constant-false-alarm-rate (CFAR) receiver in which the noise level estimate is derived from a set of contiguous time samples of the output of a logarithmic (LOG) detector as obtained from a tapped delay line. This CFAR receiver is capable of operating over a larger dynamic range of noise levels than a conventional cell averaging CFAR receiver, but with somewhat poorer detectability. The performance in stationary Gaussian noise of the cell averaging LOG/CFAR receiver with no post-detection integration is determined in this paper. For a small number of reference noise samples, results were obtained by a Monte Carlo simulation using the technique of importance sampling. For a large number of reference noise samples, a second moment analysis gave the desired results. Both these results can be summarized in the following simple formula, NLOG = 1.65NLIN - 0.65, which relates the number of reference samples required by each of the two receivers for equivalent performance. Thus, for the cell averaging LOG/CFAR receiver to give the same detection performance as the conventional cell averaging CFAR receiver, the number of reference noise samples has to be increased by up to 65 percent.     

Mismatched Filtering of Sonar Signals

A replica correlator (matched filter) is an optimum processor for a receiver employing a pulse of continuous wave (CW) signal in a white Gaussian noise background. In an active sonar, however, when the target of interest has low Doppler shift and is embedded in a high reverberation background, this is not so. High sidelobes of the correlator frequency response pass a significant portion of the signal contained in the mainlobe of the reverberation spectrum. In order to reduce the sidelobes of the correlator output spectrum and at the same time keep the increase in its 3 dB bandwidth to a small amount, we propose lengthening of the replica of the transmitted signal and weighting it by a Kaiser window. It is demonstrated that by extending the weighted replica by 50 percent compared with the transmitted signal, it is possible to reduce the sidelobe levels to at least 40 dB below the mainlobe peak, with the concomitant increase of the 3 dB band-width by less than 5 percent. The degradation in signal-to-noise ratio (SNR) performance for such a ?mismatched? filter receiver with respect to the matched filter is less than 1.5 dB.     

Mismatched-filter design for periodic binary phased signals

A mismatched filter design is introduced whereby the receiver filter coefficients are optimized with the property that all sidelobes of the cross-correlation function (CCF) are zero. Some results are given for binary sequences and the associated mismatched filters. For each length N>2 there exist at least one sequence and an associated mismatched filter with an ideal impulselike CCF, which means that there are no sidelobes at all in the receiver output signal     

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.     

A UHF True Logarithmic IF Amplifier

A UHF ?true logarithmic IF? which yields a carrier output proportional in amplitude to the logarithm of the carrier input has been constructed. Although this ?true? form has been known for a number of years, it appears to have been neglected in development. Good performance has been obtained over a 60-dB input signal range with rise and fall times of 2 ns per 20 dB. A high-level, detected dc coupled output without the use of any dc amplifier is obtained. Coherent signal processing of the output carrier is possible, and other important advantages to this type of logarithmic IF are described.     

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.     

基于I、Q信号的北斗接收机跟踪环路设计

传统的北斗接收机一般采用标量跟踪环,每个通道的卫星相互独立,在此 基础上,又发展起来了基于矢量跟踪的接收机,使每个通道的卫星跟踪不再相互独立。 提出的基于I、Q 信号观测的接收机跟踪环路,保留矢量跟踪的特点,并且采用EKF 作 为跟踪环路预处理滤波器,代替传统跟踪环路的鉴别器,可以在高动态的环境下对卫星 信号进行跟踪,提高环路的稳定性,从而可以有效地提高BDS/INS 深组合导航滤波器观 测量的估计精度。主要对高动态信号跟踪进行仿真,并与传统的标量跟踪方法和矢量跟 踪方法的跟踪能力进行比较。实验表明, 改进的矢量跟踪环能够在高动态的环境下运 行,比起传统环路有更小的跟踪误差。     

Concatenated Sequences for Spread Spectrum Systems

Concatenated sequences are suggested and developed for use in spread spectrum (SS) systems. Special receiver realization is discussed and it is shown that the concatenated sequences offer great advantage in reducing the size of the matched filter correlators (MFC) in the SS receiver. Experimental systems have been built using concatenated sequences and their performance is reported here. It has been shown that a processing gain of 60 is obtainable with a 15 × 4 concatenated sequence which requires three tapped delay lines (TDL), two of length 15 and one of 4. Thus a total of only 34 delay units (with a nonlinear interface network, only 19 delay units) are required. Teleprinter signals can be transmitted over telephone channels using a bandwidth (BW) expansion of 60 with an input signal-to-noise ratio (SNR) of only -5.1 dB, resulting in a character- error-rate of 1 in 103. Techniques have been developed to reduce the sidelobe levels in the aperiodic autocorrelation functions (ACF) of the pseudonoise (PN) codes. For 15 length and 7 length PN sequences, sidelobes are reduced by 9 dB and 9.4 dB, respectively, using transversal filters. Application of the SS systems to the problem of multiaccessing and antijamming are discussed.     

Spectral Energy Requirements and Adjacent-Channel Crosstalk in CAS

Four-frequency channels avoid the prior slot interference that handicaps single-frequency collision avoidance systems of equal message-slot capacity. Crosstalk between adjacent frequency channels is adequately limited If the receiver response to adjacent-channel signals is attenuated at least 52 dB. With 5-MHz channel separation, 48-dB crosstalk rejection was achieved in a test receiver by restricting the RF pulse rise time and decay time to 0.2 ?s minimum. Crosstalk resulting from biphase modulation can be effectively attenuated by limiting the minimum envelope transition time to 0.2 ?s. The effect of pulse rise time and decay time on the energy density spectrum is discussed.     

Adaptive Maximum Likelihood Receiver for Direct-Ranging Applications

Recent interest in direct-ranging navigation methods has prompted new research into practical receiver structures which provide improved signal processing. Maximum likelihood receivers are compared with conventional phase-locked receivers by theoretical and simulation methods as phase estimators based on signal measurements only. Signal-to-noise ratio (SNR) enhancement is examined and filter recommendations made. An adaptive maximum likelihood receiver is then developed which uses both signal measurements and a priori data to improve phase estimation accuracy. Simulation results indicate a 33.3 dB processing gain yielding a ranging error standard deviation of ±15 feet for zero dB received SNR. Nonadaptive and adaptive receivers are fabricated, flight tested, and experimental results reported.     

Mean Power Estimation with a Recursive Filter

Power estimates from smoothed detector outputs of stationary signals are analyzed for linear, square law, and logarithmic receiverdetectors. Smoothing is accomplished with a first-order recursive digital filter. lnput amplitude is assumed to be Rayleigh distributed and detector output samples independent. Expansion of results to other distributions is indicated.     

脉冲干扰对软件接收机的影响分析

卫星信号经过长距离传播,信号能量损耗严重,到达地面的功率很弱,容易受到各种干扰的影响。脉冲干扰为常见的干扰类型,所以针对不同功率、不同周期,以及不同占空比的脉冲干扰信号,通过接收前端采集受脉冲干扰的GPS L1信号,利用软件接收机及多相关器生成技术,详细分析了脉冲干扰对接收机信号捕获与跟踪性能的影响。分析结果表明,周期为1ms的脉冲干扰信号,能对接收机产生强烈的干扰效果,捕获图中的噪声明显增大;跟踪过程中,载噪比和相关值突发性减小,造成跟踪数据异常。而长周期的脉冲信号仅在脉冲到达时影响接收机的捕获和跟踪,但由于信号跟踪不能连续进行,导致伪距观测量的不连续与导航数据不能正常解码,从而干扰接收机。     

Signal Resolution via Digital Inverse Filtering

Research in numerous areas is directed toward the resolution of multiple overlapping signals in a noisy environment. These areas include radar, sonar, speech, seismology, and electrophysiology. Sometimes matched filters are used; other times inverse filters are employed. This paper discusses one approach to the analysis of the resolution of inverse filters. Our method is to compromise the trade-off between signal resolution and the output signal-to-noise ratio (SNR). A performance measure for the inverse or deconvolution filter is defined as a quantity proportional to the harmonic mean of the resolution and the SNR. An optimum output pulse duration is obtained using this criterion, where the pulse shape has been previously selected and the input signal waveform is known. In addition, upper and lower bounds for the output pulse duration are presented. Graphs are given which allow the designer to select the optimum inverse filter output pulse duration for a desired signal resolution and an estimated SNR.     

Probability Density Function at the Output of the Delay-Lock Discriminator

The first-order probability density at the output of the correlation network of the delay-lock discriminator is determined using the methods developed by Arthur, Kac, and Siegert. The analysis assumes that the input signal is stationary and Gaussian and that the discriminator is locked on. No restrictions are made on the bandpass filter characteristics in the channels.     

High Dynamic GPS Receiver Using Maximum Likelihood Estimationand Frequency Tracking

A new high dynamic global positioning system (GPS) receiver ispresented and its performance characterized by analysis,simulation, and demonstration. The demonstration receiver is abreadboard model capable of tracking a single simulated satellitesignal in pseudorange and range rate. Pseudorange and range rateestimates are made once every 20 ms, using a maximum likelihoodestimator, and are tracked by means of a third-order fadingmemory filter in a feedback configuration. The receiver trackspseudorange with rms errors of under 1 m when subjected tosimulated 50 g, 40 g/s circular trajectories. The tracking thresholdis approximately 28 dB·Hz, which provides 12 dB margin relativethe the minimum specified signal strength, assuming 3.5 dB systemnoise figure and 0 dB antenna gain.     

Detection of Known Signals in Nonstationary Noise

The basic design of a nonlinear, time-invariant filter is postulated for detecting signal pulses of known shape imbedded in nonstationary noise. The noise is a sample function of a Gaussian random process whose statistics are approximately constant during the length of a signal pulse. The parameters of the filter are optimized to maximize the output signal-to-noise ratio (SNR). The resulting nonlinear filter has the interesting property of approximating the performance of an adaptive filter in that it weights each frequency band of each input pulse by a factor that depends on the instantaneous noise power spectrum present at that time. The SNR at the output of the nonlinear filter is compared to that at the output of a matched filter. The relative performance of the nonlinear system is good when the signal pulses have large time-bandwidth products and the instantaneous noise power spectrum is colored in the signal pass band.     

Matched-Filter Response to a Linear FM Signal Transmitted Through a Phased Array

The response of a linear phased array with a matched filter connected at its output is investigated when a linear FM signal is incident on the array at an arbitrary angle. The filter is assumed to be matched to the linear FM signal. The dispersion produced by the array results in a mismatch at the receiver which depends on the scan angle and on the type of feed system used with the array. The distortion of the compressed pulse is studied for the series end-fed arrays, the series     

Monopulse Radar Air-ti-Ground Ranging

The use of a two-lobe monopulse radar for measuring slant range to the surface of the earth in the absence of discrete targets is analyzed. It is shown that tracking dispersion can be considered as the resultant of two components. One component is independent of range and results from the finite pulse length and gate length and the random nature of the return signals. The other component is due to receiver noise and increases as the signal-to-noise ratio decreases. The dispersion component independent of range is shown to be proportional to the pulse length and tracking gate length. The variable dispersion is shown to be proportional to the five halves power of the range and the three halves power of the cotangent of the depression angle of the antenna boresight axis. Performance calculations for a specific radar are carried out and compared with experimental data.     

Logarithmic Detection with Postdetection Filtering

An analysis is presented of the performance of a logarithmic detector or followed by a low-pass filter, under the assumption of a large ratio of IF bandwidth to postdetection bandwidth. This allows for the probability of detection and similar quantities to be computed in a straightforward manner. The mean and variance of the filter output are found by first deriving the autocorrelation function of the logarithmic detector output and then obtaining the power spectral ral density. A useful engineering approximation is made for the variance which is valid for all signal-to-noise ratios.